Modern inorganic chemistry

216 GROUPV

(since the pressure is high) and the unconverted hydrogen and
nitrogen are returned to the inlet and passed again over the catalyst.
In the laboratory ammonia is obtained when any ammonium
salt is heated with an alkali, either solid or in solution:

NH: + OH' -> NH 3 t + H 2 O

It is best prepared by heating an intimate mixture of solid ammonium

chloride and quicklime:

2NH 4 C1 -h CaO -> CaCl 2 + 2NH 3 + H 2 O

After drying over quicklime, calcium oxide CaO, the ammonia is
collected by upward delivery. (N.B. Both of the common drying
agents, calcium chloride and concentrated sulphuric acid, combine
with the gas.)
Ammonia is also produced when an ionic nitride is hydrolysed,
for example magnesium nitride, produced when magnesium burns in
nitrogen:

Mg 3 N 2 4- 6H 2 O -> 3Mg(OH) 2 + 2NH 3 T

PROPERTIES

Ammonia is a colourless gas at room temperature and atmospheric
pressure with a characteristic pungent smell. It is easily liquefied
either by cooling (b.p. 240 K) or under a pressure of 8-9 atmospheres
at ordinary temperature. Some of its physical and many of its
chemical properties are best understood in terms of its structure.
Like the other group head elements, nitrogen has no d orbitals
available for bond formation and it is limited to a maximum of
four single bonds. Ammonia has a basic tetrahedral arrangement
with a lone pair occupying one position:

//"N \

\ 1

X

Because of the lone pair of electrons, ammonia has a dipole moment
(high electron density at the lone pair) and this concentration of
negative charge can attract (positive) hydrogen atoms in adjacent
molecules giving fairly strong intermolecular forces, i.e. hydrogen
bonding. Consequently ammonia has a high latent heat of vaporisa-
tion and a relatively high boiling point (see Table 9.2 and p. 52),