Modern inorganic chemistry

214 GROUPV

COMPOUNDS OF GROUP V ELEMENTS

1. HYDRIDES

All Group V elements form covalent hydrides MH 3. Some physical

data for these hydrides are given below in Table 9.2. The abnormal

values of the melting and boiling points of ammonia are explained

by hydrogen bonding (p. 52). The thermal stabilities of the hydrides

decrease rapidly from ammonia to bismuthine as indicated by the

mean thermochemical bond energies of the M—H bond, and both

stibine, SbH 3 , and bismuthine, BiH 3 , are very unstable. All the

Table 9.2

PROPERTIES OF GROUP V HYDRIDES

Hvdnde„. , ,,, m.p.iK) h.p.(K). .,, , Mean , ,,, thermochemical ,-

' l ' bond enerqv (kJ mol lu)

NH 3

PH 3

AsH 3

SbH 3

BiH 3

195

140

157

185

240

183

218

256

295

391

322

247

. —

Group V hydrides are reducing agents, the reducing power increasing
from NH 3 to BiH 3 , as thermal stability decreases.
These stability changes are in accordance with the change from a
non-metal to a weak metal for the Group V elements nitrogen to
bismuth.
Nitrogen, phosphorus and arsenic form more than one hydride.
Nitrogen forms several but of these only ammonia, NH 3 , hydrazine,
N 2 H 4 and hydrogen azide N 3 H (and the ammonia derivative
hydroxylamine) will be considered. Phosphorus and arsenic form
the hydrides diphosphane P 2 H 4 and diarsane As 2 H 4 respectively,
but both of these hydrides are very unstable.

Hydrides of nitrogen

AMMONIA NH 3

Ammonia is manufactured by the direct combination of the elements

N 2 4- 3H 2 ^ 2NH 3 AH - -92.0kJmor~l

The production by this method was developed originally by Haber

after whom the process is now named. Since the reaction is reversible