Modern inorganic chemistry

288 GROUP V!

They are rapidly hydrolysed by water and the hydrolysis of solid

aluminium sulphide can be used to prepare hydrogen sulphide:

A1 2 S 3 + 6H 2 O -> 2A1(OH) 3 + 3H 2 St

Consequently they cannot be prepared by the addition of sulphide

ions to a solution of the metal salt, the hydrated metal ions being

so strongly acidic that the following reaction occurs, for example

2[A1(H 2 O) 6 ]^3 + + 3S^2 " -+ 2[Al(OH) 3 (H 2 O) 3 ]i + 3H 2 St

The sulphides of most other metals

These are practically insoluble in water, are not hydrolysed and so
may be prepared by addition of a sufficient concentration of sulphide
ion to exceed the solubility product of the particular sulphide. Some
sulphides, for example those of lead(II), copper(II) and silver(I), have
low solubility products and are precipitated by the small concentra-
tion of sulphide ions produced by passing hydrogen sulphide
through an acid solution of the metal salts; others for example those
of zinc(II), iron(II), nickel(II) and cobalt(II) are only precipitated
when sulphide ions are available in reasonable concentrations, as
they are when hydrogen sulphide is passed into an alkaline solution.
Many of these sulphides occur naturally, for example iron(II)
sulphide, FeS (magnetic pyrites), and antimony(III) sulphide, Sb 2 S 3
(stibnite). They can usually be prepared by the direct combination
of the elements, effected by heating, but this rarely produces a pure
stoichiometric compound and the product often contains a slight
excess of the metal, or of sulphur.

SELENIDES AND TELLURIDES

These closely resemble the corresponding sulphides. The alkali
metal selenides and tellurides are colourless solids, and are powerful
reducing agents in aqueous solution, being oxidised by air to the
elements selenium and tellurium respectively (cf. the reducing power
of the hydrides).

OXIDES AND OXO-ACIDS AND THEIR SALTS

The elements, sulphur, selenium and tellurium form both di- and
tri-oxides. The dioxides reflect the increasing metallic character of