The Haber Process
"The Haber Process is a method for producing ammonia developed by Germany during World War I.
The Germans used the ammonia as a source of nitrogen for making explosives. The process is
still used by industrial chemists. Before the war, Germany had imported sodium nitrate and
potassium nitrate as sources of nitrogen. But when war broke out, the Allies set up a naval
blockade around Germany, cutting off supplies of imported goods. The Germans were forced to
find another source of nitrogen for explosives.
An obvious source of nitrogen is the air. Molecular nitrogen (N2) makes up about
80% of the air. But molecular nitrogen is highly unreactive. That is why chemists had been
using compounds containing nitrogen to make more nitrogen compounds by one or more reactions.
The Germans had to come up with a nitrogen compound that could be produced in large
amounts.
"Chemist Fritz Haber solved the problem. He developed a method, called the Haber Process, for
combining molecular nitrogen from the air with molecular hydrogen to form ammonia gas,
NH3.
"The Haber Process is a good example of the use of equilibrium principles. The equation for the
reversible reaction is
N2(g) + 3H2(g) <--> 2NH3(g) + 92 kJ
Because raising the temperature will increase the speed of both the forward and reverse reactions,
a high temperature should bring the reaction to equilibrium rapidly. However, raising the
temperature favors the endothermic reaction, shifting the equilibrium in this case to the left,
lowering the yield of ammonia, and increasing the time needed to obtain a given quantity of ammonia.
In fact, at 500oC, only 0.1% of the mass at equilibrium will be ammonia if the reaction
is done at 1 atmosphere (101.3 kPa) of pressure. (The other 99.9% is, of course, a mixture of
nitrogen and hydrogen.) But by increasing the pressure, the equilibrium favors the reaction in
which fewer gas molecules are produced. Such a stress shifts the equilibrium to the right and
produced a mixture richer in ammonia.
"The process has proven successful in its commercial applications. The aim of industry is to
control reactions so that large amounts of useful product are yielded quickly. In the industrial
preparation of ammonia, the gases are compressed to high pressures. As a result, the yield of
ammonia is increased considerably even though a high temperature is maintained to make the
reaction come to equilibrium quickly. At 500oC, the yield of ammonia increases from
0.1% to 47% if the pressure is increased from 1 atm to 700 atm.
Today pressures of up to 1000 atm are used, and the temperature is kept at about 500oC.
The catalyst used is a mixture of iron, potassium oxide, and aluminum oxide. Under these
conditions, the yield of ammonia is 40% to 60%. By removing the ammonia as it is formed and
feeding in fresh supplies of nitrogen and oxygen, chemists achieve a satisfactory rate of ammonia
production."
Passage taken from:
Dorin, Henry, Peter E. Demmin, and Dorothy L. Gabel. Prentice Hall Chemistry: The Study of Matter. 3rd ed. Needham: Prentice Hall, 1990.
Back to German Technologies during World War II Page
Back to Homepage