Chemistry, Third edition

PRODUCTION OF AMMONIA BY THE HABER–BOSCH PROCESS 281

Production of ammonia by the

Haber–Bosch process

A piece of history

Up until 1913, the bulk of the world’s artificial nitrogenous fertilizers was made from

sodium nitrate obtained from Chile. Calculations showed that this supply would

soon be exhausted if demand continued to expand, and so Fritz Haber (1868–1934)

attempted to make ammonia directly from nitrogen and hydrogen gases. After 8

years, Haber produced ammonia efficiently on a small scale. Collaboration with the

brilliant chemical engineer Carl Bosch (1874–1940) enabled the production to be

scaled up, and ammonia was first manufactured on a large scale in 1913. The world’s

production of ammonia now exceeds 100 million tonnes. About 80% of all ammonia

produced is used to make fertilizers with another 5% being used to make nylon, and a

further 5% used to make explosives.

15.5

Fritz Haber. Haber’s initial results on the nitrogen–hydrogen

reaction suggested that it was impossible to produce a

significant yield of ammonia. To Haber’s embarrassment, this

was publicly challenged by the brilliant chemist Walther Nernst,

who suggested that the yield might be significant if compressed

gases were used. Haber followed up Nernst’s suggestion and

his efforts were so successful that he was awarded the Nobel

Prize for chemistry in 1918.

Haber–Bosch process

Ammonia gas is produced on a massive scale using the Haber–Bosch process. The

reaction is

N 2 (g)3H 2 (g)\===\2NH 3 (g) H^ —=92 kJ mol^1

Nitrogen gas is obtained from air, while hydrogen is produced by the reaction of

methane with steam (see page 313).

To make the Haber–Bosch process viable we require (1) a high equilibrium con-

centration of ammonia, and (2) that the equilibrium concentration of ammonia be

produced in a short time (the ‘kinetic’ factor). In originally studying this reaction,

Haber was aware of the following facts:

●AlthoughKcfor this equilibrium is a massive 4  108 mol^2 dm^6 at 298 K, the

concentration of ammonia in a mixture of nitrogen and hydrogen gases at room

temperature is negligible. This is because the rates of the forward and back reac-

tions are extremely slow so that dynamic equilibrium is never attained.