How is ammonia made in the Haber process, and why are the conditions chosen?
The Haber process for making ammonia, the conditions used, and why these conditions are a compromise.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on industrial chemistry, covering the Haber process for making ammonia, the conditions of temperature, pressure and catalyst, and why these are chosen as a compromise.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe the Haber process for making ammonia, the conditions used, and why these conditions are a compromise.
The Haber process
The word equation is: nitrogen + hydrogen to ammonia (reversible). The nitrogen comes from the air and the hydrogen comes from natural gas.
The conditions used
Why the conditions are a compromise
Because the reaction is reversible, the conditions are chosen to balance yield, rate and cost:
- Pressure (200 atm): a high pressure increases the yield of ammonia and speeds the reaction, but very high pressures need expensive, strong equipment and are dangerous, so 200 atm is a compromise.
- Temperature (450 C): a lower temperature would give a higher yield of ammonia (the forward reaction is exothermic) but too slowly; a higher temperature is faster but gives less ammonia. So 450 C is a compromise between yield and rate.
- Catalyst (iron): speeds up the reaction, so ammonia is made faster (lowering costs), without changing the yield.
Why recycling matters
Only some of the nitrogen and hydrogen react each time they pass through, so the unreacted gases are recycled back into the reactor rather than being thrown away. This makes the process more economical, because the raw materials are used efficiently and less is wasted. Recognising that recycling the unreacted gases improves the overall yield and cuts costs is a common point in questions about the Haber process and other industrial reactions.
Why ammonia matters
Ammonia is important because it is used to make fertilisers, which farmers spread on fields to provide nitrogen for plant growth, increasing crop yields to feed a growing population. Ammonia (and the nitrates made from it) supplies the nitrogen that plants need to make proteins, linking the Haber process to the nitrogen cycle and food production. Without the Haber process, there would not be enough fertiliser to grow the food the world needs, which is why this industrial reaction is so significant.
Balancing yield, rate and cost
The Haber process is a good example of how industry must balance three things: the yield (how much product), the rate (how fast), and the cost (energy, equipment and raw materials). The chosen conditions do not give the maximum possible yield, because that would be too slow or too expensive; instead they give a good amount of ammonia at a reasonable rate and cost. Being able to explain that industrial conditions are a compromise between yield, rate and cost, rather than just maximising yield, is the key idea examiners look for.
Try this
Q1. What are the two raw materials for the Haber process? [1 mark]
- Cue. Nitrogen and hydrogen.
Q2. What catalyst is used in the Haber process? [1 mark]
- Cue. Iron.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksName the raw materials for the Haber process and write the word equation for making ammonia.Show worked answer →
A Unit 5 recall question worth 4 marks. Reward: the raw materials are nitrogen (from the air) and hydrogen (from natural gas) (2); the word equation is nitrogen + hydrogen to ammonia (a reversible reaction) (2). Markers credit both raw materials and the correct (reversible) equation. A common error is to give the wrong source for nitrogen or hydrogen, or to forget it is reversible.
WJEC style4 marksThe Haber process uses about 450 degrees C and 200 atmospheres pressure with an iron catalyst. Explain why these conditions are chosen.Show worked answer →
A Unit 5 explain question worth 4 marks. Reward: a high pressure (200 atm) increases the yield of ammonia and speeds the reaction, but very high pressures are expensive and dangerous, so 200 atm is a compromise (1); a moderate temperature (450 C) is a compromise - a lower temperature would give more ammonia but too slowly, a higher temperature is faster but gives less ammonia (2); the iron catalyst speeds up the reaction (lowering costs) without changing the yield (1). Markers credit the pressure choice, the temperature compromise, and the catalyst. A common error is to say a high temperature increases the yield.
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