GROUP VI 295

SULPHUR TRIOXIDE

Sulphur trioxide was first prepared by heating iron(III) sulphate :

Fe 2 (SO 4 ) 3 -» Fe 2 O 3 + 3SO 3

It is also obtained by the dehydration of concentrated sulphuric acid

with phosphorus(V) oxide:

2H 2 SO 4 + P 4 O 10 -> 4HPO 3 + 2SO 3

and the thermal decomposition of iron(II) sulphate :

2FeSO 4 -* Fe 2 O 3 + SO 2 -h SO 3

iron(II) iron(II)

sulphate oxide

In the laboratory it is commonly prepared by the reaction between

sulphur dioxide and oxygen at high temperature in the presence of

a platinum catalyst :

2SO 2 + O 2 ^ 2SO 3

(This is the basis of the industrial manufacture of sulphuric acid
and is dealt with on p. 296.)
Sulphur trioxide can be collected as a white solid in a receiver
surrounded by a freezing mixture of ice and salt.

Properties

In the vapour state, sulphur trioxide has the formula SO 3. The
molecule is planar with all the S — O bonds short and of equal length.
The structure can be represented simply as

O

A

but is probably a resonance hybrid of several forms. Solid sulphur
trioxide exists in at least two modifications, the a and j3 forms. The
a form is an ice-like transparent solid consisting of rings of formula
S 3 O 9 (shown geometrically below):

a - SO 3

O O