Modern inorganic chemistry

232 GROUP V

00 Os .0

\ X \ ^

N---N and N— N

All the N — O bonds are of equal length.

The two oxides, NO 2 and N 2 O 4 , exist in equilibrium, the position

of which depends very greatly on temperature :

N 2 O 4 (g) ^ 2NO(g) : AH = + 57.2 kJ mol"^1

Below 262 K the solid dimer N 2 O 4 exists as a colourless solid. At

262 K colourless liquid N 2 O 4 is produced but as the temperature is

increased dissociation begins and the liquid becomes a dilute

solution of brown NO 2 in liquid N 2 O 4 and is pale brown in colour.

The liquid boils at 294 K. As the temperature is further increased

the gas gradually darkens in colour as more N 2 O 4 dissociates, this

being complete at 423 K when the gas is almost black in colour.

Above 423 K further dissociation occurs into nitrogen monoxide

and oxygen, both of which are colourless and hence the colour of

the gas slowly diminishes.

N 2 O 4 ^
(m.p. 262 K)
colourless
solid

N 2 O 4 ^

(b.p. 294 K)

pale yellow

liquid

2NO 2 ^

(100% at 423 K)

dark brown gas

2NO + O 2

(100% at 870 K)

colourless gases

Preparation

Nitrogen dioxide is prepared by heating the nitrate of a heavy metal,

usually that of lead(II):

2Pb(NO 3 ) 2 -> 2PbO + 4NO 2 T + O 2

If the mixture of oxygen and nitrogen dioxide is passed through a

U tube in a freezing mixture the dioxide condenses mainly as N 2 O 4

and the oxygen passes on.

Chemical properties

Nitrogen dioxide can be both oxidised and reduced. It is reduced by
phosphorus, charcoal and sulphur which burn in it to form their
oxides and nitrogen. Heated metals such as iron and copper also
reduce it to nitrogen but other reducing agents such as hydrogen
sulphide and aqueous iodide give nitrogen monoxide: