302 GROUP VI
the value of Ka for the first dissociation indicating that this reaction
goes virtually to completion in dilute solution. The acid exhibits
all the properties of the hydrogen ion, i.e. neutralising bases, giving
hydrogen with many metals and so on. Dilute sulphuric acid attacks
iron, but lead very soon becomes resistant due to the formation of
a superficial layer of insoluble lead sulphate.
FUMING SULPHURIC ACID (OLEUM)
When sulphur trioxide is dissolved in concentrated sulphuric acid
the pure 100% acid is first formed; then a further molecule of the
trioxide adds on:
H 2 SO 4 + SO 3 -> H 2 S 2 O 7
heptaoxodisulphuric(VI) acid
pyrosulphuric acid
or oleum
or fuming sulphuric acid
The formation of other polysulphuric acids H 2 S 3 O 10 up to
H 2 O(SO 3 )n, by the addition of more sulphur trioxide, have been
reported.
Pure sulphuric acid is a true acid. In dilute aqueous solution,
sulphuric acid is an acid because the solvent water has an affinity
for the proton:
H 2 SO 4 + H 2 O ^ H 3 O+ + HSO,:
In the pure acid the 'dihydrogen sulphate' has a proton affinity, so
that
H 2 SO 4 + H 2 SO 4 ^ H 3 SO^ + HSO;
If some polysulphuric acid is present, this can lose a proton more
easily, for example
H 2 SO 4 + H 2 S 2 O 7 ^ H 3 SO^ + HS 2 O 7 ".
Hence the strength of the acid goes up as sulphur trioxide is dis-
solved in it*. The acidity of pure and fuming sulphuric acids is
not so apparent as in ordinary aqueous acids because it is masked
by the oxidising and other properties; moreover, the conductivity
- Actually, the pure acid H 2 SO 4 always contains some H 2 S 2 O 7 , because there is
an equilibrium:
2H 2 SO 4 ^ H 2 S 2 O 7 + H 2 O
Thus water is available to take the proton, and H 2 S 2 O 7 to lose it, even in the 'pure'
acid H 2 SO 4.
