Modern inorganic chemistry

238 GROUPV

in alkalis, and dissolves only in acids (when bismuth salts are

formed), a clear indication of the more metallic nature of bismuth.

Bismuth(V) oxide is not easy to prepare; the (III) oxide (or better

a suspension of the hydroxide) must be oxidised with a strong

oxidising agent such as the peroxodisulphate ion. When this is

carried out, the bismuthate ion, [Biv(OH) 6 ]", is formed. On evapora-

tion, the sodium salt, for example, has the formula NaBiO 3. Addition

of acid to a solution of a bismuthate precipitates the (V) oxide,

Bi 2 O 5 , but this loses oxygen rapidly and forms the trioxide. The

bismuthate ion is an extremely strong oxidising agent, for example

the manganese(II) ion Mn~^ is oxidised to manganate(VII)

OXOACIDS AND THEIR SALTS

Nitrogen

NITRIC(V') ACID, HNO 3

Nitric acid is prepared in the laboratory by distilling equal weights

of potassium nitrate and concentrated sulphuric acid using an air

condenser, the stem of which dips into a flask cooled by tap water.

The reaction is:

H 2 SO 4 + KNO 3 -» KHSO 4 + HNO 3

The temperature is kept as low as possible to avoid decomposition

of the nitric acid to (brown) nitrogen dioxide. The nitric acid con-

denses out as a fuming liquid; it may be purified by redistillation

with concentrated sulphuric acid. If the nitric acid is condensed at

room temperature, it gives off dinitrogen pentoxide, N 2 O 5 (which

fumes with the atmospheric moisture), and so becomes diluted some-

what. Only if it is frozen out at 231 K (the melting point) does it form

pure nitric acid, HNO 3. "Concentrated^9 nitric acid contains about

67 % of the pure acid—this is the constant boiling mixture formed

by distilling a solution of any concentration. Hence concentrated

nitric acid is not pure nitric acid.

On the large scale, nitric acid is now made in large quantities by

the catalytic oxidation of ammonia, employing the reaction:

4NH 3 + 5O 2 -> 4NO 4- 6H 2 O: AH - - 120 kJ mol"l

The process is as follows: ammonia gas (made by the Haber process)
is liquefied under pressure, to freeze out any water, and the
anhydrous gas is then passed together with dust-free air through a