Modern inorganic chemistry

242 GROUP V

NO" + 4Zn + 15OH- + 6H 2 O -» NHJ + 4Zn(OH)^

(3) Cations:—Some of these are oxidised to a higher state by

nitric acid. For example, iron(II) (in presence of sulphuric acid) is

quantitatively oxidised to iron(III):

3Fe2+ -h NOJ + 4H+ -> 3Fe3+ + NOT + 2H 2 O

Tin(II) chloride, in presence of hydrochloric acid, is oxidised to

tin(IV) chloride, the nitrate ion in this case being reduced to

hydroxylamine and ammonia.

The noble metals such as gold and platinum, although almost

insoluble in nitric acid, are very ready to form chloro-complexes, for

example gold gives the [AuCl 4 ]~ ion very readily. Hence they can be

dissolved by aqua regia, a mixture of 3 volumes of concentrated

hydrochloric acid and 1 volume of concentrated nitric acid. The

latter oxidises the gold to the auric fgold(III)) state (Au3+), which

then appears as the ion (AuQ 4 )~ (p. 431).

NITRATES

Hydrated nitrates, and anhydrous nitrates of very electropositive
metals (for example Na, K), contain the ion NO^ which has the
structure:

o cr -o o "O o-

\+/ \ + / \ + /

N N N

o

resonance hybrids

with the three N—O distances identical. In other anhydrous metal
nitrates, prepared as on p. 233, the nitrate groups may be bonded
covalently to the metal, thus: M—ONO 2 (for example Cu(NO 3 ) 2 ,
p. 413).
Nitrates are prepared by the action of nitric acid on a metal or its
oxide, hydroxide or carbonate. All nitrates are soluble in water. On
heating, the nitrates of the alkali metals yield only oxygen and the
nitrite:

2KNO 3 -> 2KNO 2 + O 2 T

Ammonium nitrate gives dinitrogen oxide and steam:

NH 4 NO 3 -> 2H 2 O + N 2 Ot