Synthetic Inorganic Chemistry

METATHESIS 111

This type of reaction is particularly interesting when the salt

of the weak acid is sparingly soluble. Ferrous sulphide is a salt

of the weak acid H 2 S. It is very insoluble in water, but it passes

completely into solution in dilute HC1. The formation of the

un-ionized H 2 S removes S ions and thus allows the ionization

of the ferrous sulphide to continue unchecked

FeS; Fe++

2C1~

S"

2H+

i

H 2 S

Displacement of a Weak Base from Its Salt. An example of this
kind of process is given by the action of the strong base, sodium
hydroxide, upon a solution of the salt, ammonium chloride. The
salt solution is odorless, but the odor of ammonia is observed as
soon as the strong base is added.
NH 4 + Cl"
OH" Na+
IT
NH4OH
In this case, as in the case of the weak acid, the major part of the
ions of the weak base combine to form un-ionized molecules.
The strong odor is due to the escape of small amounts of am-
monia, NH3, from the solution. Ammonium hydroxide is capable
of undergoing two kinds of dissociation: the electrolytic disso-
ciation, or ionization, which we have already discussed, and a
non-electrolytic dissociation,
NH4OH *± H 2 O + NH 3

The latter sort of dissociation is subject to the same rules of equi-
librium as is ionization, and we can have un-ionized ammonium
hydroxide at the same time in equilibrium with two sets of dis-
sociation products,

NH 4 + + OH^ NH4OH ^ NH 3 + H 2 O

Therefore, whenever NH 4 + and OH" ions are brought together,
they must come to equilibrium with a large proportion of NH4OH
and the latter must come to equilibrium with NH 3 and H 2 O and,
through the NH 4 OH, the NH 3 and H 2 O must be in equilibrium
with the ions. In dilute solutions, where the proportion of H 2 O