Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of 1.00794 u (1.007825 u for hydrogen-1), hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental massHP Pavilion dv6-1299lx battery.[5] Non-remnant stars are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth.
The most common isotope of hydrogen is protium (name rarely used, symbol 1H) with a single proton and no neutrons. In ionic compounds it can take a negative charge (an anion known as a hydride and written as H−), or as a positively charged species H+HP Pavilion dv6-1299ez battery. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species. Hydrogen forms compounds with most elements and is present in water and most organic compoundsHP Pavilion dv6-1299es battery. It plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanicsHP Pavilion dv6-1299er battery.
Hydrogen gas (now known to be H2) was first artificially produced in the early 16th century, via the mixing of metals with strong acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance,[6] and that it produces water when burned, a property which later gave it its nameHP Pavilion dv6-1299eo battery, which in Greek means "water-former." At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, non-toxic, highly combustible diatomic gas with the molecular formula H2.
Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of waterHP Pavilion dv6-1299ef battery.[7] Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.
Hydrogen is a concern in metallurgy as it can embrittle many metals,[8] complicating the design of pipelines and storage tanksHP Pavilion dv6-1299ea battery.[9]
Properties
Combustion
The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Hydrogen gas (dihydrogen or molecular hydrogen)[10] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.[11] The enthalpy of combustion for hydrogen is −286 kJ/mol:[12]
2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)[note 1HP Pavilion dv6-1295el battery]
Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures spontaneously explode by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F) HP Pavilion dv6-1291es battery.[13] Pure hydrogen-oxygen flames emit ultraviolet light and are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerousHP Pavilion dv6-1290ev battery. The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of combustible materials in the ship's skin.[14] Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and many deaths were instead the result of falls or burning diesel fuelHP Pavilion dv6-1290es battery.[15]
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids. HP Pavilion dv6-1290en battery
Electron energy levels
Main article: Hydrogen atom
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale).
The ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm wavelengthHP Pavilion dv6-1290ec battery.[17]
The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the electromagnetic force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by gravityHP Pavilion dv6-1288la battery. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.[18]
A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation or the Feynman path integral formulation to calculate the probability density of the electron around the protonHP Pavilion dv6-1285es battery.[19] The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all— an illustration of how different the "planetary orbit" conception of electron motion differs from realityHP Pavilion dv6-1282et battery.
Elemental molecular forms
See also: Spin isomers of hydrogen
First tracks observed in liquid hydrogen bubble chamber at the Bevatron
There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei.[20] In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (½+½); in the parahydrogen form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 (½–½)HP Pavilion dv6-1280us battery. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form, also known as the "normal form".[21] The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy than the para form, it is unstable and cannot be purifiedHP Pavilion dv6-1280sn battery. At very low temperatures, the equilibrium state is composed almost exclusively of the para form. The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities, as discussed more fully in Spin isomers of hydrogen.[22] The ortho/para distinction also occurs in other hydrogen-containing molecules or functional groupsHP Pavilion dv6-1280ev battery, such as water and methylene, but is of little significance for their thermal properties.[23]
The uncatalyzed interconversion between para and ortho H2 increases with increasing temperature; thus rapidly condensed H2 contains large quantities of the high-energy ortho form that converts to the para form very slowlyHP Pavilion dv6-1280et battery.[24] The ortho/para ratio in condensed H2 is an important consideration in the preparation and storage of liquid hydrogen: the conversion from ortho to para is exothermic and produces enough heat to evaporate some of the hydrogen liquid, leading to loss of liquefied material. Catalysts for the ortho-para interconversionHP Pavilion dv6-1280es battery, such as ferric oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromic oxide, or some nickel[25] compounds, are used during hydrogen cooling.[26]
Further information: Hydrogen compounds
Covalent and organic compounds
While H2 is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more electronegative, such as halogens (e.g., F, Cl, Br, I), or oxygenHP Pavilion dv6-1280ep battery; in these compounds hydrogen takes on a partial positive charge.[27] When bonded to fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding called hydrogen bonding, which is critical to the stability of many biological molecules.[28][29] Hydrogen also forms compounds with less electronegative elementsHP Pavilion dv6-1280el battery, such as the metals and metalloids, in which it takes on a partial negative charge. These compounds are often known as hydrides.[30]
Hydrogen forms a vast array of compounds with carbon called the hydrocarbons, and an even vaster array with heteroatoms that, because of their general association with living things, are called organic compoundsHP Pavilion dv6-1277la battery.[31] The study of their properties is known as organic chemistry[32] and their study in the context of living organisms is known as biochemistry.[33] By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogenHP Pavilion dv6-1277el battery, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.[31] Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways, which seldom involve elementary hydrogenHP Pavilion dv6-1275la battery.
Hydrides
Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H−, and is used when hydrogen forms a compound with a more electropositive element. The existence of the hydride anionHP Pavilion dv6-1275eg battery, suggested by Gilbert N. Lewis in 1916 for group I and II salt-like hydrides, was demonstrated by Moers in 1920 with the electrolysis of molten lithium hydride (LiH), that produced a stoichiometry quantity of hydrogen at the anode.[34] For hydrides other than group I and II metalsHP Pavilion dv6-1270st battery, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group II hydrides is BeH2, which is polymeric. In lithium aluminium hydride, the AlH−
4 anion carries hydridic centers firmly attached to the Al(III).
Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, there are over 100 binary borane hydrides known, but only one binary aluminium hydrideHP Pavilion dv6-1270ss battery.[35] Binary indium hydride has not yet been identified, although larger complexes exist.[36]
In inorganic chemistry, hydrides can also serve as bridging ligands that link two metal centers in a coordination complex. This function is particularly common in group 13 elements, especially in boranes (boron hydrides) and aluminium complexes, as well as in clustered carboranesHP Pavilion dv6-1270la battery.[37]
Protons and acids
Further information: Acid–base reaction
Oxidation of hydrogen removes its electron and gives H+, which contains no electrons and a nucleus which is usually composed of one proton. That is why H+ is often called a proton. This species is central to discussion of acids. Under the Bronsted-Lowry theory, acids are proton donors, while bases are proton acceptorsHP Pavilion dv6-1270ev battery.
A bare proton, H+, cannot exist in solution or in ionic crystals, because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the electron clouds of atoms and molecules, and will remain attached to themHP Pavilion dv6-1270et battery. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or cationic hydrogen attached to other species in this fashion, and as such is denoted "H+" without any implication that any single protons exist freely as a speciesHP Pavilion dv6-1270eq battery.
To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "hydronium ion" (H3O+). However, even in this case, such solvated hydrogen cations are thought more realistically physically to be organized into clusters that form species closer to H9O+HP Pavilion dv6-1270el battery
4.[38] Other oxonium ions are found when water is in solution with other solvents.[39]
Although exotic on earth, one of the most common ions in the universe is the H+
3 ion, known as protonated molecular hydrogen or the trihydrogen cation.[40]
Isotopes
Main article: Isotopes of hydrogen
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tubeHP Pavilion dv6-1270eg battery
Protium, the most common isotope of hydrogen, has one proton and one electron. Unique among all stable isotopes, it has no neutrons (see diproton for a discussion of why others do not exist).
Hydrogen has three naturally occurring isotopes, denoted 1H, 2H and 3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in the laboratory but not observed in nature. HP Pavilion dv6-1270ec battery
1H is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.[43]
2H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleusHP Pavilion dv6-1268tx battery. Essentially all deuterium in the universe is thought to have been produced at the time of the Big Bang, and has endured since that time. Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy waterHP Pavilion dv6-1268nr battery. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-NMR spectroscopy.[44] Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusionHP Pavilion dv6-1266el battery.[45]
3H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32 years.[37] It is so radioactive that it can be used in luminous paint, making it useful in such things as watchesHP Pavilion dv6-1265tx battery. The glass prevents the small amount of radiation from getting out.[46] Small amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases; tritium has also been released during nuclear weapons tests.[47] It is used in nuclear fusion reactions,[48] as a tracer in isotope geochemistry,[49] and specialized in self-powered lighting devicesHP Pavilion dv6-1265sg battery.[50] Tritium has also been used in chemical and biological labeling experiments as a radiolabel.[51]
Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritiumHP Pavilion dv6-1264tx battery. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, but the corresponding symbol for protium, P, is already in use for phosphorus and thus is not available for protium.[52] In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry allows any of D, T, 2H, and 3H to be used, although 2H and 3H are preferredHP Pavilion dv6-1264ca battery.[53]
History
Discovery and use
Main article: Timeline of hydrogen technologies
In 1671, Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.[54][55] In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "flammable air"HP Pavilion dv6-1263cl battery. He speculated that "flammable air" was in fact identical to the hypothetical substance called "phlogiston"[56][57] and further finding in 1781 that the gas produces water when burned. He is usually given credit for its discovery as an element.[58][59] In 1783, Antoine Lavoisier gave the element the name hydrogen (from the Greek ὕδρω hydro meaning water and γενῆς genes meaning creator) HP Pavilion dv6-1261tx battery when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.[59]
Antoine-Laurent de Lavoisier
Lavoisier produced hydrogen for his famous experiments on mass conservation by reacting a flux of steam with metallic iron through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactionsHP Pavilion dv6-1260tx battery:
Fe + H2O → FeO + H2
2 Fe + 3 H2O → Fe2O3 + 3 H2
3 Fe + 4 H2O → Fe3O4 + 4 H2
Many metals such as zirconium undergo a similar reaction with water leading to the production of hydrogen.
Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask.[59] He produced solid hydrogen the next year.[59] Deuterium was discovered in December 1931 by Harold UreyHP Pavilion dv6-1260sn battery, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck.[58] Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.[59] François Isaac de Rivaz built the first internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819HP Pavilion dv6-1260sh battery. The Döbereiner's lamp and limelight were invented in 1823.[59]
The first hydrogen-filled balloon was invented by Jacques Charles in 1783.[59] Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by Henri Giffard.[59] German count Ferdinand von Zeppelin promoted the idea of rigid airships lifted by hydrogen that later were called ZeppelinsHP Pavilion dv6-1260sg battery; the first of which had its maiden flight in 1900.[59] Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the warHP Pavilion dv6-1260se battery.
The first non-stop transatlantic crossing was made by the British airship R34 in 1919. Regular passenger service resumed in the 1920s and the discovery of helium reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, H2 was used in the Hindenburg airship, which was destroyed in a midair fire over New Jersey on May 6, 1937HP Pavilion dv6-1260ev battery.[59] The incident was broadcast live on radio and filmed. Ignition of leaking hydrogen is widely assumed to be the cause, but later investigations pointed to the ignition of the aluminized fabric coating by static electricity. But the damage to hydrogen's reputation as a lifting gas was already doneHP Pavilion dv6-1260et battery.
In the same year the first hydrogen-cooled turbogenerator went into service with gaseous hydrogen as a coolant in the rotor and the stator in 1937 at Dayton, Ohio, by the Dayton Power & Light Co,[61] because of the thermal conductivity of hydrogen gas this is the most common type in its field todayHP Pavilion dv6-1260ep battery.
The nickel hydrogen battery was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).[62] For example, the ISS,[63] Mars Odyssey[64] and the Mars Global Surveyor[65] are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteriesHP Pavilion dv6-1260ej battery, which were finally replaced in May 2009, more than 19 years after launch, and 13 years over their design life.
Role in quantum theory
Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series
Because of its relatively simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structureHP Pavilion dv6-1260ei battery.[66] Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation H2+ allowed fuller understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.
One of the first quantum effects to be explicitly noticed (but not understood at the time) was a Maxwell observation involving hydrogenHP Pavilion dv6-1260ec battery, half a century before full quantum mechanical theory arrived. Maxwell observed that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theoryHP Pavilion dv6-1259tx battery, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H2 because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect. HP Pavilion dv6-1259dx battery
Natural occurrence
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy
Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms (most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as dark matter and dark energy). HP Pavilion dv6-1256tx battery This element is found in great abundance in stars and gas giant planets. Molecular clouds of H2 are associated with star formation. Hydrogen plays a vital role in powering stars through proton-proton reaction and CNO cycle nuclear fusion.[69]
Throughout the universe, hydrogen is mostly found in the atomic and plasma states whose properties are quite different from molecular hydrogenHP Pavilion dv6-1255tx battery. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the auroraHP Pavilion dv6-1255eg battery. Hydrogen is found in the neutral atomic state in the Interstellar medium. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the Universe up to redshift z=4.[70]
Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H2 (for data see table) HP Pavilion dv6-1255ee battery. However, hydrogen gas is very rare in the Earth's atmosphere (1 ppm by volume) because of its light weight, which enables it to escape from Earth's gravity more easily than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface,[71] mostly in the form of chemical compounds such as hydrocarbons and waterHP Pavilion dv6-1254tx battery.[37] Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus, as is methane, itself a hydrogen source of increasing importance.[72]
A molecular form called protonated molecular hydrogen (H3+) is found in the interstellar medium (ISM), where it is generated by ionization of molecular hydrogen from cosmic raysHP Pavilion dv6-1253cl battery. It has also been observed in the upper atmosphere of the planet Jupiter. This ion is relatively stable in the environment of outer space due to the low temperature and density. H3+ is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium.[73] Neutral triatomic hydrogen H3 can only exist in an excited form and is unstableHP Pavilion dv6-1253ca battery.[74] The hydrogen molecular ion (H2+) is a rare molecular system in the universe.
Production
For more details on this topic, see Hydrogen production.
H2 is produced in chemistry and biology laboratories, often as a by-product of other reactions; in industry for the hydrogenation of unsaturated substrates; and in nature as a means of expelling reducing equivalents in biochemical reactions.
Laboratory
In the laboratory, H2 is usually prepared by the reaction of acids on metals such as zinc with Kipp's apparatusHP Pavilion dv6-1252tx battery.
Zn + 2 H+ → Zn2+ + H2
Aluminium can also produce H2 upon treatment with bases:
2 Al + 6 H2O + 2 OH− → 2 Al(OH)−
4 + 3 H2
The electrolysis of water is a simple method of producing hydrogen. A low voltage current is run through the water, and gaseous oxygen forms at the anode while gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, howeverHP Pavilion dv6-1250us battery, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is in the range 80–94%.[75]
2 H2O(l) → 2 H2(g) + O2(g) HP Pavilion dv6-1250ss battery
In 2007, it was discovered that an alloy of aluminium and gallium in pellet form added to water could be used to generate hydrogen. The process also creates alumina, but the expensive gallium, which prevents the formation of an oxide skin on the pellets, can be re-used. This has important potential implications for a hydrogen economy, as hydrogen can be produced on-site and does not need to be transportedHP Pavilion dv6-1250sp battery.[76]
Industrial
Main article: Hydrogen production
Hydrogen can be prepared in several different ways, but economically the most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas.[77] At high temperatures (1000–1400 K, 700–1100 °C or 1300–2000 °F), steam (water vapor) reacts with methane to yield carbon monoxide and H2HP Pavilion dv6-1250sn battery.
