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Fertilisers and the Haber process
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Fertilisers feed the world's crops. Most nitrogen fertilisers start with ammonia from the Haber process. Separate Chemistry only.
Fertilisers
Plants need nitrogen (N), phosphorus (P) and potassium (K). NPK fertilisers supply them.
Ammonium nitrate and ammonium sulfate are common nitrogen fertilisers.
Ammonium nitrate and ammonium sulfate are common nitrogen fertilisers.
Making ammonium sulfate
In the lab: titrate ammonia solution with dilute sulfuric acid, then crystallise. Small scale, batch.
In industry: a continuous process starting from raw materials, using the Haber process for ammonia.
In industry: a continuous process starting from raw materials, using the Haber process for ammonia.
The compromise conditions
N2 + 3H2 ⇌ 2NH3 (exothermic)
450 °C: a lower temperature gives a better yield, but the rate would be too slow.
200 atmospheres: higher pressure helps yield and rate, but is expensive and dangerous.
Iron catalyst: speeds up the reaction.
450 °C: a lower temperature gives a better yield, but the rate would be too slow.
200 atmospheres: higher pressure helps yield and rate, but is expensive and dangerous.
Iron catalyst: speeds up the reaction.
Why isn't the Haber process run at a low temperature, even though that gives a higher yield?
- The forward reaction is exothermic, so low temperature favours ammonia.
- But at low temperature the rate is too slow.
- 450 °C is a compromise between yield and rate.
Answer: The rate would be too slow.
NPK fertilisers supply nitrogen, phosphorus and potassium. Ammonia comes from the Haber process at 450 °C, 200 atm and an iron catalyst: a compromise between yield, rate and cost.
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