Modern inorganic chemistry

240 GROUP V

A similar decomposition occurs if nitric acid is subjected to a

temperature above its boiling point.

The chemical properties of nitric acid require us to consider the

structure first. The vapour of pure nitric acid (i.e. anhydrous) is

probably composed of molecules of 'hydrogen nitrate', which

structurally is a resonance hybrid of such forms as :

In liquid nitric acid, hydrogen bonding gives a loose structure

similar to that of hydrogencarbonate ions. However, although pure

nitric acid does not attack metals readily and does not evolve

carbon dioxide from a carbonate, it is a conducting liquid, and

undergoes auto-ionisation thus :

2HNO 3 ^ H 2 NOJ + NO 3 ~

and H 2 NOJ + HNO 3 ^ NO^ + H 3 O+ + NO 3 '

The second equilibrium is the more important, giving rise to the

nitronium ion, NOJ, already mentioned as a product of the dis

sociation of dinitrogen tetroxide. Several nitronium salts have been

identified, for example nitronium chlorate(VII), (NO 2 )+(C1O 4 )~. If

pure nitric acid is dissolved in concentrated sulphuric acid, the

freezing point of the latter is depressed to an extent suggesting the

formation of four ions, thus :

HNO 3 + 2H 2 SO 4 ^ NO 2 + + H 3 O+ + 2HSO;

It is the nitronium ion which is responsible for nitrating actions
in organic chemistry which are carried out in a mixture of nitric and
sulphuric acids. When nitric acid is dissolved in water, its behaviour
is that of a strong acid, i.e. :

HNO 3 + H 2 O ^ H 3 O+ + NOs

because of the proton affinity of water. The majority of the reactions
of nitric acid are oxidations due to the nitrate ion in the presence of
hydrogen ions — and the corresponding reduction product (from the
nitrate ion) depends upon the hydrogen ion concentration and upon
the nature of the substance oxidised ; it may be nitrogen dioxide,
nitrogen oxide, dinitrogen oxide, nitrogen, hydroxylamine (NH 2 OH)
or ammonia (as ammonium ion in acid solution). The following are
some typical examples :