CH4 + H2O → CO + 3 H2
This reaction is favored at low pressures but is nonetheless conducted at high pressures (2.0 MPa, 20 atm or 600 inHg). This is because high-pressure H2 is the most marketable product and Pressure Swing Adsorption (PSA) purification systems work better at higher pressuresHP Pavilion dv6-1250sc battery. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and related compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbonHP Pavilion dv6-1250eq battery:
CH4 → C + 2 H2
Consequently, steam reforming typically employs an excess of H2O. Additional hydrogen can be recovered from the steam by use of carbon monoxide through the water gas shift reaction, especially with an iron oxide catalyst. This reaction is also a common industrial source of carbon dioxide:[77]
CO + H2O → CO2 + H2
Other important methods for H2 production include partial oxidation of hydrocarbons: HP Pavilion dv6-1250ep battery
2 CH4 + O2 → 2 CO + 4 H2
and the coal reaction, which can serve as a prelude to the shift reaction above:[77]
C + H2O → CO + H2
Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for the production of ammonia, hydrogen is generated from natural gas.[79] Electrolysis of brine to yield chlorine also produces hydrogen as a co-product. HP Pavilion dv6-1250eo battery
Thermochemical
There are more than 200 thermochemical cycles which can be used for water splitting, around a dozen of these cycles such as the iron oxide cycle, cerium(IV) oxide-cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine cycle, copper-chlorine cycle and hybrid sulfur cycle are under research and in testing phase to produce hydrogen and oxygen from water and heat without using electricityHP Pavilion dv6-1250el battery.[81] A number of laboratories (including in France, Germany, Greece, Japan, and the USA) are developing thermochemical methods to produce hydrogen from solar energy and water.[82]
Anaerobic corrosion
Under anaerobic conditions, iron and steel alloys are slowly oxidized by the protons of water concomitantly reduced in molecular hydrogen (H2). The anaerobic corrosion of iron leads first to the formation of ferrous hydroxide (green rust) and can be described by the following reactionHP Pavilion dv6-1250eg battery:
Fe + 2 H2O → Fe(OH)2 + H2
In its turn, under anaerobic conditions, the ferrous hydroxide (Fe(OH)2 ) can be oxidized by the protons of water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:
3 Fe(OH)2 → Fe3O4 + 2 H2O + H2
ferrous hydroxide → magnetite + water + hydrogen
The well crystallized magnetite (Fe3O4) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)2 ) HP Pavilion dv6-1250eb battery.
This process occurs during the anaerobic corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.
Geological occurrence: the serpentinization reaction
In the absence of atmospheric oxygen (O2), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H2) is produced during the process of serpentinization by the anaerobic oxidation by the water protons HP Pavilion dv6-1249tx battery (H+) of the ferrous (Fe2+) silicate present in the crystal lattice of the fayalite (Fe2SiO4, the olivine iron-endmember). The corresponding reaction leading to the formation of magnetite (Fe3O4), quartz (SiO2) and hydrogen (H2) is the following:
3 Fe2SiO4 + 2 H2O → 2 Fe3O4 + 3 SiO2 + 3 H2
fayalite + water → magnetite + quartz + hydrogen
This reaction closely resembles the Schikorr reaction observed in the anaerobic oxidation of the ferrous hydroxide in contact with waterHP Pavilion dv6-1249eo battery.
Applications
Consumption in processes
Large quantities of H2 are needed in the petroleum and chemical industries. The largest application of H2 is for the processing ("upgrading") of fossil fuels, and in the production of ammonia. The key consumers of H2 in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocrackingHP Pavilion dv6-1248eo battery. H2 has several other important uses. H2 is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated fats and oils (found in items such as margarine), and in the production of methanol. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H2 is also used as a reducing agent of metallic oresHP Pavilion dv6-1248ca battery.[83]
Hydrogen is highly soluble in many rare earth and transition metals[84] and is soluble in both nanocrystalline and amorphous metals.[85] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.[86] These properties may be useful when hydrogen is purified by passage through hot palladium disksHP Pavilion dv6-1247tx battery, but the gas's high solubility is a metallurgical problem, contributing to the embrittlement of many metals,[8] complicating the design of pipelines and storage tanks.[9]
Apart from its use as a reactant, H2 has wide applications in physics and engineering. It is used as a shielding gas in welding methods such as atomic hydrogen weldingHP Pavilion dv6-1247ez battery.[87][88] H2 is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas. Liquid H2 is used in cryogenic research, including superconductivity studies.[89] Because H2 is lighter than air, having a little more than 1⁄15 of the density of air, it was once widely used as a lifting gas in balloons and airshipsHP Pavilion dv6-1247et battery.[90]
In more recent applications, hydrogen is used pure or mixed with nitrogen (sometimes called forming gas) as a tracer gas for minute leak detection. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industriesHP Pavilion dv6-1247eo battery.[91] Hydrogen is an authorized food additive (E 949) that allows food package leak testing among other anti-oxidizing properties.[92]
Hydrogen's rarer isotopes also each have specific applications. Deuterium (hydrogen-2) is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions.[59] Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effectsHP Pavilion dv6-1247cl battery.[93] Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs,[94] as an isotopic label in the biosciences,[51] and as a radiation source in luminous paints.[95]
The triple point temperature of equilibrium hydrogen is a defining fixed point on the ITS-90 temperature scale at 13.8033 kelvinsHP Pavilion dv6-1246tx battery.[96]
Coolant
Hydrogen is commonly used in power stations, as a coolant in generators, due to its specific heat capacity being considerably higher than any other gas.
Energy carrier
See also: Hydrogen economy and Hydrogen infrastructure
Hydrogen is not an energy resource,[97] except in the hypothetical context of commercial nuclear fusion power plants using deuterium or tritium, a technology presently far from developmentHP Pavilion dv6-1246eo battery.[98] The Sun's energy comes from nuclear fusion of hydrogen, but this process is difficult to achieve controllably on Earth.[99] Elemental hydrogen from solar, biological, or electrical sources require more energy to make it than is obtained by burning it, so in these cases hydrogen functions as an energy carrier, like a batteryHP Pavilion dv6-1245et battery. Hydrogen may be obtained from fossil sources (such as methane), but these sources are unsustainable.[97]
The energy density per unit volume of both liquid hydrogen and compressed hydrogen gas at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel mass is higher.[97] Nevertheless, elemental hydrogen has been widely discussed in the context of energyHP Pavilion dv6-1245es battery, as a possible future carrier of energy on an economy-wide scale.[100] For example, CO2 sequestration followed by carbon capture and storage could be conducted at the point of H2 production from fossil fuels.[101] Hydrogen used in transportation would burn relatively cleanly, with some NOx emissions,[102] but without carbon emissionsHP Pavilion dv6-1245eo battery.[101] However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial.[103]
Semiconductor industry
Hydrogen is employed to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon that helps stabilizing material properties.[104] It is also a potential electron donor in various oxide materials, including ZnO,[105][106] SnO2, CdO, MgO,[107] ZrO2, HfO2, La2O3, Y2O3, TiO2, SrTiO3, LaAlO3, SiO2, Al2O3, ZrSiO4, HfSiO4, and SrZrO3HP Pavilion dv6-1245dx battery.[108]
Biological reactions
Further information: Biohydrogen and Biological hydrogen production
H2 is a product of some types of anaerobic metabolism and is produced by several microorganisms, usually via reactions catalyzed by iron- or nickel-containing enzymes called hydrogenases. These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electronsHP Pavilion dv6-1244tx battery. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water.[109]
Water splitting, in which water is decomposed into its component protons, electrons, and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms, including the alga Chlamydomonas reinhardtii and cyanobacteriaHP Pavilion dv6-1244sb battery, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast.[110] Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H2 gas even in the presence of oxygen.[111] Efforts have also been undertaken with genetically modified alga in a bioreactorHP Pavilion dv6-1243cl battery.[112]
Safety and precautions
Main article: Hydrogen safety
Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form.[113] In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquidsHP Pavilion dv6-1242tx battery.[114] Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement,[115] leading to cracks and explosions.[116] Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burnsHP Pavilion dv6-1242et battery.[117]
Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Many physical and chemical properties of hydrogen depend on the parahydrogen/orthohydrogen ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given) HP Pavilion dv6-1241et battery. Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.[113]
Oxygen ( /ˈɒksɨdʒɨn/ ok-si-jin) is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς (oxys) ("acid", literally "sharp", referring to the sour taste of acids) and -γενής (-genēs) ("producer", literally "begetter")HP Pavilion dv6-1241eo battery, because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a very pale blue, odorless, tasteless diatomic gas with the formula O2HP Pavilion dv6-1240us battery.
Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with almost all other elements. Oxygen is a strong oxidizing agent and has the second-highest electronegativity of all the elements (only fluorine has a higher electronegativity) HP Pavilion dv6-1240tx battery.[1] By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium[2] and the most abundant element by mass in the Earth's crust, making up almost half of the crust's mass.[3] Free oxygen is too chemically reactive to appear on Earth without the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from waterHP Pavilion dv6-1240sh battery. Elemental O2 only began to accumulate in the atmosphere after the evolutionary appearance of these organisms, roughly 2.5 billion years ago.[4] Diatomic oxygen gas constitutes 20.8% of the volume of air.[5]
Because it comprises most of the mass in water, oxygen comprises most of the mass of living organisms (for example, about two-thirds of the human body's mass). All major classes of structural molecules in living organisms, such as proteinsHP Pavilion dv6-1240sf battery, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphereHP Pavilion dv6-1240sb battery. Another form (allotrope) of oxygen, ozone (O3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraftHP Pavilion dv6-1240ez battery.[6]
Oxygen was independently discovered by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774, but Priestley is often given priority because his work was published first. The name oxygen was coined in 1777 by Antoine Lavoisier,[7] whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosionHP Pavilion dv6-1240ep battery. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water and other means. Uses of oxygen include the production of steel, plastics and textiles; rocket propellant; oxygen therapy; and life support in aircraft, submarines, spaceflight and divingHP Pavilion dv6-1240eo battery.
Characteristics
Structure
Oxygen discharge (spectrum) tube
At standard temperature and pressure, oxygen is a very pale blue, odorless gas with the molecular formula O2, in which the two oxygen atoms are chemically bonded to each other with a spin triplet electron configuration. This bond has a bond order of two, and is often simplified in description as a double bond[8] or as a combination of one two-electron bond and two three-electron bondsHP Pavilion dv6-1240el battery.[9]
Triplet oxygen (not to be confused with ozone, O3) is the ground state of the O2 molecule.[10] The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals.[11] These orbitals are classified as antibonding (weakening the bond order from three to two) HP Pavilion dv6-1240ek battery, so the diatomic oxygen bond is weaker than the diatomic nitrogen triple bond in which all bonding molecular orbitals are filled, but some antibonding orbitals are not.[10]
A trickle of liquid oxygen is deflected by a magnetic field, illustrating its paramagnetic property
In normal triplet form, O2 molecules are paramagnetic. That is, they form a magnet in the presence of a magnetic field—because of the spin magnetic moments of the unpaired electrons in the moleculeHP Pavilion dv6-1240ei battery, and the negative exchange energy between neighboring O2 molecules.[12] Liquid oxygen is attracted to a magnet to a sufficient extent that, in laboratory demonstrations, a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet. HP Pavilion dv6-1240ef battery
Singlet oxygen is a name given to several higher-energy species of molecular O2 in which all the electron spins are paired. It is much more reactive towards common organic molecules than is molecular oxygen per se. In nature, singlet oxygen is commonly formed from water during photosynthesis, using the energy of sunlightHP Pavilion dv6-1240ed battery.[15] It is also produced in the troposphere by the photolysis of ozone by light of short wavelength,[16] and by the immune system as a source of active oxygen.[17] Carotenoids in photosynthetic organisms (and possibly also in animals) play a major role in absorbing energy from singlet oxygen and converting it to the unexcited ground state before it can cause harm to tissuesHP Pavilion dv6-1240ec battery.[18]
Allotropes
Main article: Allotropes of oxygen
Ozone is a rare gas on Earth found mostly in the stratosphere.
The common allotrope of elemental oxygen on Earth is called dioxygen, O2. It has a bond length of 121 pm and a bond energy of 498 kJ·mol−1.[19] This is the form that is used by complex forms of life, such as animals, in cellular respiration (see Biological role) and is the form that is a major part of the Earth's atmosphere (see Occurrence) HP Pavilion dv6-1240ea battery. Other aspects of O2 are covered in the remainder of this article.
Trioxygen (O3) is usually known as ozone and is a very reactive allotrope of oxygen that is damaging to lung tissue.[20] Ozone is produced in the upper atmosphere when O2 combines with atomic oxygen made by the splitting of O2 by ultraviolet (UV) radiation.[7] Since ozone absorbs strongly in the UV region of the spectrumHP Pavilion dv6-1239tx battery, the ozone layer of the upper atmosphere functions as a protective radiation shield for the planet.[7] Near the Earth's surface, however, it is a pollutant formed as a by-product of automobile exhaust.[20] The metastable molecule tetraoxygen (O4) was discovered in 2001,[21][22] and was assumed to exist in one of the six phases of solid oxygenHP Pavilion dv6-1239et battery. It was proven in 2006 that this phase, created by pressurizing O2 to 20 GPa, is in fact a rhombohedral O8 cluster.[23] This cluster has the potential to be a much more powerful oxidizer than either O2 or O3 and may therefore be used in rocket fuel.[21][22] A metallic phase was discovered in 1990 when solid oxygen is subjected to a pressure of above 96 GPa[24] and it was shown in 1998 that at very low temperatures, this phase becomes superconducting. HP Pavilion dv6-1239es battery
Physical properties
See also: Liquid oxygen and solid oxygen
Oxygen is more soluble in water than nitrogen is; water contains approximately 1 molecule of O2 for every 2 molecules of N2, compared to an atmospheric ratio of approximately 1:4. The solubility of oxygen in water is temperature-dependent, and about twice as much (14.6 mg·L−1) dissolves at 0 °C than at 20 °C (7.6 mg·L−1). HP Pavilion dv6-1238nr battery At 25 °C and 1 standard atmosphere (101.3 kPa) of air, freshwater contains about 6.04 milliliters (mL) of oxygen per liter, whereas seawater contains about 4.95 mL per liter.[28] At 5 °C the solubility increases to 9.0 mL (50% more than at 25 °C) per liter for water and 7.2 mL (45% more) per liter for sea water.
Oxygen condenses at 90.20 K (−182.95 °C, −297.31 °F) HP Pavilion dv6-1238et battery, and freezes at 54.36 K (−218.79 °C, −361.82 °F).[29] Both liquid and solid O2 are clear substances with a light sky-blue color caused by absorption in the red (in contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid O2 is usually obtained by the fractional distillation of liquefied airHP Pavilion dv6-1238es battery.[30] Liquid oxygen may also be produced by condensation out of air, using liquid nitrogen as a coolant. It is a highly reactive substance and must be segregated from combustible materials.[31]
Isotopes and stellar origin
Main article: Isotopes of oxygen
Late in a massive star's life, 16O concentrates in the O-shell, 17O in the H-shell and oxygen-1818O in the He-shell.
Naturally occurring oxygen is composed of three stable isotopes, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance) HP Pavilion dv6-1238ca battery.[32]
Most 16O is synthesized at the end of the helium fusion process in massive stars but some is made in the neon burning process.[33] 17O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars.[33] Most 18O is produced when 14NHP Pavilion dv6-1237et battery (made abundant from CNO burning) captures a 4He nucleus, making 18O common in the helium-rich zones of evolved, massive stars.[33]
Fourteen radioisotopes have been characterized. The most stable are 15O with a half-life of 122.24 seconds and 14O with a half-life of 70.606 seconds.[32] All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 millisecondsHP Pavilion dv6-1237es battery.[32] The most common decay mode of the isotopes lighter than 16O is β+ decay[34][35][36] to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay to yield fluorine.[32]
Occurrence
Oxygen is the most abundant chemical element, by mass, in our biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium.[2] About 0.9% of the Sun's mass is oxygenHP Pavilion dv6-1237ca battery.[5] Oxygen constitutes 49.2% of the Earth's crust by mass[3] and is the major component of the world's oceans (88.8% by mass).[5] Oxygen gas is the second most common component of the Earth's atmosphere, taking up 20.8% of its volume and 23.1% of its mass (some 1015 tonnes). Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere: Mars (with 0.1% O2 by volume) HP Pavilion dv6-1236ss battery and Venus have far lower concentrations. However, the O2 surrounding these other planets is produced solely by ultraviolet radiation impacting oxygen-containing molecules such as carbon dioxide.
