How are ammonia and sulfuric acid made industrially, and why are the conditions chosen as a compromise?
Describe the Haber process and the Contact process, including their conditions, and explain why the conditions are a compromise between yield, rate and cost.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering the Haber process for ammonia and the Contact process for sulfuric acid, their reactions and conditions, the use of the products including fertilisers, and why industrial conditions are a compromise.
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What this topic is asking
WJEC topic 2.6 wants you to describe two important industrial processes: the Haber process (making ammonia) and the Contact process (making sulfuric acid). You must know the reactions and conditions, the uses of the products (especially fertilisers), and explain why the conditions are a compromise between yield, rate and cost.
The Haber process
The Haber process makes ammonia, , the basis of most fertilisers.
The ammonia is cooled and condensed out as a liquid, and the unreacted nitrogen and hydrogen are recycled back into the reactor.
Why these Haber conditions?
The conditions are chosen as a compromise:
- Temperature. The forward reaction is exothermic, so a lower temperature would give a higher yield. But a low temperature makes the rate too slow. About 450 degrees is a compromise: an acceptable yield at a reasonable rate.
- Pressure. The right-hand side has fewer gas molecules (2 versus 4), so a high pressure increases the yield. But very high pressures are expensive and dangerous, so about 200 atmospheres is a compromise.
- Catalyst. The iron catalyst speeds up the reaction (without changing the yield), helping equilibrium be reached faster.
Uses of ammonia and fertilisers
Most ammonia is used to make fertilisers, which replace nutrients that crops take from the soil and so increase crop yields. NPK fertilisers supply the three key plant nutrients: nitrogen (N), phosphorus (P) and potassium (K). Ammonia can be reacted with acids to make ammonium salts such as ammonium nitrate, a common nitrogen fertiliser.
The Contact process
The Contact process makes sulfuric acid, , one of the most widely used industrial chemicals.
As with the Haber process, the temperature is a compromise between yield (favoured by lower temperature for this exothermic step) and rate (favoured by higher temperature).
Judging industrial processes
When evaluating an industrial process you weigh up the yield (how much product), the rate (how fast), the cost (energy, equipment, raw materials) and the environmental impact. The best conditions are rarely those that give the highest yield, because rate and cost matter too.
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 sample5 marksThe Haber process makes ammonia from nitrogen and hydrogen. State the raw materials, the conditions used, and explain why a temperature of about 450 degrees Celsius is chosen rather than a much lower temperature.Show worked answer β
A Unit 2.6 extended question. Reward: the raw materials are nitrogen (from the air) and hydrogen (from natural gas). The conditions are about 450 degrees Celsius, about 200 atmospheres pressure, and an iron catalyst. The forward reaction is exothermic, so a lower temperature would give a higher yield of ammonia, but at a low temperature the rate would be too slow. The chosen temperature of about 450 degrees is a compromise that gives a reasonable yield at an acceptable rate. Markers credit the raw materials, the three conditions, and the compromise argument linking low temperature to high yield but slow rate. A common slip is to claim a high temperature gives more product.
WJEC sample3 marksAmmonia from the Haber process is used to make fertilisers. Explain why fertilisers are important and name the elements plants need from NPK fertilisers.Show worked answer β
A Unit 2.6 application question. Reward: fertilisers replace the nutrients that crops remove from the soil, so they help plants grow and increase crop yields to feed a growing population. NPK fertilisers supply the three key elements nitrogen (N), phosphorus (P) and potassium (K). Markers credit replacing soil nutrients and increasing crop yield, and naming nitrogen, phosphorus and potassium. A common error is to name the wrong elements or to forget why fertilisers are needed.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) β WJEC (2016)