Commercial production and use
When required in tonnage quantities, oxygen is prepared by the fractional distillation of liquid air. Of the main components of air, oxygen has the highest boiling point and therefore is less volatile than nitrogen and argon. The process takes advantage of the fact that when a compressed gas is allowed to expand, it cools. Major steps in the operation include the following: (1) Air is filtered to remove particulates; (2) moisture and carbon dioxide are removed by absorption in alkali; (3) the air is compressed and the heat of compression removed by ordinary cooling procedures; (4) the compressed and cooled air is passed into coils contained in a chamber; (5) a portion of the compressed air (at about 200 atmospheres pressure) is allowed to expand in the chamber, cooling the coils; (6) the expanded gas is returned to the compressor with multiple subsequent expansion and compression steps resulting finally in liquefaction of the compressed air at a temperature of −196 °C; (7) the liquid air is allowed to warm to distill first the light rare gases, then the nitrogen, leaving liquid oxygen. Multiple fractionations will produce a product pure enough (99.5 percent) for most industrial purposes.
The steel industry is the largest consumer of pure oxygen in “blowing” high carbon steel—that is, volatilizing carbon dioxide and other nonmetal impurities in a more rapid and more easily controlled process than if air were used. The treatment of sewage by oxygen holds promise for more efficient treatment of liquid effluents than other chemical processes. Incineration of wastes in closed systems using pure oxygen has become important. The so-called LOX of rocket oxidizer fuels is liquid oxygen; the consumption of LOX depends upon the activity of space programs. Pure oxygen is used in submarines and diving bells.
Commercial oxygen or oxygen-enriched air has replaced ordinary air in the chemical industry for the manufacture of such oxidation-controlled chemicals as acetylene, ethylene oxide, and methanol. Medical applications of oxygen include use in oxygen tents, inhalators, and pediatric incubators. Oxygen-enriched gaseous anesthetics ensure life support during general anesthesia. Oxygen is significant in a number of industries that use kilns.
Chemical properties and reactions
The large values of the electronegativity and the electron affinity of oxygen are typical of elements that show only nonmetallic behaviour. In all of its compounds, oxygen assumes a negative oxidation state as is expected from the two half-filled outer orbitals. When these orbitals are filled by electron transfer, the oxide ion O2− is created. In peroxides (species containing the ion O22−) it is assumed that each oxygen has a charge of −1. This property of accepting electrons by complete or partial transfer defines an oxidizing agent. When such an agent reacts with an electron-donating substance, its own oxidation state is lowered. The change (lowering), from the zero to the −2 state in the case of oxygen, is called a reduction. Oxygen may be thought of as the “original” oxidizing agent, the nomenclature used to describe oxidation and reduction being based upon this behaviour typical of oxygen.
As described in the section on allotropy, oxygen forms the diatomic species, O2, under normal conditions and, as well, the triatomic species ozone, O3. There is some evidence for a very unstable tetratomic species, O4. In the molecular diatomic form there are two unpaired electrons that lie in antibonding orbitals. The paramagnetic behaviour of oxygen confirms the presence of such electrons.
The intense reactivity of ozone is sometimes explained by suggesting that one of the three oxygen atoms is in an “atomic” state; on reacting, this atom is dissociated from the O3 molecule, leaving molecular oxygen.
The molecular species, O2, is not especially reactive at normal (ambient) temperatures and pressures. The atomic species, O, is far more reactive. The energy of dissociation (O2 → 2O) is large at 117.2 kilocalories per mole.
Oxygen has an oxidation state of −2 in most of its compounds. It forms a large range of covalently bonded compounds, among which are oxides of nonmetals, such as water (H2O), sulfur dioxide (SO2), and carbon dioxide (CO2); organic compounds such as alcohols, aldehydes, and carboxylic acids; common acids such as sulfuric (H2SO4), carbonic (H2CO3), and nitric (HNO3); and corresponding salts, such as sodium sulfate (Na2SO4), sodium carbonate (Na2CO3), and sodium nitrate (NaNO3). Oxygen is present as the oxide ion, O2-, in the crystalline structure of solid metallic oxides such as calcium oxide, CaO. Metallic superoxides, such as potassium superoxide, KO2, contain the O2- ion, whereas metallic peroxides, such as barium peroxide, BaO2, contain the O22- ion.
|melting point||−218.4 °C (−361.1 °F)|
|boiling point||−183.0 °C (−297.4 °F)|
|density (1 atm, 0 °C)||1.429 g/litre|
|oxidation states||−1, −2, +2 (in compounds with fluorine)|