The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on EarthHP Pavilion dv6-1236et battery: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration and decay remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate of roughly 1/2000th of the entire atmospheric oxygen per yearHP Pavilion dv6-1236eo battery.
Cold water holds more dissolved O2.
Free oxygen also occurs in solution in the world's water bodies. The increased solubility of O2 at lower temperatures (see Physical properties) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content.[40] Polluted water may have reduced amounts of O2 in itHP Pavilion dv6-1235ss battery, depleted by decaying algae and other biomaterials through a process called eutrophication. Scientists assess this aspect of water quality by measuring the water's biochemical oxygen demand, or the amount of O2 needed to restore it to a normal concentration. HP Pavilion dv6-1235sb battery
Biological role
Main article: Dioxygen in biological reactions
Photosynthesis and respiration
Photosynthesis splits water to liberate O2 and fixes CO2 into sugar.
In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, Green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on earth and the rest is produced by terrestrial plants. HP Pavilion dv6-1235et battery Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.[43]
A simplified overall formula for photosynthesis is:[44]
6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2 (or simply carbon dioxide + water + sunlight → glucose + dioxygen) HP Pavilion dv6-1235es battery
Photolytic oxygen evolution occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons.[45] Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize ATP via photophosphorylation.[46] The O2 remaining after oxidation of the water molecule is released into the atmosphere. HP Pavilion dv6-1235eo battery
Molecular dioxygen, O2, is essential for cellular respiration in all aerobic organisms. Oxygen is used in mitochondria to help generate adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified asHP Pavilion dv6-1235ee battery:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 2880 kJ·mol−1
In vertebrates, O2 diffuses through membranes in the lungs and into red blood cells. Hemoglobin binds O2, changing its color from bluish red to bright red[20] (CO2 is released from another part of hemoglobin through the Bohr effect). Other animals use hemocyanin (molluscs and some arthropods) or hemerythrin (spiders and lobsters).[38] A liter of blood can dissolve 200 cm3 of O2. HP Pavilion dv6-1235ec battery
Reactive oxygen species, such as superoxide ion (O−
2) and hydrogen peroxide (H2O2), are dangerous by-products of oxygen use in organisms.[38] Parts of the immune system of higher organisms, however, create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in the hypersensitive response of plants against pathogen attack. HP Pavilion dv6-1234tx battery
An adult human in rest inhales 1.8 to 2.4 grams of oxygen per minute.[48] This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.[49]
Content in body
The oxygen content in the body of a living organism is usually highest in the respiratory system, and decreases along any arterial system, peripheral tissues and venous system, respectivelyHP Pavilion dv6-1234nr battery. Oxygen content in this sense is often given as the partial pressure, which is the pressure which oxygen would have if it alone occupied the volume.[53]
Build-up in the atmosphere
Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved. Free oxygen first appeared in significant quantities during the Paleoproterozoic eon (between 2.5 and 1.6 billion years ago). At first, the oxygen combined with dissolved iron in the oceans to form banded iron formationsHP Pavilion dv6-1234eo battery. Free oxygen started to outgas from the oceans 2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago.[54]
The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the Great Oxygenation Event(oxygen catastrophe) about 2.4 billion years agoHP Pavilion dv6-1233et battery. However, cellular respiration using O2 enables aerobic organisms to produce much more ATP than anaerobic organisms, helping the former to dominate Earth's biosphere.[55] Photosynthesis and cellular respiration of O2 allowed for the evolution of eukaryotic cells and ultimately complex multicellular organisms such as plants and animalsHP Pavilion dv6-1233eo battery.
Since the beginning of the Cambrian period 540 million years ago, O2 levels have fluctuated between 15% and 30% by volume.[56] Towards the end of the Carboniferous period (about 300 million years ago) atmospheric O2 levels reached a maximum of 35% by volumeHP Pavilion dv6-1232ss battery,[56] which may have contributed to the large size of insects and amphibians at this time.[57] Human activities, including the burning of 7 billion tonnes of fossil fuels each year have had very little effect on the amount of free oxygen in the atmosphere.[12] At the current rate of photosynthesis it would take about 2,000 years to regenerate the entire O2 in the present atmosphereHP Pavilion dv6-1232et battery.[58]
History
Early experiments
Philo's experiment inspired later investigators.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neckHP Pavilion dv6-1232eo battery.[59] Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respirationHP Pavilion dv6-1231eo battery.[60]
In the late 17th century, Robert Boyle proved that air is necessary for combustion. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus or just nitroaereus.[61] In one experiment he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjectsHP Pavilion dv6-1230us battery.[62] From this he surmised that nitroaereus is consumed in both respiration and combustion.
Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it.[61] He also thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the bodyHP Pavilion dv6-1230ss battery.[61] Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione".[62]
Phlogiston theory
Main article: Phlogiston theory
Stahl helped develop and popularize the phlogiston theory.
Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as an element.[26] This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processesHP Pavilion dv6-1230sl battery.
Established in 1667 by the German alchemist J. J. Becher, and modified by the chemist Georg Ernst Stahl by 1731,[63] phlogiston theory stated that all combustible materials were made of two parts. One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calxHP Pavilion dv6-1230sf battery.[60]
Highly combustible materials that leave little residue, such as wood or coal, were thought to be made mostly of phlogiston; whereas non-combustible substances that corrode, such as iron, contained very little. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the ideaHP Pavilion dv6-1230sb battery; instead, it was based on observations of what happens when something burns, that most common objects appear to become lighter and seem to lose something in the process.[60] The fact that a substance like wood actually gains overall weight in burning was hidden by the buoyancy of the gaseous combustion products. Indeed one of the first clues that the phlogiston theory was incorrect was that metalsHP Pavilion dv6-1230ew battery, too, gain weight in rusting (when they were supposedly losing phlogiston).
Discovery
Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards.
Oxygen was first discovered by Swedish pharmacist Carl Wilhelm Scheele. He had produced oxygen gas by heating mercuric oxide and various nitrates by about 1772.[5][60] Scheele called the gas "fire air" because it was the only known supporter of combustionHP Pavilion dv6-1230et battery, and wrote an account of this discovery in a manuscript he titled Treatise on Air and Fire, which he sent to his publisher in 1775. However, that document was not published until 1777.[64]
Joseph Priestley is usually given priority in the discovery.
In the meantime, on August 1, 1774, an experiment conducted by the British clergyman Joseph Priestley focused sunlight on mercuric oxide (HgO) inside a glass tube, which liberated a gas he named "dephlogisticated air"HP Pavilion dv6-1230er battery.[5] He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it. After breathing the gas himself, he wrote: "The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards." HP Pavilion dv6-1230eq battery Priestley published his findings in 1775 in a paper titled "An Account of Further Discoveries in Air" which was included in the second volume of his book titled Experiments and Observations on Different Kinds of Air.[60][65] Because he published his findings first, Priestley is usually given priority in the discoveryHP Pavilion dv6-1230eo battery.
The noted French chemist Antoine Laurent Lavoisier later claimed to have discovered the new substance independently. However, Priestley visited Lavoisier in October 1774 and told him about his experiment and how he liberated the new gas. Scheele also posted a letter to Lavoisier on September 30, 1774 that described his own discovery of the previously unknown substanceHP Pavilion dv6-1230ek battery, but Lavoisier never acknowledged receiving it (a copy of the letter was found in Scheele's belongings after his death).[64]
Lavoisier's contribution
What Lavoisier did indisputably do (although this was disputed at the time) was to conduct the first adequate quantitative experiments on oxidation and give the first correct explanation of how combustion worksHP Pavilion dv6-1230ej battery.[5] He used these and similar experiments, all started in 1774, to discredit the phlogiston theory and to prove that the substance discovered by Priestley and Scheele was a chemical element.
Antoine Lavoisier discredited the Phlogiston theory.
In one experiment, Lavoisier observed that there was no overall increase in weight when tin and air were heated in a closed containerHP Pavilion dv6-1230ed battery.[5] He noted that air rushed in when he opened the container, which indicated that part of the trapped air had been consumed. He also noted that the tin had increased in weight and that increase was the same as the weight of the air that rushed back in. This and other experiments on combustion were documented in his book Sur la combustion en général, which was published in 1777. HP Pavilion dv6-1230ec battery In that work, he proved that air is a mixture of two gases; 'vital air', which is essential to combustion and respiration, and azote (Gk. ἄζωτον "lifeless"), which did not support either. Azote later became nitrogen in English, although it has kept the name in French and several other European languagesHP Pavilion dv6-1230ca battery.[5]
Lavoisier renamed 'vital air' to oxygène in 1777 from the Greek roots ὀξύς (oxys) (acid, literally "sharp," from the taste of acids) and -γενής (-genēs) (producer, literally begetter), because he mistakenly believed that oxygen was a constituent of all acids.[7] Chemists (notably Sir Humphrey Davy in 1812) eventually determined that Lavoisier was wrong in this regardHP Pavilion dv6-1229tx battery (it is in fact hydrogen that forms the basis for acid chemistry), but by that time it was too late, the name had taken.
Oxygen entered the English language despite opposition by English scientists and the fact that the Englishman Priestley had first isolated the gas and written about it. This is partly due to a poem praising the gas titled "Oxygen" in the popular book The Botanic Garden (1791) by Erasmus Darwin, grandfather of Charles Darwin. HP Pavilion dv6-1229sf battery
Later history
Robert H. Goddard and a liquid oxygen-gasoline rocket
John Dalton's original atomic hypothesis assumed that all elements were monoatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed that water's formula was HO, giving the atomic mass of oxygen as 8 times that of hydrogen, instead of the modern value of about 16HP Pavilion dv6-1229et battery.[66] In 1805, Joseph Louis Gay-Lussac and Alexander von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen; and by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the assumption of diatomic elemental molecules. HP Pavilion dv6-1229er battery
By the late 19th century scientists realized that air could be liquefied, and its components isolated, by compressing and cooling it. Using a cascade method, Swiss chemist and physicist Raoul Pierre Pictet evaporated liquid sulfur dioxide in order to liquefy carbon dioxide, which in turn was evaporated to cool oxygen gas enough to liquefy itHP Pavilion dv6-1228tx battery. He sent a telegram on December 22, 1877 to the French Academy of Sciences in Paris announcing his discovery of liquid oxygen.[69] Just two days later, French physicist Louis Paul Cailletet announced his own method of liquefying molecular oxygen.[69] Only a few drops of the liquid were produced in either case so no meaningful analysis could be conductedHP Pavilion dv6-1228sf battery. Oxygen was liquified in stable state for the first time on March 29, 1883 by Polish scientists from Jagiellonian University, Zygmunt Wróblewski and Karol Olszewski.[70]
In 1891 Scottish chemist James Dewar was able to produce enough liquid oxygen to study.[12] The first commercially viable process for producing liquid oxygen was independently developed in 1895 by German engineer Carl von Linde and British engineer William HampsonHP Pavilion dv6-1228ez battery. Both men lowered the temperature of air until it liquefied and then distilled the component gases by boiling them off one at a time and capturing them.[71] Later, in 1901, oxyacetylene welding was demonstrated for the first time by burning a mixture of acetylene and compressed O2. This method of welding and cutting metal later became commonHP Pavilion dv6-1227sf battery.[71]
In 1923 the American scientist Robert H. Goddard became the first person to develop a rocket engine; the engine used gasoline for fuel and liquid oxygen as the oxidizer. Goddard successfully flew a small liquid-fueled rocket 56 m at 97 km/h on March 16, 1926 in Auburn, Massachusetts, USA. HP Pavilion dv6-1225tx battery
Industrial production
See also: Air separation, Oxygen evolution, and fractional distillation
Two major methods are employed to produce 100 million tonnes of O2 extracted from air for industrial uses annually.[64] The most common method is to fractionally distill liquefied air into its various components, with N2 distilling as a vapor while O2 is left as a liquid. HP Pavilion dv6-1225sv battery
Hofmann electrolysis apparatus used in electrolysis of water.
The other major method of producing O2 gas involves passing a stream of clean, dry air through one bed of a pair of identical zeolite molecular sieves, which absorbs the nitrogen and delivers a gas stream that is 90% to 93% O2.[64] Simultaneously, nitrogen gas is released from the other nitrogen-saturated zeolite bed, by reducing the chamber operating pressure and diverting part of the oxygen gas from the producer bed through it, in the reverse direction of flowHP Pavilion dv6-1225ss battery. After a set cycle time the operation of the two beds is interchanged, thereby allowing for a continuous supply of gaseous oxygen to be pumped through a pipeline. This is known as pressure swing adsorption. Oxygen gas is increasingly obtained by these non-cryogenic technologies (see also the related vacuum swing adsorption). HP Pavilion dv6-1225so battery
Oxygen gas can also be produced through electrolysis of water into molecular oxygen and hydrogen. DC electricity must be used: if AC is used, the gases in each limb consist of hydrogen and oxygen in the explosive ratio 2:1. Contrary to popular belief, the 2:1 ratio observed in the DC electrolysis of acidified water does not prove that the empirical formula of water is H2O unless certain assumptions are made about the molecular formulae of hydrogen and oxygen themselvesHP Pavilion dv6-1225sf battery. A similar method is the electrocatalytic O2 evolution from oxides and oxoacids. Chemical catalysts can be used as well, such as in chemical oxygen generators or oxygen candles that are used as part of the life-support equipment on submarines, and are still part of standard equipment on commercial airliners in case of depressurization emergenciesHP Pavilion dv6-1225ew battery. Another air separation technology involves forcing air to dissolve through ceramic membranes based on zirconium dioxide by either high pressure or an electric current, to produce nearly pure O2 gas.[41]
In large quantities, the price of liquid oxygen in 2001 was approximately $0.21/kg.[74] Since the primary cost of production is the energy cost of liquefying the air, the production cost will change as energy cost variesHP Pavilion dv6-1225et battery.
