Modern inorganic chemistry

GROUPV 217

facts at one time made use of in refrigeration employing ammonia.
The great solubility of ammonia in water (1 volume of water
dissolves 1300 volumes of ammonia at 273K) can be attributed to
hydrogen bonding between ammonia and water molecules. (N.B.
Concentrated ammonia solution has a density of 0.880 gem"^3 and
contains 35 % of ammonia.) The reaction :

NH 3 4- H 2 O^NH 3 .H 2 O

is exothermic and can easily be reversed by heat, all the ammonia

being evolved on boiling.

A second competing reaction also occurs:

NH 3 .H 2 O -^NH^ + OH~

For this second reaction K 298 = 1.81 x 10~^5 and hence pK 6 for

ammonia solution is 4.75. The entity NH 3. H 2 O is often referred

to as ammonium hydroxide, NH 4 OH, a formula which would imply

that either nitrogen has a covalency of five, an impossible arrange-

ment, or that NH 4 OH existed as the ions NH^ and OH~. It is

possible to crystallise two hydrates from concentrated ammonia

solution but neither of these hydrates is ionic. Hence use of the

term "ammonium hydroxide' is to be discouraged in favour of

'ammonia solution'.

CHEMICAL PROPERTIES OF AMMONIA

These may, for convenience, be divided into a number of topics but
all are closely related depending very largely on the presence of the
lone pair of electrons on the nitrogen atom.

Ammonia as a donor molecule. Because of the presence of the lone
pair of electrons on the nitrogen atom, ammonia can behave as an
electron pair donor. For example, ammonia abstracts a proton from
a water molecule producing the tetrahedral ammonium, NH^, ion
and forms the compounds H 3 N—>A1C1 3 and H 3 N—>BC1 3.
The commonly observed behaviour of ammonia as a ligand is
due to the lone pair of electrons on the nitrogen atom, and ammonia
forms numerous complex ammines with both transition elements
and typical metals; the bonding varies from weak ion-dipole
attraction to strong covalent bonding. (For examples of ammonia
as a ligand, see pp. 46, 363.) The formation of the ammine
CaCl 2. 8NH 3 explains why calcium chloride cannot be used to dry
ammonia gas.