The Haber process makes ammonia using the reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. In industry, a temperature of around 450 °C and a pressure of around 200 atmospheres are used, along with an iron catalyst. Explain how each of these three conditions affects the rate of reaction and/or the position of equilibrium, and why a compromise temperature is chosen rather than a very low temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Haber process is used industrially to manufacture ammonia on a large scale. The reaction is carried out in a closed system so that dynamic equilibrium can be established.
Model answer (5 marks)
The iron catalyst provides an alternative pathway with a lower activation energy, so the rate of reaction increases.
High pressure brings the gaseous molecules closer together, increasing the frequency of successful collisions and therefore the rate of reaction.
High pressure also shifts the equilibrium towards the side with fewer moles of gas – from 4 moles (N₂ + 3H₂) to 2 moles (2NH₃), giving a higher yield of ammonia.
A very low temperature would favour the exothermic forward reaction, shifting equilibrium to the right and increasing the theoretical yield.
However, at a very low temperature the rate of reaction would be too slow, so a compromise temperature of about 450 °C is chosen to give a reasonable rate while still maintaining a good equilibrium yield.
High pressure brings the gaseous molecules closer together, increasing the frequency of successful collisions and therefore the rate of reaction.
High pressure also shifts the equilibrium towards the side with fewer moles of gas – from 4 moles (N₂ + 3H₂) to 2 moles (2NH₃), giving a higher yield of ammonia.
A very low temperature would favour the exothermic forward reaction, shifting equilibrium to the right and increasing the theoretical yield.
However, at a very low temperature the rate of reaction would be too slow, so a compromise temperature of about 450 °C is chosen to give a reasonable rate while still maintaining a good equilibrium yield.
Examiner tips
- Mention the catalyst’s effect on activation energy first; then pressure’s dual role (rate and equilibrium); finally explain the temperature compromise – equilibrium vs. rate.
Common mistakes
- Confusing the catalyst’s effect with pressure; writing that pressure only affects the rate; ignoring the equilibrium shift; claiming a low temperature gives a higher rate instead of a higher yield.
Mark scheme (5 marks)
- The iron catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy
- High pressure increases the rate of reaction because gaseous particles are closer together, leading to more frequent successful collisions
- High pressure shifts the equilibrium position towards the side with the smaller number of moles of gas, i.e. towards the products / towards ammonia (4 moles → 2 moles)
- A low temperature would shift the equilibrium position in the direction of the exothermic reaction (towards ammonia / to the right), giving a greater yield of ammonia
- A very low temperature would make the rate of reaction too slow, so a compromise temperature of around 450 °C is used to give an acceptable rate of reaction whilst still producing a reasonable yield
Key terms in this question
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