For reasons of economy, oxygen is often transported in bulk as a liquid in specially insulated tankers, since one litre of liquefied oxygen is equivalent to 840 liters of gaseous oxygen at atmospheric pressure and 20 °C (68 °F).[64] Such tankers are used to refill bulk liquid oxygen storage containersHP Pavilion dv6-1225er battery, which stand outside hospitals and other institutions with a need for large volumes of pure oxygen gas. Liquid oxygen is passed through heat exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is also stored and shipped in smaller cylinders containing the compressed gas; a form that is useful in certain portable medical applications and oxy-fuel welding and cutting. HP Pavilion dv6-1225ei battery
Applications
See also: Breathing gas, Redox, and Combustion
Medical
An oxygen concentrator in an emphysema patient's house
Main article: Oxygen therapy
Uptake of O2 from the air is the essential purpose of respiration, so oxygen supplementation is used in medicine. Treatment not only increases oxygen levels in the patient's blood, but has the secondary effect of decreasing resistance to blood flow in many types of diseased lungs, easing work load on the heartHP Pavilion dv6-1225ef battery. Oxygen therapy is used to treat emphysema, pneumonia, some heart disorders (congestive heart failure), some disorders that cause increased pulmonary artery pressure, and any disease that impairs the body's ability to take up and use gaseous oxygenHP Pavilion dv6-1225ee battery.[75]
Treatments are flexible enough to be used in hospitals, the patient's home, or increasingly by portable devices. Oxygen tents were once commonly used in oxygen supplementation, but have since been replaced mostly by the use of oxygen masks or nasal cannulasHP Pavilion dv6-1224eo battery.[76]
Hyperbaric (high-pressure) medicine uses special oxygen chambers to increase the partial pressure of O2 around the patient and, when needed, the medical staff.[77] Carbon monoxide poisoning, gas gangrene, and decompression sickness (the 'bends') are sometimes treated using these devicesHP Pavilion dv6-1223sl battery.[78] Increased O2 concentration in the lungs helps to displace carbon monoxide from the heme group of hemoglobin.[79][80] Oxygen gas is poisonous to the anaerobic bacteria that cause gas gangrene, so increasing its partial pressure helps kill them.[81][82] Decompression sickness occurs in divers who decompress too quickly after a dive, resulting in bubbles of inert gas, mostly nitrogen and helium, forming in their bloodHP Pavilion dv6-1223sf battery. Increasing the pressure of O2 as soon as possible is part of the treatment.
Oxygen is also used medically for patients who require mechanical ventilation, often at concentrations above the 21% found in ambient air.
Life support and recreational use
Low pressure pure O2 is used in space suits.
A notable application of O2 as a low-pressure breathing gas is in modern space suits, which surround their occupant's body with pressurized air. These devices use nearly pure oxygen at about one third normal pressure, resulting in a normal blood partial pressure of O2.[85][86] This trade-off of higher oxygen concentration for lower pressure is needed to maintain flexible spacesuitsHP Pavilion dv6-1223eo battery.
Scuba divers and submariners also rely on artificially delivered O2, but most often use normal pressure, and/or mixtures of oxygen and air. Pure or nearly pure O2 use in diving at higher-than-sea-level pressures is usually limited to rebreather, decompression, or emergency treatment use at relatively shallow depths (~6 meters depth, or less). HP Pavilion dv6-1222tx battery Deeper diving requires significant dilution of O2 with other gases, such as nitrogen or helium, to help prevent oxygen toxicity.[87]
People who climb mountains or fly in non-pressurized fixed-wing aircraft sometimes have supplemental O2 supplies.[89] Passengers traveling in (pressurized) commercial airplanes have an emergency supply of O2 automatically supplied to them in case of cabin depressurizationHP Pavilion dv6-1222sl battery. Sudden cabin pressure loss activates chemical oxygen generators above each seat, causing oxygen masks to drop. Pulling on the masks "to start the flow of oxygen" as cabin safety instructions dictate, forces iron filings into the sodium chlorate inside the canister.[41] A steady stream of oxygen gas is then produced by the exothermic reactionHP Pavilion dv6-1222et battery.
Oxygen, as a supposed mild euphoric, has a history of recreational use in oxygen bars and in sports. Oxygen bars are establishments, found in Japan, California, and Las Vegas, Nevada since the late 1990s that offer higher than normal O2 exposure for a fee.[90] Professional athletes, especially in American footballHP Pavilion dv6-1222eo battery, also sometimes go off field between plays to wear oxygen masks in order to get a "boost" in performance. The pharmacological effect is doubtful; a placebo effect is a more likely explanation.[90] Available studies support a performance boost from enriched O2 mixtures only if they are breathed during aerobic exerciseHP Pavilion dv6-1221tx battery.[91]
Other recreational uses that do not involve breathing the gas include pyrotechnic applications, such as George Goble's five-second ignition of barbecue grills.[92]
Industrial
Most commercially produced O2 is used to smelt iron into steel.
Smelting of iron ore into steel consumes 55% of commercially produced oxygen.[41] In this process, O2 is injected through a high-pressure lance into molten iron, which removes sulfur impurities and excess carbon as the respective oxides, SO2 and CO2HP Pavilion dv6-1221sl battery. The reactions are exothermic, so the temperature increases to 1,700 °C.[41]
Another 25% of commercially produced oxygen is used by the chemical industry.[41] Ethylene is reacted with O2 to create ethylene oxide, which, in turn, is converted into ethylene glycol; the primary feeder material used to manufacture a host of products, including antifreeze and polyester polymers (the precursors of many plastics and fabrics). HP Pavilion dv6-1221eo battery
Most of the remaining 20% of commercially produced oxygen is used in medical applications, metal cutting and welding, as an oxidizer in rocket fuel, and in water treatment.[41] Oxygen is used in oxyacetylene welding burning acetylene with O2 to produce a very hot flame. In this process, metal up to 60 cm thick is first heated with a small oxy-acetylene flame and then quickly cut by a large stream of O2. HP Pavilion dv6-1221ax battery Larger rockets use liquid oxygen as their oxidizer, which is mixed and ignited with the fuel for propulsion.[citation needed]
Scientific
500 million years of climate change vs 18O
Paleoclimatologists measure the ratio of oxygen-18 and oxygen-16 in the shells and skeletons of marine organisms to determine what the climate was like millions of years ago (see oxygen isotope ratio cycle). Seawater molecules that contain the lighter isotope, oxygen-16, evaporate at a slightly faster rate than water molecules containing the 12% heavier oxygen-18HP Pavilion dv6-1220tx battery; this disparity increases at lower temperatures.[94] During periods of lower global temperatures, snow and rain from that evaporated water tends to be higher in oxygen-16, and the seawater left behind tends to be higher in oxygen-18. Marine organisms then incorporate more oxygen-18 into their skeletons and shells than they would in a warmer climateHP Pavilion dv6-1220st battery.[94] Paleoclimatologists also directly measure this ratio in the water molecules of ice core samples that are up to several hundreds of thousands of years old.
Planetary geologists have measured different abundances of oxygen isotopes in samples from the Earth, the Moon, Mars, and meteorites, but were long unable to obtain reference values for the isotope ratios in the SunHP Pavilion dv6-1220ss battery, believed to be the same as those of the primordial solar nebula. However, analysis of a silicon wafer exposed to the solar wind in space and returned by the crashed Genesis spacecraft has shown that the Sun has a higher proportion of oxygen-16 than does the EarthHP Pavilion dv6-1220sp battery. The measurement implies that an unknown process depleted oxygen-16 from the Sun's disk of protoplanetary material prior to the coalescence of dust grains that formed the Earth.[95]
Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760 nm. Some remote sensing scientists have proposed using the measurement of the radiance coming from vegetation canopies in those bands to characterize plant health status from a satellite platformHP Pavilion dv6-1220so battery.[96] This approach exploits the fact that in those bands it is possible to discriminate the vegetation's reflectance from its fluorescence, which is much weaker. The measurement is technically difficult owing to the low signal-to-noise ratio and the physical structure of vegetation; but it has been proposed as a possible method of monitoring the carbon cycle from satellites on a global scaleHP Pavilion dv6-1220sh battery.
Compounds
Main article: Compounds of oxygen
Water (H2O) is the most familiar oxygen compound.
The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides.[97] Compounds containing oxygen in other oxidation states are very uncommon: −1/2 (superoxides), −1/3 (ozonides), 0 (elemental, hypofluorous acid), +1/2 (dioxygenyl), +1 (dioxygen difluoride), and +2 (oxygen difluoride) HP Pavilion dv6-1220sd battery.
Oxides and other inorganic compounds
Water (H2O) is the oxide of hydrogen and the most familiar oxygen compound. Hydrogen atoms are covalently bonded to oxygen in a water molecule but also have an additional attraction (about 23.3 kJ·mol−1 per hydrogen atom) to an adjacent oxygen atom in a separate molecule. HP Pavilion dv6-1220sb battery These hydrogen bonds between water molecules hold them approximately 15% closer than what would be expected in a simple liquid with just van der Waals forces.[99][100]
Oxides, such as iron oxide or rust form when oxygen combines with other elements.
Due to its electronegativity, oxygen forms chemical bonds with almost all other elements at elevated temperatures to give corresponding oxides. However, some elements readily form oxides at standard conditions for temperature and pressureHP Pavilion dv6-1220sa battery; the rusting of iron is an example. The surface of metals like aluminium and titanium are oxidized in the presence of air and become coated with a thin film of oxide that passivates the metal and slows further corrosion. Some of the transition metal oxides are found in nature as non-stoichiometric compounds, with a slightly less metal than the chemical formula would showHP Pavilion dv6-1220ez battery. For example, the natural occurring FeO (wüstite) is actually written as Fe1 − xO, where x is usually around 0.05.[101]
Oxygen as a compound is present in the atmosphere in trace quantities in the form of carbon dioxide (CO2). The earth's crustal rock is composed in large part of oxides of silicon (silica SiO2, found in granite and sand), aluminium (aluminium oxide Al2O3, in bauxite and corundum), iron (iron(III) oxide Fe2O3HP Pavilion dv6-1220ew battery, in hematite and rust), and calcium carbonate (in limestone). The rest of the Earth's crust is also made of oxygen compounds, in particular various complex silicates (in silicate minerals). The Earth's mantle, of much larger mass than the crust, is largely composed of silicates of magnesium and ironHP Pavilion dv6-1220ev battery.
Water-soluble silicates in the form of Na4SiO4, Na2SiO3, and Na2Si2O5 are used as detergents and adhesives.[102]
Oxygen also acts as a ligand for transition metals, forming metal–O2 bonds with the iridium atom in Vaska's complex,[103] with the platinum in PtF6,[104] and with the iron center of the heme group of hemoglobin.
Organic compounds and biomolecules
Acetone is an important feeder material in the chemical industryHP Pavilion dv6-1220er battery.
Oxygen
Carbon
Hydrogen
Oxygen represents more than 40% of the molecular mass of the ATP molecule.
Among the most important classes of organic compounds that contain oxygen are (where "R" is an organic group): alcohols (R-OH); ethers (R-O-R); ketones (R-CO-R); aldehydes (R-CO-H); carboxylic acids (R-COOH); esters (R-COO-R); acid anhydrides (R-CO-O-CO-R); and amides (R-C(O)-NR2) HP Pavilion dv6-1220ek battery. There are many important organic solvents that contain oxygen, including: acetone, methanol, ethanol, isopropanol, furan, THF, diethyl ether, dioxane, ethyl acetate, DMF, DMSO, acetic acid, and formic acid. Acetone ((CH3)2CO) and phenol (C6H5OH) are used as feeder materials in the synthesis of many different substancesHP Pavilion dv6-1220ei battery. Other important organic compounds that contain oxygen are: glycerol, formaldehyde, glutaraldehyde, citric acid, acetic anhydride, and acetamide. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.
Oxygen reacts spontaneously with many organic compounds at or below room temperature in a process called autoxidationHP Pavilion dv6-1220eg battery.[105] Most of the organic compounds that contain oxygen are not made by direct action of O2. Organic compounds important in industry and commerce that are made by direct oxidation of a precursor include ethylene oxide and peracetic acid.[102]
The element is found in almost all biomolecules that are important to (or generated by) life. Only a few common complex biomolecules, such as squalene and the carotenes, contain no oxygenHP Pavilion dv6-1220ec battery. Of the organic compounds with biological relevance, carbohydrates contain the largest proportion by mass of oxygen. All fats, fatty acids, amino acids, and proteins contain oxygen (due to the presence of carbonyl groups in these acids and their ester residues). Oxygen also occurs in phosphate (PO3−4) groups in the biologically important energy-carrying molecules ATP and ADPHP Pavilion dv6-1219tx battery, in the backbone and the purines (except adenine) and pyrimidines of RNA and DNA, and in bones as calcium phosphate and hydroxylapatite.
Safety and precautions
Toxicity
Main article: Oxygen toxicity
Main symptoms of oxygen toxicity[106]
Oxygen toxicity occurs when the lungs take in a higher than normal O2 partial pressure, which can occur in deep scuba diving.
Oxygen gas (O2) can be toxic at elevated partial pressures, leading to convulsions and other health problems. Oxygen toxicity usually begins to occur at partial pressures more than 50 kilopascals (kPa), or 2.5 times the normal sea-level O2 partial pressure of about 21 kPa (equal to about 50% oxygen composition at standard pressure) HP Pavilion dv6-1219ez battery. This is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30%–50% O2 by volume (about 30 kPa at standard pressure).[26] (although this figure also is subject to wide variation, depending on type of mask) HP Pavilion dv6-1218tx battery.
At one time, premature babies were placed in incubators containing O2-rich air, but this practice was discontinued after some babies were blinded by it.[26][109]
Breathing pure O2 in space applications, such as in some modern space suits, or in early spacecraft such as Apollo, causes no damage due to the low total pressures used.[85][110] In the case of spacesuitsHP Pavilion dv6-1218ss battery, the O2 partial pressure in the breathing gas is, in general, about 30 kPa (1.4 times normal), and the resulting O2 partial pressure in the astronaut's arterial blood is only marginally more than normal sea-level O2 partial pressure (for more information on this, see space suit and arterial blood gas) HP Pavilion dv6-1218ca battery.
Oxygen toxicity to the lungs and central nervous system can also occur in deep scuba diving and surface supplied diving.[26][87] Prolonged breathing of an air mixture with an O2 partial pressure more than 60 kPa can eventually lead to permanent pulmonary fibrosis.[111] Exposure to a O2 partial pressures greater than 160 kPa (about 1.6 atm) may lead to convulsions (normally fatal for divers) HP Pavilion dv6-1217tx battery. Acute oxygen toxicity (causing seizures, its most feared effect for divers) can occur by breathing an air mixture with 21% O2 at 66 m or more of depth; the same thing can occur by breathing 100% O2 at only 6 m.
Combustion and other hazards
The interior of the Apollo 1 Command Module. Pure O2 at higher than normal pressure and a spark led to a fire and the loss of the Apollo 1 crewHP Pavilion dv6-1217ss battery.
Highly concentrated sources of oxygen promote rapid combustion. Fire and explosion hazards exist when concentrated oxidants and fuels are brought into close proximity; however, an ignition event, such as heat or a spark, is needed to trigger combustion.[115] Oxygen itself is not the fuel, but the oxidant. Combustion hazards also apply to compounds of oxygen with a high oxidative potentialHP Pavilion dv6-1217er battery, such as peroxides, chlorates, nitrates, perchlorates, and dichromates because they can donate oxygen to a fire.
Concentrated O2 will allow combustion to proceed rapidly and energetically.[115] Steel pipes and storage vessels used to store and transmit both gaseous and liquid oxygen will act as a fuel; and therefore the design and manufacture of O2 systems requires special training to ensure that ignition sources are minimizedHP Pavilion dv6-1217ax battery.[115] The fire that killed the Apollo 1 crew in a launch pad test spread so rapidly because the capsule was pressurized with pure O2 but at slightly more than atmospheric pressure, instead of the 1⁄3 normal pressure that would be used in a mission. HP Pavilion dv6-1216tx battery
Liquid oxygen spills, if allowed to soak into organic matter, such as wood, petrochemicals, and asphalt can cause these materials to detonate unpredictably on subsequent mechanical impact.[115] As with other cryogenic liquids, on contact with the human body it can cause frostbites to the skin and the eyesHP Pavilion dv6-1216ss battery.
