Modern inorganic chemistry

GROUP V 221
Liquid ammonia. This can be prepared by compressing ammonia
gas. It has a boiling point of 240 K and is an excellent solvent for
many inorganic and organic substances as well as for the alkali
metals. Liquid ammonia is slightly ionised:

2NH 3 ^-NH + + NH 2 -
(cf.2H 2 O ^H 3 O+ + OH")
Liquid ammonia, like water, is only a poor conductor of electricity.
Ammonium salts dissolved in water behave as acids giving the ion
NH^, whilst amides which give the ion NH^ behave as bases. Thus
the reaction:

NH 4 C1 4- KNH 2 -> KC1| + 2NH 3

acid base salt solvent

is a neutralisation in liquid ammonia (p. 90).
Solutions of alkali metals in liquid ammonia are used in organic
chemistry as reducing agents. The deep blue solutions effectively
contain solvated electrons (p. 126), for example

Na -» Na+ + e~

e~ + xNH 3 -»e-(NH 3 )x

Ammonium salts. Ammonium salts can be prepared by the direct

neutralisation of acid by ammonia. The salts are similar to alkali

metal salts and are composed of discrete ions. Most ammonium

salts are soluble in water. Since ammonia is volatile and readily

oxidisable the behaviour of ammonium salts to heat is particularly

interesting.

If the acid of the salt is also volatile, as in the chloride and the

carbonate, dissociation occurs causing the salt to sublime:

NH 4 C1 ^ NH 3 + HC1

The extent of dissociation at a given temperature can be determined
by measuring the density of the vapour. Since anhydrous sulphuric
acid is less volatile than hydrogen chloride, ammonium sulphate
does not readily sublime on heating; some ammonia is evolved to
leave the hydrogensulphate:

(NH 4 ) 2 SO 4 -> NH 4 HSO 4 4- NH 3 T
If the acid of the ammonium salt is an oxidising agent, then on
heating the salt, mutual oxidation and reduction occurs. The oxida-
tion products can be nitrogen or one of its oxides and the reactions
can be explosive, for example: