Modern inorganic chemistry

260 GROUP VI

THE ELEMENTS: OCCURRENCE AND EXTRACTION

OXYGEN

Oxygen occurs free in the atmosphere (21% by volume. 23% by

weight). The proportion is constant-over the earth's surface; it is

also constant for many miles upwards, because the turbulence of

the atmosphere prevents the tendency for the lighter gases, for

example helium, to increase in amount at higher altitudes.

Water contains 89% by weight of oxygen, and the outer crust of

the earth contains about 47%; hence air, earth and sea together

contain about 50 % by weight of oxygen.

On the industrial scale oxygen is obtained by the fractional dis-

tillation of air. A common laboratory method for the preparation

of oxygen is by the decomposition of hydrogen peroxide, H 2 O 2 , a

reaction catalysed by manganese(IV) oxide:

2H 2 O 2 — 2H 2 O + O 2 T

A similar decomposition of the chlorate(I) (hypochlorite) ion, OC1~,

catalysed by both light and cobalt(II) ions, is less commonly used:

2C1CT -»2Cr 4- O 2 T

Oxygen can also be prepared by the thermal decomposition of

certain solid compounds containing it. These include oxides of the

more noble metals, for example of mercury or silver:

2HgO -> 2Hg + O 2 T

certain higher oxides, for example of lead(IV) and manganese(IV):

2PbO 2 ^ 2PbO + O 2 T

peroxides, for example of barium:

2BaO 2 ^ 2BaO + O 2 T

and certain oxosalts, notably the nitrates, chlorates(V), iodates(V)
and manganates(VII) of alkali metals.
Pure oxygen is conveniently prepared by the thermal decom-
position of potassium manganate(VII):

2KMnO 4 -» K 2 MnO 4 + MnO 2 + O 2 T

Oxygen can be produced by certain reactions in solution, for example
the oxidation of hydrogen peroxide by potassium manganate(VII)
acidified with sulphuric acid:

2MnO 4 ~ + 5H 2 O 2 + 6H 3 O+ -» 2Mn^2 + + 14H 2 O 4- 5O 2 t