The most common isotope of hydrogen is protium (name rarely used, symbol 1H) with a single proton and no neutrons. In ionic compounds it can take a negative charge (an anion known as a hydride and written as H−), or as a positively charged species H+HP Pavilion dv6-1299ez battery. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species. Hydrogen forms compounds with most elements and is present in water and most organic compoundsHP Pavilion dv6-1299es battery. It plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanicsHP Pavilion dv6-1299er battery.
Hydrogen gas (now known to be H2) was first artificially produced in the early 16th century, via the mixing of metals with strong acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance,[6] and that it produces water when burned, a property which later gave it its nameHP Pavilion dv6-1299eo battery, which in Greek means "water-former." At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, non-toxic, highly combustible diatomic gas with the molecular formula H2.
Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of waterHP Pavilion dv6-1299ef battery.[7] Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.
Hydrogen is a concern in metallurgy as it can embrittle many metals,[8] complicating the design of pipelines and storage tanksHP Pavilion dv6-1299ea battery.[9]
Properties
Combustion
The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Hydrogen gas (dihydrogen or molecular hydrogen)[10] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.[11] The enthalpy of combustion for hydrogen is −286 kJ/mol:[12]
2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)[note 1HP Pavilion dv6-1295el battery]
Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures spontaneously explode by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F) HP Pavilion dv6-1291es battery.[13] Pure hydrogen-oxygen flames emit ultraviolet light and are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerousHP Pavilion dv6-1290ev battery. The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of combustible materials in the ship's skin.[14] Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and many deaths were instead the result of falls or burning diesel fuelHP Pavilion dv6-1290es battery.[15]
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids. HP Pavilion dv6-1290en battery
Electron energy levels
Main article: Hydrogen atom
Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the Bohr model radius (image not to scale).
The ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm wavelengthHP Pavilion dv6-1290ec battery.[17]
The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the electromagnetic force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by gravityHP Pavilion dv6-1288la battery. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.[18]
A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation or the Feynman path integral formulation to calculate the probability density of the electron around the protonHP Pavilion dv6-1285es battery.[19] The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all— an illustration of how different the "planetary orbit" conception of electron motion differs from realityHP Pavilion dv6-1282et battery.
Elemental molecular forms
See also: Spin isomers of hydrogen
First tracks observed in liquid hydrogen bubble chamber at the Bevatron
There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei.[20] In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (½+½); in the parahydrogen form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 (½–½)HP Pavilion dv6-1280us battery. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form, also known as the "normal form".[21] The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy than the para form, it is unstable and cannot be purifiedHP Pavilion dv6-1280sn battery. At very low temperatures, the equilibrium state is composed almost exclusively of the para form. The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities, as discussed more fully in Spin isomers of hydrogen.[22] The ortho/para distinction also occurs in other hydrogen-containing molecules or functional groupsHP Pavilion dv6-1280ev battery, such as water and methylene, but is of little significance for their thermal properties.[23]
The uncatalyzed interconversion between para and ortho H2 increases with increasing temperature; thus rapidly condensed H2 contains large quantities of the high-energy ortho form that converts to the para form very slowlyHP Pavilion dv6-1280et battery.[24] The ortho/para ratio in condensed H2 is an important consideration in the preparation and storage of liquid hydrogen: the conversion from ortho to para is exothermic and produces enough heat to evaporate some of the hydrogen liquid, leading to loss of liquefied material. Catalysts for the ortho-para interconversionHP Pavilion dv6-1280es battery, such as ferric oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromic oxide, or some nickel[25] compounds, are used during hydrogen cooling.[26]
Further information: Hydrogen compounds
Covalent and organic compounds
While H2 is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more electronegative, such as halogens (e.g., F, Cl, Br, I), or oxygenHP Pavilion dv6-1280ep battery; in these compounds hydrogen takes on a partial positive charge.[27] When bonded to fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding called hydrogen bonding, which is critical to the stability of many biological molecules.[28][29] Hydrogen also forms compounds with less electronegative elementsHP Pavilion dv6-1280el battery, such as the metals and metalloids, in which it takes on a partial negative charge. These compounds are often known as hydrides.[30]
Hydrogen forms a vast array of compounds with carbon called the hydrocarbons, and an even vaster array with heteroatoms that, because of their general association with living things, are called organic compoundsHP Pavilion dv6-1277la battery.[31] The study of their properties is known as organic chemistry[32] and their study in the context of living organisms is known as biochemistry.[33] By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogenHP Pavilion dv6-1277el battery, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.[31] Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways, which seldom involve elementary hydrogenHP Pavilion dv6-1275la battery.
Hydrides
Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H−, and is used when hydrogen forms a compound with a more electropositive element. The existence of the hydride anionHP Pavilion dv6-1275eg battery, suggested by Gilbert N. Lewis in 1916 for group I and II salt-like hydrides, was demonstrated by Moers in 1920 with the electrolysis of molten lithium hydride (LiH), that produced a stoichiometry quantity of hydrogen at the anode.[34] For hydrides other than group I and II metalsHP Pavilion dv6-1270st battery, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group II hydrides is BeH2, which is polymeric. In lithium aluminium hydride, the AlH−
4 anion carries hydridic centers firmly attached to the Al(III).
Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, there are over 100 binary borane hydrides known, but only one binary aluminium hydrideHP Pavilion dv6-1270ss battery.[35] Binary indium hydride has not yet been identified, although larger complexes exist.[36]
In inorganic chemistry, hydrides can also serve as bridging ligands that link two metal centers in a coordination complex. This function is particularly common in group 13 elements, especially in boranes (boron hydrides) and aluminium complexes, as well as in clustered carboranesHP Pavilion dv6-1270la battery.[37]
Protons and acids
Further information: Acid–base reaction
Oxidation of hydrogen removes its electron and gives H+, which contains no electrons and a nucleus which is usually composed of one proton. That is why H+ is often called a proton. This species is central to discussion of acids. Under the Bronsted-Lowry theory, acids are proton donors, while bases are proton acceptorsHP Pavilion dv6-1270ev battery.
A bare proton, H+, cannot exist in solution or in ionic crystals, because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the electron clouds of atoms and molecules, and will remain attached to themHP Pavilion dv6-1270et battery. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or cationic hydrogen attached to other species in this fashion, and as such is denoted "H+" without any implication that any single protons exist freely as a speciesHP Pavilion dv6-1270eq battery.
To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "hydronium ion" (H3O+). However, even in this case, such solvated hydrogen cations are thought more realistically physically to be organized into clusters that form species closer to H9O+HP Pavilion dv6-1270el battery
4.[38] Other oxonium ions are found when water is in solution with other solvents.[39]
Although exotic on earth, one of the most common ions in the universe is the H+
3 ion, known as protonated molecular hydrogen or the trihydrogen cation.[40]
Isotopes
Main article: Isotopes of hydrogen
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tubeHP Pavilion dv6-1270eg battery
Protium, the most common isotope of hydrogen, has one proton and one electron. Unique among all stable isotopes, it has no neutrons (see diproton for a discussion of why others do not exist).
Hydrogen has three naturally occurring isotopes, denoted 1H, 2H and 3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in the laboratory but not observed in nature. HP Pavilion dv6-1270ec battery
1H is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.[43]
2H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleusHP Pavilion dv6-1268tx battery. Essentially all deuterium in the universe is thought to have been produced at the time of the Big Bang, and has endured since that time. Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy waterHP Pavilion dv6-1268nr battery. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1H-NMR spectroscopy.[44] Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusionHP Pavilion dv6-1266el battery.[45]
3H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32 years.[37] It is so radioactive that it can be used in luminous paint, making it useful in such things as watchesHP Pavilion dv6-1265tx battery. The glass prevents the small amount of radiation from getting out.[46] Small amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases; tritium has also been released during nuclear weapons tests.[47] It is used in nuclear fusion reactions,[48] as a tracer in isotope geochemistry,[49] and specialized in self-powered lighting devicesHP Pavilion dv6-1265sg battery.[50] Tritium has also been used in chemical and biological labeling experiments as a radiolabel.[51]
Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritiumHP Pavilion dv6-1264tx battery. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, but the corresponding symbol for protium, P, is already in use for phosphorus and thus is not available for protium.[52] In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry allows any of D, T, 2H, and 3H to be used, although 2H and 3H are preferredHP Pavilion dv6-1264ca battery.[53]
History
Discovery and use
Main article: Timeline of hydrogen technologies
In 1671, Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.[54][55] In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "flammable air"HP Pavilion dv6-1263cl battery. He speculated that "flammable air" was in fact identical to the hypothetical substance called "phlogiston"[56][57] and further finding in 1781 that the gas produces water when burned. He is usually given credit for its discovery as an element.[58][59] In 1783, Antoine Lavoisier gave the element the name hydrogen (from the Greek ὕδρω hydro meaning water and γενῆς genes meaning creator) HP Pavilion dv6-1261tx battery when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.[59]
Antoine-Laurent de Lavoisier
Lavoisier produced hydrogen for his famous experiments on mass conservation by reacting a flux of steam with metallic iron through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactionsHP Pavilion dv6-1260tx battery:
Fe + H2O → FeO + H2
2 Fe + 3 H2O → Fe2O3 + 3 H2
3 Fe + 4 H2O → Fe3O4 + 4 H2
Many metals such as zirconium undergo a similar reaction with water leading to the production of hydrogen.
Hydrogen was liquefied for the first time by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask.[59] He produced solid hydrogen the next year.[59] Deuterium was discovered in December 1931 by Harold UreyHP Pavilion dv6-1260sn battery, and tritium was prepared in 1934 by Ernest Rutherford, Mark Oliphant, and Paul Harteck.[58] Heavy water, which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.[59] François Isaac de Rivaz built the first internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819HP Pavilion dv6-1260sh battery. The Döbereiner's lamp and limelight were invented in 1823.[59]
The first hydrogen-filled balloon was invented by Jacques Charles in 1783.[59] Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by Henri Giffard.[59] German count Ferdinand von Zeppelin promoted the idea of rigid airships lifted by hydrogen that later were called ZeppelinsHP Pavilion dv6-1260sg battery; the first of which had its maiden flight in 1900.[59] Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the warHP Pavilion dv6-1260se battery.
The first non-stop transatlantic crossing was made by the British airship R34 in 1919. Regular passenger service resumed in the 1920s and the discovery of helium reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, H2 was used in the Hindenburg airship, which was destroyed in a midair fire over New Jersey on May 6, 1937HP Pavilion dv6-1260ev battery.[59] The incident was broadcast live on radio and filmed. Ignition of leaking hydrogen is widely assumed to be the cause, but later investigations pointed to the ignition of the aluminized fabric coating by static electricity. But the damage to hydrogen's reputation as a lifting gas was already doneHP Pavilion dv6-1260et battery.
In the same year the first hydrogen-cooled turbogenerator went into service with gaseous hydrogen as a coolant in the rotor and the stator in 1937 at Dayton, Ohio, by the Dayton Power & Light Co,[61] because of the thermal conductivity of hydrogen gas this is the most common type in its field todayHP Pavilion dv6-1260ep battery.
The nickel hydrogen battery was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).[62] For example, the ISS,[63] Mars Odyssey[64] and the Mars Global Surveyor[65] are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the Hubble Space Telescope is also powered by nickel-hydrogen batteriesHP Pavilion dv6-1260ej battery, which were finally replaced in May 2009, more than 19 years after launch, and 13 years over their design life.
Role in quantum theory
Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the Balmer series
Because of its relatively simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structureHP Pavilion dv6-1260ei battery.[66] Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation H2+ allowed fuller understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.
One of the first quantum effects to be explicitly noticed (but not understood at the time) was a Maxwell observation involving hydrogenHP Pavilion dv6-1260ec battery, half a century before full quantum mechanical theory arrived. Maxwell observed that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theoryHP Pavilion dv6-1259tx battery, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H2 because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect. HP Pavilion dv6-1259dx battery
Natural occurrence
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy
Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms (most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as dark matter and dark energy). HP Pavilion dv6-1256tx battery This element is found in great abundance in stars and gas giant planets. Molecular clouds of H2 are associated with star formation. Hydrogen plays a vital role in powering stars through proton-proton reaction and CNO cycle nuclear fusion.[69]
Throughout the universe, hydrogen is mostly found in the atomic and plasma states whose properties are quite different from molecular hydrogenHP Pavilion dv6-1255tx battery. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the auroraHP Pavilion dv6-1255eg battery. Hydrogen is found in the neutral atomic state in the Interstellar medium. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the Universe up to redshift z=4.[70]
Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H2 (for data see table) HP Pavilion dv6-1255ee battery. However, hydrogen gas is very rare in the Earth's atmosphere (1 ppm by volume) because of its light weight, which enables it to escape from Earth's gravity more easily than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface,[71] mostly in the form of chemical compounds such as hydrocarbons and waterHP Pavilion dv6-1254tx battery.[37] Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus, as is methane, itself a hydrogen source of increasing importance.[72]
A molecular form called protonated molecular hydrogen (H3+) is found in the interstellar medium (ISM), where it is generated by ionization of molecular hydrogen from cosmic raysHP Pavilion dv6-1253cl battery. It has also been observed in the upper atmosphere of the planet Jupiter. This ion is relatively stable in the environment of outer space due to the low temperature and density. H3+ is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium.[73] Neutral triatomic hydrogen H3 can only exist in an excited form and is unstableHP Pavilion dv6-1253ca battery.[74] The hydrogen molecular ion (H2+) is a rare molecular system in the universe.
Production
For more details on this topic, see Hydrogen production.
H2 is produced in chemistry and biology laboratories, often as a by-product of other reactions; in industry for the hydrogenation of unsaturated substrates; and in nature as a means of expelling reducing equivalents in biochemical reactions.
Laboratory
In the laboratory, H2 is usually prepared by the reaction of acids on metals such as zinc with Kipp's apparatusHP Pavilion dv6-1252tx battery.
Zn + 2 H+ → Zn2+ + H2
Aluminium can also produce H2 upon treatment with bases:
2 Al + 6 H2O + 2 OH− → 2 Al(OH)−
4 + 3 H2
The electrolysis of water is a simple method of producing hydrogen. A low voltage current is run through the water, and gaseous oxygen forms at the anode while gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, howeverHP Pavilion dv6-1250us battery, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is in the range 80–94%.[75]
2 H2O(l) → 2 H2(g) + O2(g) HP Pavilion dv6-1250ss battery
In 2007, it was discovered that an alloy of aluminium and gallium in pellet form added to water could be used to generate hydrogen. The process also creates alumina, but the expensive gallium, which prevents the formation of an oxide skin on the pellets, can be re-used. This has important potential implications for a hydrogen economy, as hydrogen can be produced on-site and does not need to be transportedHP Pavilion dv6-1250sp battery.[76]
Industrial
Main article: Hydrogen production
Hydrogen can be prepared in several different ways, but economically the most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas.[77] At high temperatures (1000–1400 K, 700–1100 °C or 1300–2000 °F), steam (water vapor) reacts with methane to yield carbon monoxide and H2HP Pavilion dv6-1250sn battery.
