The Haber process is used to manufacture ammonia, which can be used to make NPK fertilisers. Describe the Haber process and explain why NPK fertilisers are important in agriculture.

AQA GCSE Chemistry (8462) — 4.10.4 The Haber process and the use of NPK fertilisers (Chem only) · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The Haber process synthesises ammonia by reacting nitrogen gas from the air with hydrogen gas derived from natural gas (methane) under high temperature and pressure in the presence of an iron catalyst. The reaction is:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Ammonia is then converted into compounds such as ammonium nitrate, ammonium sulphate and urea, which are used as NPK fertilisers. These fertilisers supply the essential nutrients nitrogen, phosphorus and potassium that crops lose to the soil during growth, thereby improving agricultural productivity.

Examiner tips

  • Use the exact reaction equation and mention the catalyst, temperature and pressure; link ammonia to NPK fertilisers; explain the role of each element in crop growth.

Common mistakes

  • Omitting the catalyst or reaction conditions; confusing nitrogen source; failing to state why NPK fertilisers are important.

Mark scheme (4 marks)

  1. The Haber process uses nitrogen and hydrogen as reactants
  2. The nitrogen is obtained from the air / the hydrogen is obtained from natural gas (methane)
  3. The reaction is reversible and the ammonia produced is used to make fertilisers
  4. NPK fertilisers contain compounds of nitrogen, phosphorus and potassium which improve agricultural productivity / replace essential elements removed from the soil by crops

Key terms in this question

Haber process · NPK fertilisers · ammonia

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