A company manufactures NPK fertilisers using ammonia from the Haber process. Explain why the conditions used in the Haber process are described as a compromise.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Haber process reacts nitrogen and hydrogen together to produce ammonia. The reaction is reversible. Industrial plants operate at approximately 450 °C and 200 atmospheres pressure, using an iron catalyst.
Model answer (4 marks)
Higher pressure shifts the equilibrium toward ammonia, increasing the yield.
Very high pressure is expensive and dangerous to maintain, so a lower pressure is used.
Lower temperature favours ammonia formation, so the temperature is kept relatively low.
Very low temperature would make the reaction rate too slow, so a higher temperature is chosen to give an acceptable rate.
Very high pressure is expensive and dangerous to maintain, so a lower pressure is used.
Lower temperature favours ammonia formation, so the temperature is kept relatively low.
Very low temperature would make the reaction rate too slow, so a higher temperature is chosen to give an acceptable rate.
Examiner tips
- Use the exact terms ‘higher pressure’, ‘lower temperature’, ‘equilibrium’, ‘rate’
- Show the cause–effect link between pressure/temperature and yield
- Mention cost/danger and rate as trade‑offs
Common mistakes
- Confusing ‘high temperature’ with ‘low temperature’ for equilibrium
- Omitting the rate argument
- Using vague words like ‘good’ instead of ‘acceptable’
Mark scheme (4 marks)
- Higher pressure increases the yield of ammonia / shifts equilibrium towards ammonia
- Very high pressure is expensive / dangerous to maintain, so a lower pressure is used
- Lower temperature increases the yield of ammonia / equilibrium favours ammonia at lower temperature
- Very low temperature makes the rate of reaction too slow, so a higher temperature is used to give an acceptable rate
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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