CH4 + H2O → CO + 3 H2
This reaction is favored at low pressures but is nonetheless conducted at high pressures (2.0 MPa, 20 atm or 600 inHg). This is because high-pressure H2 is the most marketable product and Pressure Swing Adsorption (PSA) purification systems work better at higher pressuresHP Pavilion dv6-1250sc battery. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and related compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbonHP Pavilion dv6-1250eq battery:
CH4 → C + 2 H2
Consequently, steam reforming typically employs an excess of H2O. Additional hydrogen can be recovered from the steam by use of carbon monoxide through the water gas shift reaction, especially with an iron oxide catalyst. This reaction is also a common industrial source of carbon dioxide:[77]
CO + H2O → CO2 + H2
Other important methods for H2 production include partial oxidation of hydrocarbons: HP Pavilion dv6-1250ep battery
2 CH4 + O2 → 2 CO + 4 H2
and the coal reaction, which can serve as a prelude to the shift reaction above:[77]
C + H2O → CO + H2
Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for the production of ammonia, hydrogen is generated from natural gas.[79] Electrolysis of brine to yield chlorine also produces hydrogen as a co-product. HP Pavilion dv6-1250eo battery
Thermochemical
There are more than 200 thermochemical cycles which can be used for water splitting, around a dozen of these cycles such as the iron oxide cycle, cerium(IV) oxide-cerium(III) oxide cycle, zinc zinc-oxide cycle, sulfur-iodine cycle, copper-chlorine cycle and hybrid sulfur cycle are under research and in testing phase to produce hydrogen and oxygen from water and heat without using electricityHP Pavilion dv6-1250el battery.[81] A number of laboratories (including in France, Germany, Greece, Japan, and the USA) are developing thermochemical methods to produce hydrogen from solar energy and water.[82]
Anaerobic corrosion
Under anaerobic conditions, iron and steel alloys are slowly oxidized by the protons of water concomitantly reduced in molecular hydrogen (H2). The anaerobic corrosion of iron leads first to the formation of ferrous hydroxide (green rust) and can be described by the following reactionHP Pavilion dv6-1250eg battery:
Fe + 2 H2O → Fe(OH)2 + H2
In its turn, under anaerobic conditions, the ferrous hydroxide (Fe(OH)2 ) can be oxidized by the protons of water to form magnetite and molecular hydrogen. This process is described by the Schikorr reaction:
3 Fe(OH)2 → Fe3O4 + 2 H2O + H2
ferrous hydroxide → magnetite + water + hydrogen
The well crystallized magnetite (Fe3O4) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)2 ) HP Pavilion dv6-1250eb battery.
This process occurs during the anaerobic corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table.
Geological occurrence: the serpentinization reaction
In the absence of atmospheric oxygen (O2), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H2) is produced during the process of serpentinization by the anaerobic oxidation by the water protons HP Pavilion dv6-1249tx battery (H+) of the ferrous (Fe2+) silicate present in the crystal lattice of the fayalite (Fe2SiO4, the olivine iron-endmember). The corresponding reaction leading to the formation of magnetite (Fe3O4), quartz (SiO2) and hydrogen (H2) is the following:
3 Fe2SiO4 + 2 H2O → 2 Fe3O4 + 3 SiO2 + 3 H2
fayalite + water → magnetite + quartz + hydrogen
This reaction closely resembles the Schikorr reaction observed in the anaerobic oxidation of the ferrous hydroxide in contact with waterHP Pavilion dv6-1249eo battery.
Applications
Consumption in processes
Large quantities of H2 are needed in the petroleum and chemical industries. The largest application of H2 is for the processing ("upgrading") of fossil fuels, and in the production of ammonia. The key consumers of H2 in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocrackingHP Pavilion dv6-1248eo battery. H2 has several other important uses. H2 is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated fats and oils (found in items such as margarine), and in the production of methanol. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H2 is also used as a reducing agent of metallic oresHP Pavilion dv6-1248ca battery.[83]
Hydrogen is highly soluble in many rare earth and transition metals[84] and is soluble in both nanocrystalline and amorphous metals.[85] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.[86] These properties may be useful when hydrogen is purified by passage through hot palladium disksHP Pavilion dv6-1247tx battery, but the gas's high solubility is a metallurgical problem, contributing to the embrittlement of many metals,[8] complicating the design of pipelines and storage tanks.[9]
Apart from its use as a reactant, H2 has wide applications in physics and engineering. It is used as a shielding gas in welding methods such as atomic hydrogen weldingHP Pavilion dv6-1247ez battery.[87][88] H2 is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas. Liquid H2 is used in cryogenic research, including superconductivity studies.[89] Because H2 is lighter than air, having a little more than 1⁄15 of the density of air, it was once widely used as a lifting gas in balloons and airshipsHP Pavilion dv6-1247et battery.[90]
In more recent applications, hydrogen is used pure or mixed with nitrogen (sometimes called forming gas) as a tracer gas for minute leak detection. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industriesHP Pavilion dv6-1247eo battery.[91] Hydrogen is an authorized food additive (E 949) that allows food package leak testing among other anti-oxidizing properties.[92]
Hydrogen's rarer isotopes also each have specific applications. Deuterium (hydrogen-2) is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions.[59] Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effectsHP Pavilion dv6-1247cl battery.[93] Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs,[94] as an isotopic label in the biosciences,[51] and as a radiation source in luminous paints.[95]
The triple point temperature of equilibrium hydrogen is a defining fixed point on the ITS-90 temperature scale at 13.8033 kelvinsHP Pavilion dv6-1246tx battery.[96]
Coolant
Hydrogen is commonly used in power stations, as a coolant in generators, due to its specific heat capacity being considerably higher than any other gas.
Energy carrier
See also: Hydrogen economy and Hydrogen infrastructure
Hydrogen is not an energy resource,[97] except in the hypothetical context of commercial nuclear fusion power plants using deuterium or tritium, a technology presently far from developmentHP Pavilion dv6-1246eo battery.[98] The Sun's energy comes from nuclear fusion of hydrogen, but this process is difficult to achieve controllably on Earth.[99] Elemental hydrogen from solar, biological, or electrical sources require more energy to make it than is obtained by burning it, so in these cases hydrogen functions as an energy carrier, like a batteryHP Pavilion dv6-1245et battery. Hydrogen may be obtained from fossil sources (such as methane), but these sources are unsustainable.[97]
The energy density per unit volume of both liquid hydrogen and compressed hydrogen gas at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel mass is higher.[97] Nevertheless, elemental hydrogen has been widely discussed in the context of energyHP Pavilion dv6-1245es battery, as a possible future carrier of energy on an economy-wide scale.[100] For example, CO2 sequestration followed by carbon capture and storage could be conducted at the point of H2 production from fossil fuels.[101] Hydrogen used in transportation would burn relatively cleanly, with some NOx emissions,[102] but without carbon emissionsHP Pavilion dv6-1245eo battery.[101] However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial.[103]
Semiconductor industry
Hydrogen is employed to saturate broken ("dangling") bonds of amorphous silicon and amorphous carbon that helps stabilizing material properties.[104] It is also a potential electron donor in various oxide materials, including ZnO,[105][106] SnO2, CdO, MgO,[107] ZrO2, HfO2, La2O3, Y2O3, TiO2, SrTiO3, LaAlO3, SiO2, Al2O3, ZrSiO4, HfSiO4, and SrZrO3HP Pavilion dv6-1245dx battery.[108]
Biological reactions
Further information: Biohydrogen and Biological hydrogen production
H2 is a product of some types of anaerobic metabolism and is produced by several microorganisms, usually via reactions catalyzed by iron- or nickel-containing enzymes called hydrogenases. These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electronsHP Pavilion dv6-1244tx battery. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water.[109]
Water splitting, in which water is decomposed into its component protons, electrons, and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms, including the alga Chlamydomonas reinhardtii and cyanobacteriaHP Pavilion dv6-1244sb battery, have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast.[110] Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H2 gas even in the presence of oxygen.[111] Efforts have also been undertaken with genetically modified alga in a bioreactorHP Pavilion dv6-1243cl battery.[112]
Safety and precautions
Main article: Hydrogen safety
Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form.[113] In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquidsHP Pavilion dv6-1242tx battery.[114] Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement,[115] leading to cracks and explosions.[116] Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burnsHP Pavilion dv6-1242et battery.[117]
Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Many physical and chemical properties of hydrogen depend on the parahydrogen/orthohydrogen ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given) HP Pavilion dv6-1241et battery. Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.[113]
Oxygen ( /ˈɒksɨdʒɨn/ ok-si-jin) is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς (oxys) ("acid", literally "sharp", referring to the sour taste of acids) and -γενής (-genēs) ("producer", literally "begetter")HP Pavilion dv6-1241eo battery, because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a very pale blue, odorless, tasteless diatomic gas with the formula O2HP Pavilion dv6-1240us battery.
Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with almost all other elements. Oxygen is a strong oxidizing agent and has the second-highest electronegativity of all the elements (only fluorine has a higher electronegativity) HP Pavilion dv6-1240tx battery.[1] By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium[2] and the most abundant element by mass in the Earth's crust, making up almost half of the crust's mass.[3] Free oxygen is too chemically reactive to appear on Earth without the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from waterHP Pavilion dv6-1240sh battery. Elemental O2 only began to accumulate in the atmosphere after the evolutionary appearance of these organisms, roughly 2.5 billion years ago.[4] Diatomic oxygen gas constitutes 20.8% of the volume of air.[5]
Because it comprises most of the mass in water, oxygen comprises most of the mass of living organisms (for example, about two-thirds of the human body's mass). All major classes of structural molecules in living organisms, such as proteinsHP Pavilion dv6-1240sf battery, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphereHP Pavilion dv6-1240sb battery. Another form (allotrope) of oxygen, ozone (O3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraftHP Pavilion dv6-1240ez battery.[6]
Oxygen was independently discovered by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774, but Priestley is often given priority because his work was published first. The name oxygen was coined in 1777 by Antoine Lavoisier,[7] whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosionHP Pavilion dv6-1240ep battery. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water and other means. Uses of oxygen include the production of steel, plastics and textiles; rocket propellant; oxygen therapy; and life support in aircraft, submarines, spaceflight and divingHP Pavilion dv6-1240eo battery.
Characteristics
Structure
Oxygen discharge (spectrum) tube
At standard temperature and pressure, oxygen is a very pale blue, odorless gas with the molecular formula O2, in which the two oxygen atoms are chemically bonded to each other with a spin triplet electron configuration. This bond has a bond order of two, and is often simplified in description as a double bond[8] or as a combination of one two-electron bond and two three-electron bondsHP Pavilion dv6-1240el battery.[9]
Triplet oxygen (not to be confused with ozone, O3) is the ground state of the O2 molecule.[10] The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals.[11] These orbitals are classified as antibonding (weakening the bond order from three to two) HP Pavilion dv6-1240ek battery, so the diatomic oxygen bond is weaker than the diatomic nitrogen triple bond in which all bonding molecular orbitals are filled, but some antibonding orbitals are not.[10]
A trickle of liquid oxygen is deflected by a magnetic field, illustrating its paramagnetic property
In normal triplet form, O2 molecules are paramagnetic. That is, they form a magnet in the presence of a magnetic field—because of the spin magnetic moments of the unpaired electrons in the moleculeHP Pavilion dv6-1240ei battery, and the negative exchange energy between neighboring O2 molecules.[12] Liquid oxygen is attracted to a magnet to a sufficient extent that, in laboratory demonstrations, a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet. HP Pavilion dv6-1240ef battery
Singlet oxygen is a name given to several higher-energy species of molecular O2 in which all the electron spins are paired. It is much more reactive towards common organic molecules than is molecular oxygen per se. In nature, singlet oxygen is commonly formed from water during photosynthesis, using the energy of sunlightHP Pavilion dv6-1240ed battery.[15] It is also produced in the troposphere by the photolysis of ozone by light of short wavelength,[16] and by the immune system as a source of active oxygen.[17] Carotenoids in photosynthetic organisms (and possibly also in animals) play a major role in absorbing energy from singlet oxygen and converting it to the unexcited ground state before it can cause harm to tissuesHP Pavilion dv6-1240ec battery.[18]
Allotropes
Main article: Allotropes of oxygen
Ozone is a rare gas on Earth found mostly in the stratosphere.
The common allotrope of elemental oxygen on Earth is called dioxygen, O2. It has a bond length of 121 pm and a bond energy of 498 kJ·mol−1.[19] This is the form that is used by complex forms of life, such as animals, in cellular respiration (see Biological role) and is the form that is a major part of the Earth's atmosphere (see Occurrence) HP Pavilion dv6-1240ea battery. Other aspects of O2 are covered in the remainder of this article.
Trioxygen (O3) is usually known as ozone and is a very reactive allotrope of oxygen that is damaging to lung tissue.[20] Ozone is produced in the upper atmosphere when O2 combines with atomic oxygen made by the splitting of O2 by ultraviolet (UV) radiation.[7] Since ozone absorbs strongly in the UV region of the spectrumHP Pavilion dv6-1239tx battery, the ozone layer of the upper atmosphere functions as a protective radiation shield for the planet.[7] Near the Earth's surface, however, it is a pollutant formed as a by-product of automobile exhaust.[20] The metastable molecule tetraoxygen (O4) was discovered in 2001,[21][22] and was assumed to exist in one of the six phases of solid oxygenHP Pavilion dv6-1239et battery. It was proven in 2006 that this phase, created by pressurizing O2 to 20 GPa, is in fact a rhombohedral O8 cluster.[23] This cluster has the potential to be a much more powerful oxidizer than either O2 or O3 and may therefore be used in rocket fuel.[21][22] A metallic phase was discovered in 1990 when solid oxygen is subjected to a pressure of above 96 GPa[24] and it was shown in 1998 that at very low temperatures, this phase becomes superconducting. HP Pavilion dv6-1239es battery
Physical properties
See also: Liquid oxygen and solid oxygen
Oxygen is more soluble in water than nitrogen is; water contains approximately 1 molecule of O2 for every 2 molecules of N2, compared to an atmospheric ratio of approximately 1:4. The solubility of oxygen in water is temperature-dependent, and about twice as much (14.6 mg·L−1) dissolves at 0 °C than at 20 °C (7.6 mg·L−1). HP Pavilion dv6-1238nr battery At 25 °C and 1 standard atmosphere (101.3 kPa) of air, freshwater contains about 6.04 milliliters (mL) of oxygen per liter, whereas seawater contains about 4.95 mL per liter.[28] At 5 °C the solubility increases to 9.0 mL (50% more than at 25 °C) per liter for water and 7.2 mL (45% more) per liter for sea water.
Oxygen condenses at 90.20 K (−182.95 °C, −297.31 °F) HP Pavilion dv6-1238et battery, and freezes at 54.36 K (−218.79 °C, −361.82 °F).[29] Both liquid and solid O2 are clear substances with a light sky-blue color caused by absorption in the red (in contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid O2 is usually obtained by the fractional distillation of liquefied airHP Pavilion dv6-1238es battery.[30] Liquid oxygen may also be produced by condensation out of air, using liquid nitrogen as a coolant. It is a highly reactive substance and must be segregated from combustible materials.[31]
Isotopes and stellar origin
Main article: Isotopes of oxygen
Late in a massive star's life, 16O concentrates in the O-shell, 17O in the H-shell and oxygen-1818O in the He-shell.
Naturally occurring oxygen is composed of three stable isotopes, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance) HP Pavilion dv6-1238ca battery.[32]
Most 16O is synthesized at the end of the helium fusion process in massive stars but some is made in the neon burning process.[33] 17O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars.[33] Most 18O is produced when 14NHP Pavilion dv6-1237et battery (made abundant from CNO burning) captures a 4He nucleus, making 18O common in the helium-rich zones of evolved, massive stars.[33]
Fourteen radioisotopes have been characterized. The most stable are 15O with a half-life of 122.24 seconds and 14O with a half-life of 70.606 seconds.[32] All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 millisecondsHP Pavilion dv6-1237es battery.[32] The most common decay mode of the isotopes lighter than 16O is β+ decay[34][35][36] to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay to yield fluorine.[32]
Occurrence
Oxygen is the most abundant chemical element, by mass, in our biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium.[2] About 0.9% of the Sun's mass is oxygenHP Pavilion dv6-1237ca battery.[5] Oxygen constitutes 49.2% of the Earth's crust by mass[3] and is the major component of the world's oceans (88.8% by mass).[5] Oxygen gas is the second most common component of the Earth's atmosphere, taking up 20.8% of its volume and 23.1% of its mass (some 1015 tonnes). Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere: Mars (with 0.1% O2 by volume) HP Pavilion dv6-1236ss battery and Venus have far lower concentrations. However, the O2 surrounding these other planets is produced solely by ultraviolet radiation impacting oxygen-containing molecules such as carbon dioxide.
