Haber process
The general principles that affect all industrial proceses involving equilibria are illustrated by the Haber process. In this process, ammonia is manufacured by the direct combination of nitrogen and hydrogen. An equilibrium mixture is formed:
N2(g) + 3H2(g) 2NH3(g) ΔH = –92 kJ mol–1
What conditions will favour the production of ammonia?
The production of ammonia is favoured by high pressure and low temperature.
What might be the drawback in using the conditions you've suggested?
A low temperature would mean that the rate of conversion of nitrogen and hydrogen to ammonia would be slow.
How might this problem be minimised?
A catalyst could be used that would speed up the reaction.
The percentage by volume of ammonia in the equilibrium mixture for the reaction between nitrogen and hydrogen at a range of temperatures and pressures is given in the table below (The Essential Chemical Industry, 5th edn, 2010).
| Pressure/atm | 373 K | 473 K | 573 K | 673 K | 773 K |
| 10 | – | 50.7 | 14.7 | 3.9 | 1.2 |
| 25 | 91.7 | 63.6 | 27.4 | 8.7 | 2.9 |
| 50 | 94.5 | 74.0 | 39.5 | 15.3 | 5.6 |
| 100 | 96.7 | 81.7 | 52.5 | 25.2 | 10.6 |
| 200 | 98.4 | 89.0 | 66.7 | 38.8 | 18.3 |
| 400 | 99.4 | 94.6 | 79.7 | 55.4 | 31.9 |
| 1000 | – | 98.3 | 92.6 | 79.8 | 57.5 |
A wide range of conditions are used depending on the construction of the reactor. Temperatures vary between 600 K and 700 K and pressure between 50 and 100 atmospheres. A catalyst of iron mixed with promoters such as potassium oxide or aluminium oxide is a vital part of the process.
What would be the approximate percentage of ammonia produced in a plant operating at 700 K and 75 atmospheres?
About 20%.
Here is a generalised flow diagram for the Haber process.
[INSERT EII_CI_05_ima]
Why are the gases leaving the catalytic converter hot?
The formation of ammonia from nitrogen and hydrogen is an exothermic process.
What is the purpose of the heat exchanger?
The heat exchanger transfers energy from the hot gases leaving the catalytic converter to the cold gases about to enter the converter. It cools the gases leaving and heats up the gases about to enter the converter.
The ammonia gas is removed by cooling the gases leaving the converter to –50 ºC. What property of ammonia makes this an efficient method of separating it from unreacted gases?
Ammonia has a boiling point above –50 ºC. It boils at –33 ºC. The ammonia condenses and can be tapped off as liquid, separating it from the unreacted nitrogen and hydrogen gas.
The process for making ammonia from its elements was discovered by Fritz Haber. Carl Bosch was responsible for developing an industrial plant to manufacture ammonia. Both chemists were awarded a Nobel prize for chemistry for their work on ammonia synthesis.
[INSERT EII_CI_06_ima]
Why was the work of Haber and Bosch to synthesise ammonia felt to be important?
Ammonia reacts with acids to form ammonium salts. It can also be oxidised to form nitric acid, which can be used to form nitrates. Both ammonium salts and nitrates can be used as fertilisers, so the production of ammonia was the starting point for the manufacture of fertilisers.