AQA GCSE Chemistry 4.10.4 The Haber Process and the Use of NPK Fertilisers Exam Questions

About AQA GCSE Chemistry 4.10.4 The Haber Process and the Use of NPK Fertilisers

AQA GCSE Chemistry 4.10.4 is separate Chemistry only content. It covers the industrial production of ammonia by the Haber process and the use of ammonia to make nitrogen-based fertilisers. You learn where the raw materials come from, the conditions used and, at Higher tier, how rate, equilibrium position and cost lead to compromise conditions. You also study how NPK fertilisers are made from ammonia, phosphate rock and potassium compounds. It is assessed on Paper 2 and draws on equilibrium (4.6.2), rates (4.6.1) and percentage yield (4.3.3). Explaining the choice of 450 °C and 200 atmospheres is a classic 6 mark question.

What you need to know

• The Haber process N2(g) + 3H2(g) ⇌ 2NH3(g). Nitrogen comes from the air and hydrogen is obtained from natural gas or other sources. The forward reaction is exothermic.
• Conditions The purified gases are passed over an iron catalyst at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres). On cooling, ammonia liquefies and is removed; unreacted nitrogen and hydrogen are recycled.
• Compromise conditions (Higher tier only) A lower temperature gives a higher equilibrium yield (exothermic forward reaction) but a slower rate, so 450 °C is a compromise. A higher pressure gives a higher yield (fewer gas molecules on the right) and faster rate, but very high pressures are expensive and dangerous. The catalyst increases rate but does not change yield.
• NPK fertilisers Formulations containing salts of nitrogen, phosphorus and potassium in appropriate percentages, to improve agricultural productivity.
• Making the compounds Ammonia is used to make nitric acid and ammonium salts, for example ammonium nitrate and ammonium sulfate. Phosphate rock is treated with nitric acid (giving phosphoric acid and calcium nitrate), sulfuric acid (single superphosphate) or phosphoric acid (triple superphosphate). Potassium chloride and potassium sulfate are mined.
• Lab versus industry Ammonium sulfate can be made in the laboratory by titrating ammonia solution with dilute sulfuric acid and crystallising, on a small scale. Industry uses a continuous process with raw materials made in several stages.

Common mistakes in 4.10.4 The Haber Process and the Use of NPK Fertilisers

• Saying high temperature increases the yield of ammonia. Because the forward reaction is exothermic, high temperature decreases the equilibrium yield; it is used for rate.
• Saying the iron catalyst increases yield. It increases rate only.
• Forgetting that unreacted gases are recycled, so overall conversion is high despite a low single-pass yield.
• Saying hydrogen comes from the air. Nitrogen comes from the air; hydrogen usually comes from natural gas.
• Thinking NPK stands for names of compounds. It stands for the elements nitrogen, phosphorus and potassium.

Try three 4.10.4 The Haber Process and the Use of NPK Fertilisers exam-style questions