1. 1
   A chemical plant manufactures ammonia using the Haber process, combining nitrogen extracted from the air with hydrogen obtained from natural gas, using an iron catalyst under specific conditions of temperature and pressure.

   (a)

   Which equation correctly represents this reaction?

   [1 mark]

   • A2N2 + 3H2 ⇌ 2NH3
   • BN2 + H2 ⇌ NH3
   • CN2 + 3H2 ⇌ 2NH3
   • DN2 + 3H2 → N2H3

   (b)

   The plant operates the Haber process at approximately 450°C and 200 atmospheres pressure, using an iron catalyst. What is the role of the iron catalyst in this process?

   [1 mark]

   • AIt removes hydrogen from the reaction mixture
   • BIt shifts the equilibrium position to produce more ammonia
   • CIt reacts with nitrogen to directly form ammonia
   • DIt increases the rate of reaction without being used up

   (c)

   Describe what happens to the ammonia produced in the reaction vessel, and to any unreacted nitrogen and hydrogen.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Engineers reviewing the Haber process at the plant are analysing why the commercial conditions of 450°C and 200 atmospheres represent a compromise, rather than using the temperature and pressure that would give the maximum possible equilibrium yield.

   (a)

   Engineers reviewing the Haber process note that the forward reaction (forming ammonia) is exothermic. If the temperature were increased well above 450°C, what would happen to the equilibrium yield of ammonia?

   [1 mark]

   • AThe yield of ammonia would increase
   • BThe yield of ammonia would decrease
   • CThe yield of ammonia would stay exactly the same
   • DThe reaction would stop being reversible

   (b)

   Since a lower temperature would actually increase the equilibrium yield of ammonia (because the forward reaction is exothermic), why does the plant not simply use a much lower temperature than 450°C?

   [1 mark]

   • AA lower temperature would destroy the iron catalyst
   • BA lower temperature would make the reaction irreversible
   • CA lower temperature would make the reaction proceed too slowly to be commercially useful
   • DA lower temperature is not possible for gases

   (c)

   Explain why the plant uses 200 atmospheres of pressure rather than a much higher pressure, even though higher pressure would increase the equilibrium yield of ammonia.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fertiliser manufacturer produces NPK fertiliser by combining ammonium nitrate, potassium chloride, and a phosphate compound made by treating phosphate rock with nitric acid.

   (a)

   Explain what the letters N, P and K represent in an NPK fertiliser, and why all three are included.

   [3 marks]

   (b)

   Explain why phosphate rock cannot be added directly to the fertiliser mixture, and describe the process used to convert it into a form that can be used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A national government is planning to build a new integrated fertiliser plant. The plant will manufacture ammonia by the Haber process using nitrogen from the air and hydrogen from natural gas, and will use some of the ammonia to make ammonium nitrate, while combining this with mined potassium chloride and phosphate rock treated with sulfuric acid to produce a complete NPK fertiliser.

   (a)

   Discuss the sequence of raw materials and processes involved in producing this fertiliser, and explain why the Haber process conditions must balance rate of reaction against equilibrium yield.

   [6 marks]

   (b)

   Local environmental campaigners argue that the new fertiliser plant should use a much lower operating pressure in the Haber process to reduce energy consumption and the risk of accidents, even though this would reduce the yield of ammonia. Evaluate this proposal, discussing the trade-offs the plant operators would need to consider.

   [6 marks]

   Total for question 4: 12 marks

End of questions