Modern inorganic chemistry

GROUP VI 267

This is a modification of the process originally devised by Deacon;

further reference is made on p. 317.

Sulphur, selenium and tellurium

These elements are generally unaffected by non-oxidising acids

(behaviour expected for non-metallic elements) but they do react

when heated with concentrated sulphuric and nitric acids, both

powerful oxidising agents. Sulphur is oxidised to sulphur dioxide by

hot concentrated sulphuric acid,

S + 2H 2 SO 4 -> 2H 2 O + 3SO 2

and to sulphuric acid by hot concentrated nitric acid,

S + 6HNO 3 -> H 2 SO 4 + 6NO 2 t 4- 2H 2 O

With concentrated nitric acid, selenium and tellurium form only

their + 4 oxoacids, H 2 SeO 3 and H 2 TeO 3 respectively, indicating a

tendency for the higher oxidation states to become less stable as the

atomic number of the element is increased (cf. Group V, Chapter 9).

Polonium

The more metallic nature of polonium is shown by the fact that it

dissolves not only in concentrated nitric and sulphuric acids but

also in hydrofluoric and hydrochloric acids.

3. REACTIONS WITH ALKALIS

Oxygen does not read with alkalis. Sulphur dissolves slowly in
strong alkalis to give a mixture of sulphite [sulphate(IV)] and sul-
phide initially:

3S + 6OH~ -> 2S^2 ~ + SO|" + 3H 2 O
However, the sulphide ion can attach to itself further atoms of
sulphur to give polysulphide ions, for example 82", 83", and so
these are found in solution also. Further, the sulphite ion can add
on a sulphur atom to give the thiosulphate ion, S 2 O3~ which is
also found in the reaction mixture.
Selenium and tellurium react similarly, forming selenides and
selenates(IV), and tellurides and tellurates(IV) respectively. Like
the sulphide ion, S^2 ~, the ions Se^2 ~ and Te^2 ~ form polyanions
but to a much lesser extent.