Modern inorganic chemistry

GROUP V! 287

The two oxides formed with hydrogen, H 2 O and H 2 O 2 , have

already been discussed, but it should be emphasised that hydrogen

peroxide and the peroxides formed from it contain the —O—O—

linkage. The oxidising power of these peroxides lies in the oxygen

of the peroxo-group, unlike the dioxides (see above).

SULPHIDES

1. The alkali metal sulphides

These are ionic solids and can exist as the anhydrous salts (prepared

by heating together sulphur with excess of the alkali metal) or as

hydrates, for example Na 2 S.9H 2 O. Since hydrogen sulphide is a

weak acid these salts are hydrolysed in water,

S^2 ~ 4- H 2 O-»HS'

HS~ + H 2 O-»H 2 S-h OH~

and smell of hydrogen sulphide. Aqueous solutions of these salts are

conveniently prepared by the action of hydrogen sulphide on the

alkali metal hydroxide ; if excess hydrogen sulphide is used the

hydrogensulphide is formed, for example NaHS. Solutions of these

sulphides can dissolve sulphur to give coloured polysulphides, for

example Na 2 S 4 containing anionic sulphur chains.

2. The sulphides of alkaline earth metals

These are similar to those of the alkali metals but are rather less
soluble in water. However, calcium sulphide, for example, is not
precipitated by addition of sulphide ions to a solution of a calcium
salt, since in acid solution the equilibrium position

H 2 S + Ca^2 + ^CaS + 2H+

is very much to the left and in neutral, or alkaline solution the
soluble hydrogensulphide is formed, for example

CaS + H?O -> Ca^2 + + HS" + OH~

3. The sulphides of aluminium and chromium

These can be prepared by the direct combination of the elements.