The Haber Process (Edexcel GCSE Chemistry: Combined Science)

• Ammonia is manufactured using The Haber Process which occurs in five stages
• Stage 1: H₂ and N₂ are obtained from natural gas and the air respectively and are pumped into the compressor through pipe
• Stage 2: the gases are compressed to about 200 atmospheres inside the compressor
• Stage 3: the pressurised gases are pumped into a tank containing layers of catalytic iron beds at a temperature of 450°C. Some of the hydrogen and nitrogen react to form ammonia:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• Stage 4: unreacted H₂ and N₂ and product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels
• Stage 5: the unreacted H₂ and N₂ gases are recycled back into the system and start over again

The production of ammonia by the Haber Process

Conditions

Temperature: 450ºC

• A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
• If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
• However at a lower temperature the rate of reaction is very slow
• So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly

• A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
• A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made
• However high pressures can be dangerous and very expensive equipment is needed
• So 200 atm is a compromise pressure between a lower yield of products being made safely and economically

Catalyst

• A catalyst of iron is used to speed up the reaction

Choosing the conditions for the Haber Process