Chemistry Reference
In-Depth Information
across a range of uses, is produced in very large
quantities. The major process used for making
ammonia is the
Haber process
. This process was
developed by the German scientist Fritz Haber in
1913 (Figure 11.13). He was awarded a Nobel Prize
in 1918 for his work. The process involves reacting
nitrogen and hydrogen. It was first developed to
satisfy the need for explosives during World War I, as
explosives can be made from ammonia. We now have
many more uses for this important gas including the
manufacture of nitric acid and of fertilisers such as
ammonium nitrate.
This reaction is a
reversible
reaction and special
conditions are employed to ensure that the reaction
proceeds to the right (the forward reaction),
producing hydrogen and carbon monoxide. The
process is carried out at a temperature of 750 °C, at a
pressure of 30 atmospheres with a catalyst of nickel.
These conditions enable the maximum amount of
hydrogen to be produced at an economic cost.
The carbon monoxide produced is then allowed
to reduce some of the unreacted steam to produce
more hydrogen gas.
carbon + steam
hydrogen + carbon
monoxide
dioxide
CO(
g
) + H
2
O(
g
)
H
2
(
g
)
+ CO
2
(
g
)
Making ammonia
In the Haber process, nitrogen and hydrogen in
the correct proportions (1 : 3) are pressurised to
approximately 200 atmospheres and passed over a
catalyst of freshly produced, finely divided iron at
a temperature of between 350 °C and 500 °C. The
reaction in the Haber process is:
nitrogen + hydrogen
ammonia
N
2
(
g
) + 3H
2
(
g
)
2NH
3
(
g
) Δ
H
=
−92 kJ mol
−1
The reaction is exothermic.
The industrial conditions employed ensure that
sufficient ammonia is produced at a fast enough rate.
The principles which underpin equilibrium processes
such as this are discussed below and in Chapter 12
(the Contact process for manufacturing sulfuric acid,
p. 199).
Under these conditions the gas mixture leaving
the reaction vessel contains about 15% ammonia,
which is removed by cooling and condensing it as
a liquid. The unreacted nitrogen and hydrogen are
re-circulated into the reaction vessel to react together
once more to produce further quantities of ammonia.
The 15% of ammonia produced does not seem a
great deal. The reason for this is the reversible nature
of the reaction. Once the ammonia is made from
nitrogen and hydrogen, it decomposes to produce
nitrogen and hydrogen. There comes a point when
the rate at which the nitrogen and hydrogen react
to produce ammonia is equal to the rate at which
the ammonia decomposes. This situation is called a
chemical equilibrium
. Because the processes
Figure 11.13
Fritz Haber (1868-1934).
Obtaining nitrogen
The nitrogen needed in the Haber process is
obtained from the atmosphere by fractional
distillation of liquid air (p. 174).
Obtaining hydrogen
The hydrogen needed in the Haber process is obtained
from the reaction between methane and steam.
methane + steam
hydrogen + carbon monoxide
CH
4
(
g
) + H
2
O(
g
)
3H
2
(
g
)
+
CO(
g
)
This process is known as
steam re-forming
.