The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on EarthHP Pavilion dv6-1236et battery: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration and decay remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate of roughly 1/2000th of the entire atmospheric oxygen per yearHP Pavilion dv6-1236eo battery.
Cold water holds more dissolved O2.
Free oxygen also occurs in solution in the world's water bodies. The increased solubility of O2 at lower temperatures (see Physical properties) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content.[40] Polluted water may have reduced amounts of O2 in itHP Pavilion dv6-1235ss battery, depleted by decaying algae and other biomaterials through a process called eutrophication. Scientists assess this aspect of water quality by measuring the water's biochemical oxygen demand, or the amount of O2 needed to restore it to a normal concentration. HP Pavilion dv6-1235sb battery
Biological role
Main article: Dioxygen in biological reactions
Photosynthesis and respiration
Photosynthesis splits water to liberate O2 and fixes CO2 into sugar.
In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, Green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on earth and the rest is produced by terrestrial plants. HP Pavilion dv6-1235et battery Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.[43]
A simplified overall formula for photosynthesis is:[44]
6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2 (or simply carbon dioxide + water + sunlight → glucose + dioxygen) HP Pavilion dv6-1235es battery
Photolytic oxygen evolution occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons.[45] Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize ATP via photophosphorylation.[46] The O2 remaining after oxidation of the water molecule is released into the atmosphere. HP Pavilion dv6-1235eo battery
Molecular dioxygen, O2, is essential for cellular respiration in all aerobic organisms. Oxygen is used in mitochondria to help generate adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified asHP Pavilion dv6-1235ee battery:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 2880 kJ·mol−1
In vertebrates, O2 diffuses through membranes in the lungs and into red blood cells. Hemoglobin binds O2, changing its color from bluish red to bright red[20] (CO2 is released from another part of hemoglobin through the Bohr effect). Other animals use hemocyanin (molluscs and some arthropods) or hemerythrin (spiders and lobsters).[38] A liter of blood can dissolve 200 cm3 of O2. HP Pavilion dv6-1235ec battery
Reactive oxygen species, such as superoxide ion (O−
2) and hydrogen peroxide (H2O2), are dangerous by-products of oxygen use in organisms.[38] Parts of the immune system of higher organisms, however, create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in the hypersensitive response of plants against pathogen attack. HP Pavilion dv6-1234tx battery
An adult human in rest inhales 1.8 to 2.4 grams of oxygen per minute.[48] This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.[49]
Content in body
The oxygen content in the body of a living organism is usually highest in the respiratory system, and decreases along any arterial system, peripheral tissues and venous system, respectivelyHP Pavilion dv6-1234nr battery. Oxygen content in this sense is often given as the partial pressure, which is the pressure which oxygen would have if it alone occupied the volume.[53]
Build-up in the atmosphere
Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved. Free oxygen first appeared in significant quantities during the Paleoproterozoic eon (between 2.5 and 1.6 billion years ago). At first, the oxygen combined with dissolved iron in the oceans to form banded iron formationsHP Pavilion dv6-1234eo battery. Free oxygen started to outgas from the oceans 2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago.[54]
The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the Great Oxygenation Event(oxygen catastrophe) about 2.4 billion years agoHP Pavilion dv6-1233et battery. However, cellular respiration using O2 enables aerobic organisms to produce much more ATP than anaerobic organisms, helping the former to dominate Earth's biosphere.[55] Photosynthesis and cellular respiration of O2 allowed for the evolution of eukaryotic cells and ultimately complex multicellular organisms such as plants and animalsHP Pavilion dv6-1233eo battery.
Since the beginning of the Cambrian period 540 million years ago, O2 levels have fluctuated between 15% and 30% by volume.[56] Towards the end of the Carboniferous period (about 300 million years ago) atmospheric O2 levels reached a maximum of 35% by volumeHP Pavilion dv6-1232ss battery,[56] which may have contributed to the large size of insects and amphibians at this time.[57] Human activities, including the burning of 7 billion tonnes of fossil fuels each year have had very little effect on the amount of free oxygen in the atmosphere.[12] At the current rate of photosynthesis it would take about 2,000 years to regenerate the entire O2 in the present atmosphereHP Pavilion dv6-1232et battery.[58]
History
Early experiments
Philo's experiment inspired later investigators.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neckHP Pavilion dv6-1232eo battery.[59] Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respirationHP Pavilion dv6-1231eo battery.[60]
In the late 17th century, Robert Boyle proved that air is necessary for combustion. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus or just nitroaereus.[61] In one experiment he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjectsHP Pavilion dv6-1230us battery.[62] From this he surmised that nitroaereus is consumed in both respiration and combustion.
Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it.[61] He also thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the bodyHP Pavilion dv6-1230ss battery.[61] Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione".[62]
Phlogiston theory
Main article: Phlogiston theory
Stahl helped develop and popularize the phlogiston theory.
Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as an element.[26] This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, which was then the favored explanation of those processesHP Pavilion dv6-1230sl battery.
Established in 1667 by the German alchemist J. J. Becher, and modified by the chemist Georg Ernst Stahl by 1731,[63] phlogiston theory stated that all combustible materials were made of two parts. One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calxHP Pavilion dv6-1230sf battery.[60]
Highly combustible materials that leave little residue, such as wood or coal, were thought to be made mostly of phlogiston; whereas non-combustible substances that corrode, such as iron, contained very little. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the ideaHP Pavilion dv6-1230sb battery; instead, it was based on observations of what happens when something burns, that most common objects appear to become lighter and seem to lose something in the process.[60] The fact that a substance like wood actually gains overall weight in burning was hidden by the buoyancy of the gaseous combustion products. Indeed one of the first clues that the phlogiston theory was incorrect was that metalsHP Pavilion dv6-1230ew battery, too, gain weight in rusting (when they were supposedly losing phlogiston).
Discovery
Carl Wilhelm Scheele beat Priestley to the discovery but published afterwards.
Oxygen was first discovered by Swedish pharmacist Carl Wilhelm Scheele. He had produced oxygen gas by heating mercuric oxide and various nitrates by about 1772.[5][60] Scheele called the gas "fire air" because it was the only known supporter of combustionHP Pavilion dv6-1230et battery, and wrote an account of this discovery in a manuscript he titled Treatise on Air and Fire, which he sent to his publisher in 1775. However, that document was not published until 1777.[64]
Joseph Priestley is usually given priority in the discovery.
In the meantime, on August 1, 1774, an experiment conducted by the British clergyman Joseph Priestley focused sunlight on mercuric oxide (HgO) inside a glass tube, which liberated a gas he named "dephlogisticated air"HP Pavilion dv6-1230er battery.[5] He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it. After breathing the gas himself, he wrote: "The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards." HP Pavilion dv6-1230eq battery Priestley published his findings in 1775 in a paper titled "An Account of Further Discoveries in Air" which was included in the second volume of his book titled Experiments and Observations on Different Kinds of Air.[60][65] Because he published his findings first, Priestley is usually given priority in the discoveryHP Pavilion dv6-1230eo battery.
The noted French chemist Antoine Laurent Lavoisier later claimed to have discovered the new substance independently. However, Priestley visited Lavoisier in October 1774 and told him about his experiment and how he liberated the new gas. Scheele also posted a letter to Lavoisier on September 30, 1774 that described his own discovery of the previously unknown substanceHP Pavilion dv6-1230ek battery, but Lavoisier never acknowledged receiving it (a copy of the letter was found in Scheele's belongings after his death).[64]
Lavoisier's contribution
What Lavoisier did indisputably do (although this was disputed at the time) was to conduct the first adequate quantitative experiments on oxidation and give the first correct explanation of how combustion worksHP Pavilion dv6-1230ej battery.[5] He used these and similar experiments, all started in 1774, to discredit the phlogiston theory and to prove that the substance discovered by Priestley and Scheele was a chemical element.
Antoine Lavoisier discredited the Phlogiston theory.
In one experiment, Lavoisier observed that there was no overall increase in weight when tin and air were heated in a closed containerHP Pavilion dv6-1230ed battery.[5] He noted that air rushed in when he opened the container, which indicated that part of the trapped air had been consumed. He also noted that the tin had increased in weight and that increase was the same as the weight of the air that rushed back in. This and other experiments on combustion were documented in his book Sur la combustion en général, which was published in 1777. HP Pavilion dv6-1230ec battery In that work, he proved that air is a mixture of two gases; 'vital air', which is essential to combustion and respiration, and azote (Gk. ἄζωτον "lifeless"), which did not support either. Azote later became nitrogen in English, although it has kept the name in French and several other European languagesHP Pavilion dv6-1230ca battery.[5]
Lavoisier renamed 'vital air' to oxygène in 1777 from the Greek roots ὀξύς (oxys) (acid, literally "sharp," from the taste of acids) and -γενής (-genēs) (producer, literally begetter), because he mistakenly believed that oxygen was a constituent of all acids.[7] Chemists (notably Sir Humphrey Davy in 1812) eventually determined that Lavoisier was wrong in this regardHP Pavilion dv6-1229tx battery (it is in fact hydrogen that forms the basis for acid chemistry), but by that time it was too late, the name had taken.
Oxygen entered the English language despite opposition by English scientists and the fact that the Englishman Priestley had first isolated the gas and written about it. This is partly due to a poem praising the gas titled "Oxygen" in the popular book The Botanic Garden (1791) by Erasmus Darwin, grandfather of Charles Darwin. HP Pavilion dv6-1229sf battery
Later history
Robert H. Goddard and a liquid oxygen-gasoline rocket
John Dalton's original atomic hypothesis assumed that all elements were monoatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed that water's formula was HO, giving the atomic mass of oxygen as 8 times that of hydrogen, instead of the modern value of about 16HP Pavilion dv6-1229et battery.[66] In 1805, Joseph Louis Gay-Lussac and Alexander von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen; and by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the assumption of diatomic elemental molecules. HP Pavilion dv6-1229er battery
By the late 19th century scientists realized that air could be liquefied, and its components isolated, by compressing and cooling it. Using a cascade method, Swiss chemist and physicist Raoul Pierre Pictet evaporated liquid sulfur dioxide in order to liquefy carbon dioxide, which in turn was evaporated to cool oxygen gas enough to liquefy itHP Pavilion dv6-1228tx battery. He sent a telegram on December 22, 1877 to the French Academy of Sciences in Paris announcing his discovery of liquid oxygen.[69] Just two days later, French physicist Louis Paul Cailletet announced his own method of liquefying molecular oxygen.[69] Only a few drops of the liquid were produced in either case so no meaningful analysis could be conductedHP Pavilion dv6-1228sf battery. Oxygen was liquified in stable state for the first time on March 29, 1883 by Polish scientists from Jagiellonian University, Zygmunt Wróblewski and Karol Olszewski.[70]
In 1891 Scottish chemist James Dewar was able to produce enough liquid oxygen to study.[12] The first commercially viable process for producing liquid oxygen was independently developed in 1895 by German engineer Carl von Linde and British engineer William HampsonHP Pavilion dv6-1228ez battery. Both men lowered the temperature of air until it liquefied and then distilled the component gases by boiling them off one at a time and capturing them.[71] Later, in 1901, oxyacetylene welding was demonstrated for the first time by burning a mixture of acetylene and compressed O2. This method of welding and cutting metal later became commonHP Pavilion dv6-1227sf battery.[71]
In 1923 the American scientist Robert H. Goddard became the first person to develop a rocket engine; the engine used gasoline for fuel and liquid oxygen as the oxidizer. Goddard successfully flew a small liquid-fueled rocket 56 m at 97 km/h on March 16, 1926 in Auburn, Massachusetts, USA. HP Pavilion dv6-1225tx battery
Industrial production
See also: Air separation, Oxygen evolution, and fractional distillation
Two major methods are employed to produce 100 million tonnes of O2 extracted from air for industrial uses annually.[64] The most common method is to fractionally distill liquefied air into its various components, with N2 distilling as a vapor while O2 is left as a liquid. HP Pavilion dv6-1225sv battery
Hofmann electrolysis apparatus used in electrolysis of water.
The other major method of producing O2 gas involves passing a stream of clean, dry air through one bed of a pair of identical zeolite molecular sieves, which absorbs the nitrogen and delivers a gas stream that is 90% to 93% O2.[64] Simultaneously, nitrogen gas is released from the other nitrogen-saturated zeolite bed, by reducing the chamber operating pressure and diverting part of the oxygen gas from the producer bed through it, in the reverse direction of flowHP Pavilion dv6-1225ss battery. After a set cycle time the operation of the two beds is interchanged, thereby allowing for a continuous supply of gaseous oxygen to be pumped through a pipeline. This is known as pressure swing adsorption. Oxygen gas is increasingly obtained by these non-cryogenic technologies (see also the related vacuum swing adsorption). HP Pavilion dv6-1225so battery
Oxygen gas can also be produced through electrolysis of water into molecular oxygen and hydrogen. DC electricity must be used: if AC is used, the gases in each limb consist of hydrogen and oxygen in the explosive ratio 2:1. Contrary to popular belief, the 2:1 ratio observed in the DC electrolysis of acidified water does not prove that the empirical formula of water is H2O unless certain assumptions are made about the molecular formulae of hydrogen and oxygen themselvesHP Pavilion dv6-1225sf battery. A similar method is the electrocatalytic O2 evolution from oxides and oxoacids. Chemical catalysts can be used as well, such as in chemical oxygen generators or oxygen candles that are used as part of the life-support equipment on submarines, and are still part of standard equipment on commercial airliners in case of depressurization emergenciesHP Pavilion dv6-1225ew battery. Another air separation technology involves forcing air to dissolve through ceramic membranes based on zirconium dioxide by either high pressure or an electric current, to produce nearly pure O2 gas.[41]
In large quantities, the price of liquid oxygen in 2001 was approximately $0.21/kg.[74] Since the primary cost of production is the energy cost of liquefying the air, the production cost will change as energy cost variesHP Pavilion dv6-1225et battery.
