Properties
Superoxides are compounds in which the oxidation number of oxygen is −½ and the valence ½. The O-O bond distance in O2− is 1.33 Å, vs. 1.21 Å in O2 and 1.49 Å in O22−.
The salts CsO2, RbO2, KO2, and NaO2 are prepared by the direct reaction of O2 with the respective alkali metal. The overall trend corresponds to a reduction in the bond order from 2 (O2), to 1.5 (O2−), to 1 (O22−).
The alkali salts of O2− are orange-yellow in color and quite stable, provided they are kept dry. Upon dissolution of these salts in water, however, the dissolved O2− undergoes disproportionation (dismutation) extremely rapidly:
- 2 O2− + 2 H2O → O2 + H2O2 + 2 OH−
In this process O2− acts as a Brønsted base, initially forming the radical HO2·. But the pKa of its conjugate acid, hydrogen superoxide (HO2·, also known as "hydroperoxyl" or "perhydroxy radical"), is 4.88 so that at neutral pH 7 the vast majority of superoxide is in the anionic form, O2−.
Salts also decompose in the solid state, but this process requires heating:
- 2 NaO2 → Na2O2 + O2
This reaction is the basis of the use of potassium superoxide as an oxygen source in chemical oxygen generators, such as those used on the space shuttle and on submarines. Superoxides are also used in firefighters' oxygen tanks in order to provide a readily available source of oxygen.
Read more about this topic: Superoxide
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