224 GROUP Vabove the surface (or it can be immersed if a high current density is
used). The Raschig process—the reaction of ammonia with chlor-
amine (p. 220)—gives lower yields and the hydrazine is not anhydrous.
Pure hydrazine is a colourless liquid, melting point 275 K, and
boiling point 387 K. It is surprisingly stable for an endothermic
compound (A/ff = + 50.6 kJ moP^1 ). Each nitrogen atom has a
lone pair of electrons and either one or both nitrogen atoms are
able to accept protons to give N 2 H 5 h and the less stable N 2 H^.
The base strength of hydrazine is, however, lower than that of
ammonia. As might be expected, hydrazine is readily soluble in
water from which the hydrate N 2 H 4 .H 2 O can be crystallised.
Hydrazine, unlike ammonia, will burn in air with evolution of
much heat:
N 2 H 4 + O 2 -> N 2 -f 2H 2 O
This reaction has been carefully studied with the aim of obtaining
the enthalpy of combustion as electrical energy, and successful
hydrazine-air fuel cells have been developed using potassium
hydroxide as the electrolyte. The hydrazine fuel, however, has the
disadvantage that it is expensive and poisonous.
In aqueous solution hydrazine can behave either as an oxidising
or reducing agent. Powerful reducing agents such as zinc reduce
hydrazine to ammonia, while chlorine oxidises it to give nitrogen:
N 2 H 5 + 4- C1 2 -> N 2 T + 5H+ 4- 4CT
Hydrazine and its alkylated derivatives are used as rocket fuels;
in organic chemistry, substituted phenylhydrazines are important
in the characterisation of sugars and other compounds, for example
aldehydes and ketones containing the carbonyl group ^c=O.Hydrogen azide (hydrazoic acid), HN 3
Hydrazoic acid has no resemblance to either ammonia or hydrazine.
-i-
It has a structure involving resonance between H—N=N=N and
+
H—N—N=N. It is prepared by the oxidation of hydrazine in
strongly acid solution; the oxidising agent used is usually nitrous
acid (i.e. sodium nitrite is added to the acid solution of hydrazine):
N 2 HJ + HNO 2 -> HN 3 -f H+ + 2H 2 O
Pure hydrazoic acid is a colourless liquid, b.p. 310 K. It is very
ready to detonate violently when subjected to even slight shock,
and so is used in aqueous solution. It is a weak acid, reacting with
alkali to give azides, which contain the ion NJ.