GCSE Chemistry · AQA 8462 · Topic 4.10.4
The Haber process and the use of NPK fertilisers
Key points
- The Haber process makes ammonia from nitrogen (from the air) and hydrogen (from natural gas): . The forward reaction is exothermic.
- Conditions: an iron catalyst, about 450 °C and 200 atmospheres. On cooling, ammonia liquefies and is removed; unreacted gases are recycled.
- (Higher) A lower temperature would give a higher equilibrium yield but the rate would be too slow, so 450 °C is a compromise. Higher pressure increases yield and rate but is expensive and needs strong equipment.
- NPK fertilisers contain compounds of nitrogen, phosphorus and potassium to improve crop growth.
- Ammonia is used to make ammonium salts and nitric acid. Phosphate rock is treated with nitric acid or sulfuric acid to make soluble phosphates; potassium chloride and potassium sulfate are mined.
- Industrial production uses many steps and raw materials; in the lab, fertiliser salts such as ammonium sulfate are made by titration and crystallisation.
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Fertilisers help farmers grow enough food for the world's population, and most nitrogen fertilisers start with ammonia from the Haber process. You need the conditions, the reasons for them and how NPK fertilisers are made.
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