296 GROUP Vt
This form melts at 290 K, and boils at 318 K. The ft form, obtained
when the a form is allowed to stand for a long time at a temperature
below 298 K. exists as asbestos-like, silky, felted needles and has a
structure consisting of SO 4 tetrahedra linked together in long chains.
Solid sulphur trioxide reacts explosively with liquid water :
SO 3 + H 2 O -* H 2 SO 4 : A/f = - 88 kJ mol"^1
and it fumes strongly in moist air. The gas sulphur trioxide does not
readily dissolve in water, but it reacts with concentrated sulphuric
acid, thus :
H 2 SO 4 + SO 3 -> H 2 S 2 O 7
H 2 S 2 O 7 + SO 3 -» H 2 S 3 O 10
and so on.
Sulphur trioxide unites exothermically with basic oxides to give
sulphates, for example
CaO + SO 3 -* CaSO 4
Sulphur trioxide is used on an industrial scale for sulphonating
organic compounds.
SULPHURIC ACID, H 2 SO 4
Sulphuric acid is probably the most important chemical substance
not found naturally. Its manufacture is therefore important ; the
total world production is about 25 000 000 tons a year.
Manufacture
The different methods of manufacturing sulphuric acid are essen-
tially the same in principle and consist of three distinct processes :
I. Production of sulphur dioxide.
- Conversion of sulphur dioxide to sulphur trioxide.
- Conversion of sulphur trioxide to sulphuric acid.
- Sulphur dioxide is obtained in the following three ways :
(a) By burning elemental sulphur (imported) :
S + O 2 -» SO 2
(b) As a by-product of the roasting process in the extraction of
certain metals from their sulphide ores, for example
2ZnS + 3O, -> 2ZnO + 2SO 2 T