For reasons of economy, oxygen is often transported in bulk as a liquid in specially insulated tankers, since one litre of liquefied oxygen is equivalent to 840 liters of gaseous oxygen at atmospheric pressure and 20 °C (68 °F).[64] Such tankers are used to refill bulk liquid oxygen storage containersHP Pavilion dv6-1225er battery, which stand outside hospitals and other institutions with a need for large volumes of pure oxygen gas. Liquid oxygen is passed through heat exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is also stored and shipped in smaller cylinders containing the compressed gas; a form that is useful in certain portable medical applications and oxy-fuel welding and cutting. HP Pavilion dv6-1225ei battery
Applications
See also: Breathing gas, Redox, and Combustion
Medical
An oxygen concentrator in an emphysema patient's house
Main article: Oxygen therapy
Uptake of O2 from the air is the essential purpose of respiration, so oxygen supplementation is used in medicine. Treatment not only increases oxygen levels in the patient's blood, but has the secondary effect of decreasing resistance to blood flow in many types of diseased lungs, easing work load on the heartHP Pavilion dv6-1225ef battery. Oxygen therapy is used to treat emphysema, pneumonia, some heart disorders (congestive heart failure), some disorders that cause increased pulmonary artery pressure, and any disease that impairs the body's ability to take up and use gaseous oxygenHP Pavilion dv6-1225ee battery.[75]
Treatments are flexible enough to be used in hospitals, the patient's home, or increasingly by portable devices. Oxygen tents were once commonly used in oxygen supplementation, but have since been replaced mostly by the use of oxygen masks or nasal cannulasHP Pavilion dv6-1224eo battery.[76]
Hyperbaric (high-pressure) medicine uses special oxygen chambers to increase the partial pressure of O2 around the patient and, when needed, the medical staff.[77] Carbon monoxide poisoning, gas gangrene, and decompression sickness (the 'bends') are sometimes treated using these devicesHP Pavilion dv6-1223sl battery.[78] Increased O2 concentration in the lungs helps to displace carbon monoxide from the heme group of hemoglobin.[79][80] Oxygen gas is poisonous to the anaerobic bacteria that cause gas gangrene, so increasing its partial pressure helps kill them.[81][82] Decompression sickness occurs in divers who decompress too quickly after a dive, resulting in bubbles of inert gas, mostly nitrogen and helium, forming in their bloodHP Pavilion dv6-1223sf battery. Increasing the pressure of O2 as soon as possible is part of the treatment.
Oxygen is also used medically for patients who require mechanical ventilation, often at concentrations above the 21% found in ambient air.
Life support and recreational use
Low pressure pure O2 is used in space suits.
A notable application of O2 as a low-pressure breathing gas is in modern space suits, which surround their occupant's body with pressurized air. These devices use nearly pure oxygen at about one third normal pressure, resulting in a normal blood partial pressure of O2.[85][86] This trade-off of higher oxygen concentration for lower pressure is needed to maintain flexible spacesuitsHP Pavilion dv6-1223eo battery.
Scuba divers and submariners also rely on artificially delivered O2, but most often use normal pressure, and/or mixtures of oxygen and air. Pure or nearly pure O2 use in diving at higher-than-sea-level pressures is usually limited to rebreather, decompression, or emergency treatment use at relatively shallow depths (~6 meters depth, or less). HP Pavilion dv6-1222tx battery Deeper diving requires significant dilution of O2 with other gases, such as nitrogen or helium, to help prevent oxygen toxicity.[87]
People who climb mountains or fly in non-pressurized fixed-wing aircraft sometimes have supplemental O2 supplies.[89] Passengers traveling in (pressurized) commercial airplanes have an emergency supply of O2 automatically supplied to them in case of cabin depressurizationHP Pavilion dv6-1222sl battery. Sudden cabin pressure loss activates chemical oxygen generators above each seat, causing oxygen masks to drop. Pulling on the masks "to start the flow of oxygen" as cabin safety instructions dictate, forces iron filings into the sodium chlorate inside the canister.[41] A steady stream of oxygen gas is then produced by the exothermic reactionHP Pavilion dv6-1222et battery.
Oxygen, as a supposed mild euphoric, has a history of recreational use in oxygen bars and in sports. Oxygen bars are establishments, found in Japan, California, and Las Vegas, Nevada since the late 1990s that offer higher than normal O2 exposure for a fee.[90] Professional athletes, especially in American footballHP Pavilion dv6-1222eo battery, also sometimes go off field between plays to wear oxygen masks in order to get a "boost" in performance. The pharmacological effect is doubtful; a placebo effect is a more likely explanation.[90] Available studies support a performance boost from enriched O2 mixtures only if they are breathed during aerobic exerciseHP Pavilion dv6-1221tx battery.[91]
Other recreational uses that do not involve breathing the gas include pyrotechnic applications, such as George Goble's five-second ignition of barbecue grills.[92]
Industrial
Most commercially produced O2 is used to smelt iron into steel.
Smelting of iron ore into steel consumes 55% of commercially produced oxygen.[41] In this process, O2 is injected through a high-pressure lance into molten iron, which removes sulfur impurities and excess carbon as the respective oxides, SO2 and CO2HP Pavilion dv6-1221sl battery. The reactions are exothermic, so the temperature increases to 1,700 °C.[41]
Another 25% of commercially produced oxygen is used by the chemical industry.[41] Ethylene is reacted with O2 to create ethylene oxide, which, in turn, is converted into ethylene glycol; the primary feeder material used to manufacture a host of products, including antifreeze and polyester polymers (the precursors of many plastics and fabrics). HP Pavilion dv6-1221eo battery
Most of the remaining 20% of commercially produced oxygen is used in medical applications, metal cutting and welding, as an oxidizer in rocket fuel, and in water treatment.[41] Oxygen is used in oxyacetylene welding burning acetylene with O2 to produce a very hot flame. In this process, metal up to 60 cm thick is first heated with a small oxy-acetylene flame and then quickly cut by a large stream of O2. HP Pavilion dv6-1221ax battery Larger rockets use liquid oxygen as their oxidizer, which is mixed and ignited with the fuel for propulsion.[citation needed]
Scientific
500 million years of climate change vs 18O
Paleoclimatologists measure the ratio of oxygen-18 and oxygen-16 in the shells and skeletons of marine organisms to determine what the climate was like millions of years ago (see oxygen isotope ratio cycle). Seawater molecules that contain the lighter isotope, oxygen-16, evaporate at a slightly faster rate than water molecules containing the 12% heavier oxygen-18HP Pavilion dv6-1220tx battery; this disparity increases at lower temperatures.[94] During periods of lower global temperatures, snow and rain from that evaporated water tends to be higher in oxygen-16, and the seawater left behind tends to be higher in oxygen-18. Marine organisms then incorporate more oxygen-18 into their skeletons and shells than they would in a warmer climateHP Pavilion dv6-1220st battery.[94] Paleoclimatologists also directly measure this ratio in the water molecules of ice core samples that are up to several hundreds of thousands of years old.
Planetary geologists have measured different abundances of oxygen isotopes in samples from the Earth, the Moon, Mars, and meteorites, but were long unable to obtain reference values for the isotope ratios in the SunHP Pavilion dv6-1220ss battery, believed to be the same as those of the primordial solar nebula. However, analysis of a silicon wafer exposed to the solar wind in space and returned by the crashed Genesis spacecraft has shown that the Sun has a higher proportion of oxygen-16 than does the EarthHP Pavilion dv6-1220sp battery. The measurement implies that an unknown process depleted oxygen-16 from the Sun's disk of protoplanetary material prior to the coalescence of dust grains that formed the Earth.[95]
Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760 nm. Some remote sensing scientists have proposed using the measurement of the radiance coming from vegetation canopies in those bands to characterize plant health status from a satellite platformHP Pavilion dv6-1220so battery.[96] This approach exploits the fact that in those bands it is possible to discriminate the vegetation's reflectance from its fluorescence, which is much weaker. The measurement is technically difficult owing to the low signal-to-noise ratio and the physical structure of vegetation; but it has been proposed as a possible method of monitoring the carbon cycle from satellites on a global scaleHP Pavilion dv6-1220sh battery.
Compounds
Main article: Compounds of oxygen
Water (H2O) is the most familiar oxygen compound.
The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides.[97] Compounds containing oxygen in other oxidation states are very uncommon: −1/2 (superoxides), −1/3 (ozonides), 0 (elemental, hypofluorous acid), +1/2 (dioxygenyl), +1 (dioxygen difluoride), and +2 (oxygen difluoride) HP Pavilion dv6-1220sd battery.
Oxides and other inorganic compounds
Water (H2O) is the oxide of hydrogen and the most familiar oxygen compound. Hydrogen atoms are covalently bonded to oxygen in a water molecule but also have an additional attraction (about 23.3 kJ·mol−1 per hydrogen atom) to an adjacent oxygen atom in a separate molecule. HP Pavilion dv6-1220sb battery These hydrogen bonds between water molecules hold them approximately 15% closer than what would be expected in a simple liquid with just van der Waals forces.[99][100]
Oxides, such as iron oxide or rust form when oxygen combines with other elements.
Due to its electronegativity, oxygen forms chemical bonds with almost all other elements at elevated temperatures to give corresponding oxides. However, some elements readily form oxides at standard conditions for temperature and pressureHP Pavilion dv6-1220sa battery; the rusting of iron is an example. The surface of metals like aluminium and titanium are oxidized in the presence of air and become coated with a thin film of oxide that passivates the metal and slows further corrosion. Some of the transition metal oxides are found in nature as non-stoichiometric compounds, with a slightly less metal than the chemical formula would showHP Pavilion dv6-1220ez battery. For example, the natural occurring FeO (wüstite) is actually written as Fe1 − xO, where x is usually around 0.05.[101]
Oxygen as a compound is present in the atmosphere in trace quantities in the form of carbon dioxide (CO2). The earth's crustal rock is composed in large part of oxides of silicon (silica SiO2, found in granite and sand), aluminium (aluminium oxide Al2O3, in bauxite and corundum), iron (iron(III) oxide Fe2O3HP Pavilion dv6-1220ew battery, in hematite and rust), and calcium carbonate (in limestone). The rest of the Earth's crust is also made of oxygen compounds, in particular various complex silicates (in silicate minerals). The Earth's mantle, of much larger mass than the crust, is largely composed of silicates of magnesium and ironHP Pavilion dv6-1220ev battery.
Water-soluble silicates in the form of Na4SiO4, Na2SiO3, and Na2Si2O5 are used as detergents and adhesives.[102]
Oxygen also acts as a ligand for transition metals, forming metal–O2 bonds with the iridium atom in Vaska's complex,[103] with the platinum in PtF6,[104] and with the iron center of the heme group of hemoglobin.
Organic compounds and biomolecules
Acetone is an important feeder material in the chemical industryHP Pavilion dv6-1220er battery.
Oxygen
Carbon
Hydrogen
Oxygen represents more than 40% of the molecular mass of the ATP molecule.
Among the most important classes of organic compounds that contain oxygen are (where "R" is an organic group): alcohols (R-OH); ethers (R-O-R); ketones (R-CO-R); aldehydes (R-CO-H); carboxylic acids (R-COOH); esters (R-COO-R); acid anhydrides (R-CO-O-CO-R); and amides (R-C(O)-NR2) HP Pavilion dv6-1220ek battery. There are many important organic solvents that contain oxygen, including: acetone, methanol, ethanol, isopropanol, furan, THF, diethyl ether, dioxane, ethyl acetate, DMF, DMSO, acetic acid, and formic acid. Acetone ((CH3)2CO) and phenol (C6H5OH) are used as feeder materials in the synthesis of many different substancesHP Pavilion dv6-1220ei battery. Other important organic compounds that contain oxygen are: glycerol, formaldehyde, glutaraldehyde, citric acid, acetic anhydride, and acetamide. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.
Oxygen reacts spontaneously with many organic compounds at or below room temperature in a process called autoxidationHP Pavilion dv6-1220eg battery.[105] Most of the organic compounds that contain oxygen are not made by direct action of O2. Organic compounds important in industry and commerce that are made by direct oxidation of a precursor include ethylene oxide and peracetic acid.[102]
The element is found in almost all biomolecules that are important to (or generated by) life. Only a few common complex biomolecules, such as squalene and the carotenes, contain no oxygenHP Pavilion dv6-1220ec battery. Of the organic compounds with biological relevance, carbohydrates contain the largest proportion by mass of oxygen. All fats, fatty acids, amino acids, and proteins contain oxygen (due to the presence of carbonyl groups in these acids and their ester residues). Oxygen also occurs in phosphate (PO3−4) groups in the biologically important energy-carrying molecules ATP and ADPHP Pavilion dv6-1219tx battery, in the backbone and the purines (except adenine) and pyrimidines of RNA and DNA, and in bones as calcium phosphate and hydroxylapatite.
Safety and precautions
Toxicity
Main article: Oxygen toxicity
Main symptoms of oxygen toxicity[106]
Oxygen toxicity occurs when the lungs take in a higher than normal O2 partial pressure, which can occur in deep scuba diving.
Oxygen gas (O2) can be toxic at elevated partial pressures, leading to convulsions and other health problems. Oxygen toxicity usually begins to occur at partial pressures more than 50 kilopascals (kPa), or 2.5 times the normal sea-level O2 partial pressure of about 21 kPa (equal to about 50% oxygen composition at standard pressure) HP Pavilion dv6-1219ez battery. This is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30%–50% O2 by volume (about 30 kPa at standard pressure).[26] (although this figure also is subject to wide variation, depending on type of mask) HP Pavilion dv6-1218tx battery.
At one time, premature babies were placed in incubators containing O2-rich air, but this practice was discontinued after some babies were blinded by it.[26][109]
Breathing pure O2 in space applications, such as in some modern space suits, or in early spacecraft such as Apollo, causes no damage due to the low total pressures used.[85][110] In the case of spacesuitsHP Pavilion dv6-1218ss battery, the O2 partial pressure in the breathing gas is, in general, about 30 kPa (1.4 times normal), and the resulting O2 partial pressure in the astronaut's arterial blood is only marginally more than normal sea-level O2 partial pressure (for more information on this, see space suit and arterial blood gas) HP Pavilion dv6-1218ca battery.
Oxygen toxicity to the lungs and central nervous system can also occur in deep scuba diving and surface supplied diving.[26][87] Prolonged breathing of an air mixture with an O2 partial pressure more than 60 kPa can eventually lead to permanent pulmonary fibrosis.[111] Exposure to a O2 partial pressures greater than 160 kPa (about 1.6 atm) may lead to convulsions (normally fatal for divers) HP Pavilion dv6-1217tx battery. Acute oxygen toxicity (causing seizures, its most feared effect for divers) can occur by breathing an air mixture with 21% O2 at 66 m or more of depth; the same thing can occur by breathing 100% O2 at only 6 m.
Combustion and other hazards
The interior of the Apollo 1 Command Module. Pure O2 at higher than normal pressure and a spark led to a fire and the loss of the Apollo 1 crewHP Pavilion dv6-1217ss battery.
Highly concentrated sources of oxygen promote rapid combustion. Fire and explosion hazards exist when concentrated oxidants and fuels are brought into close proximity; however, an ignition event, such as heat or a spark, is needed to trigger combustion.[115] Oxygen itself is not the fuel, but the oxidant. Combustion hazards also apply to compounds of oxygen with a high oxidative potentialHP Pavilion dv6-1217er battery, such as peroxides, chlorates, nitrates, perchlorates, and dichromates because they can donate oxygen to a fire.
Concentrated O2 will allow combustion to proceed rapidly and energetically.[115] Steel pipes and storage vessels used to store and transmit both gaseous and liquid oxygen will act as a fuel; and therefore the design and manufacture of O2 systems requires special training to ensure that ignition sources are minimizedHP Pavilion dv6-1217ax battery.[115] The fire that killed the Apollo 1 crew in a launch pad test spread so rapidly because the capsule was pressurized with pure O2 but at slightly more than atmospheric pressure, instead of the 1⁄3 normal pressure that would be used in a mission. HP Pavilion dv6-1216tx battery
Liquid oxygen spills, if allowed to soak into organic matter, such as wood, petrochemicals, and asphalt can cause these materials to detonate unpredictably on subsequent mechanical impact.[115] As with other cryogenic liquids, on contact with the human body it can cause frostbites to the skin and the eyesHP Pavilion dv6-1216ss battery.
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