Modern inorganic chemistry

218 GROUPV

Ammonia as a base. The ammonia molecule has a powerful affinity

for protons and hence.

1. ammonia gas will react with gaseous hydrogen containing
   compounds which are acidic, for example hydrogen chloride:

NH 3 + HC1 c°o! NH 4 Cl(i.e. NH + CP)

heai

(N.B. A trace of water is required to make the forward reaction

proceed at a realistic rate.)

2. ammonia will react with aqueous acids, for example

2NH 3 + H 2 SO 4 (aq) -> (NH 4 ) 2 SO 4

which is more correctly written

2NH 3 + 2H 3 O+ + SOJ- -> 2NH^ + 2H 2 O + SOJ-

Aqueous ammonia can also behave as a weak base giving hydroxide

ions in solution. However, addition of aqueous ammonia to a

solution of a cation which normally forms an insoluble hydroxide

may not always precipitate the latter, because (a) the ammonia may

form a complex ammine with the cation and (b) because the con-

centration of hydroxide ions available in aqueous ammonia may be

insufficient to exceed the solubility product of the cation hydroxide.

Effects (a) and (b) may operate simultaneously. The hydroxyl ion

concentration of aqueous ammonia can be further reduced by the

addition of ammonium chloride; hence this mixture can be used to

precipitate the hydroxides of, for example, aluminium and chrom-

ium(III) but not nickel(II) or cobalt(II).

Because of ammine formation, when ammonia solution is added

slowly to a metal ion in solution, the hydroxide may first be precipi-

tated and then redissolve when excess ammonia solution is added;

this is due to the formation of a complex ammine ion, for example

with copper(II) and nickel(II) salts in aqueous solution.

Ammonia as a reducing agent. Ammonia gas will not burn in air
but it does burn in oxygen with a yellowish flame after ignition.
A convenient apparatus is shown in Figure 9.3, By reversing the
gas supplies it can easily be shown that oxygen will also burn in
ammonia.
In the presence of catalyst, usually platinum, ammonia is oxidised
by oxygen (and air) to nitrogen oxide. NO. This reaction, used to
obtain nitric acid from ammonia (p. 238), can be demonstrated in
the laboratory using the apparatus shown in Figure 9.4; the oxygen
rate should be slow.