The Haber Process
Practise The Haber Process for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) chemistry topic: the Haber process, N₂ + 3H₂ ⇌ 2NH₃, run at about 450 °C, 200 atm and an iron catalyst.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The Haber process makes ammonia, which is used to produce nitrogen-based fertilisers. This is a Triple-only topic.
The raw materials are nitrogen from the air and hydrogen from natural gas. They are purified and passed over an iron catalyst at 450 °C and 200 atmospheres pressure. The reaction is reversible: N₂ + 3H₂ ⇌ 2NH₃.
On cooling, the ammonia liquefies and is removed, and the unreacted nitrogen and hydrogen are recycled. The conditions are a compromise. A lower temperature would give a higher yield because the forward reaction is exothermic, but the rate would be too slow, so 450 °C balances yield against rate.
Revision notes
The process
Nitrogen from the air and hydrogen from natural gas are purified and mixed.
They are passed over an iron catalyst at 450 °C and 200 atmospheres. The reaction is reversible: N₂ + 3H₂ ⇌ 2NH₃. On cooling the ammonia liquefies and is removed; unreacted gases are recycled.
Why the temperature is a compromise
The forward reaction is exothermic, so a lower temperature would shift the equilibrium right and give a higher yield.
But a lower temperature means a slower rate. 450 °C is a compromise giving an acceptable yield in an acceptable time.
Why high pressure is used
There are four molecules of gas on the left and two on the right.
High pressure shifts the equilibrium towards the side with fewer molecules, increasing the yield of ammonia. Very high pressures are expensive and dangerous, so 200 atmospheres is used.
Key points
- Nitrogen comes from the air.
- Hydrogen comes from natural gas.
- An iron catalyst is used.
- The conditions are 450 °C and 200 atmospheres.
- The temperature is a compromise between yield and rate.
- High pressure increases the yield of ammonia.
Worked examples
Example 1
State the source of each raw material used in the Haber process. [2 marks]
Model answer
Nitrogen is obtained from the air (1 mark). Hydrogen is obtained from natural gas (1 mark).
Example 2
Explain why a temperature of 450 °C is used rather than a lower temperature. [3 marks]
Model answer
The forward reaction is exothermic, so a lower temperature would shift the equilibrium to the right and give a higher yield (1 mark). However, a lower temperature would also make the rate of reaction too slow (1 mark). 450 °C is a compromise giving an acceptable yield in an acceptable time (1 mark).
Example 3
Explain why a high pressure is used in the Haber process. [3 marks]
Model answer
There are four molecules of gas on the left of the equation and two on the right (1 mark). High pressure shifts the equilibrium towards the side with fewer gas molecules (1 mark), increasing the yield of ammonia (1 mark).
Common mistakes
Saying the catalyst increases the yield.
A catalyst increases the rate only; it does not change the equilibrium position.
Not explaining the compromise.
Both the yield argument and the rate argument are needed.
Forgetting to count gas molecules for the pressure explanation.
Four on the left, two on the right — that is why pressure helps.
Saying unreacted gases are wasted.
They are recycled back into the reactor.
Exam tips
- Give both sides of the temperature compromise.
- Count the gas molecules when explaining pressure.
- State that the catalyst affects rate only.
- Remember this is a Triple-only topic.
Key terms
- Haber process
- The industrial process making ammonia.
- Compromise
- Conditions balancing yield against rate and cost.
- Reversible reaction
- A reaction proceeding in both directions.
- Recycled
- Returned to the reactor for another pass.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.