Modern inorganic chemistry

GROUP VI 289

the elements. At room temperature, sulphur dioxide is a gas, boiling

point 263 K, selenium dioxide is a volatile solid which sublimes at

588 K under 1 atmosphere pressure, and tellurium dioxide is a

colourless, apparently ionic, crystalline dimorphic solid.

Sulphur

SULPHUR DIOXIDE, SO 2

Sulphur dioxide is formed together with a little of the trioxide when

sulphur burns in air:

S + O 2 -> SO 2

2S + 3O 2 -* 2SO 3

It can be prepared by the reduction of hot concentrated sulphuric

acid by a metal. Copper is used since it does not also liberate

hydrogen from the acid:

Cu + 2H 2 SO 4 -> CuSO 4 + 2H 2 O + SO 2 T

The equation is not strictly representative of the reaction for the

acid is reduced further and a black deposit consisting of copper(I)

and copper(II) sulphides is also produced.

Sulphur dioxide is also produced by the action of an acid (usually

concentrated sulphuric since it is involatile) on a sulphite or

hydrogensulphite, for example

2HSO 3 - + H 2 SO 4 -> SOJ- + 2H 2 O 4- 2SO 2 t

On the industrial scale it is produced in large quantities for the
manufacture of sulphuric acid and the production methods are
dealt with later. It was once estimated that more than 4 000 000 tons
of sulphur dioxide a year entered the atmosphere of Britain from
the burning of coal and oil.
The molecule of sulphur dioxide has a bent structure. Both S—O
distances are equal and short and since sulphur can expand its outer
quantum level beyond eight, double bonds between the atom» are
likely; i.e.