1. 1
   In a sealed industrial reactor, nitrogen gas and hydrogen gas are pumped in together at high pressure. Over time, ammonia gas forms, but the reactor never contains only ammonia: nitrogen and hydrogen remain present alongside it, and their combined mass stays constant.

   (a)

   Which term correctly describes this reaction?

   [1 mark]

   • AAn irreversible reaction
   • BA reversible reaction
   • CA neutralisation reaction
   • DA precipitation reaction

   (b)

   Which balanced symbol equation correctly represents the reaction occurring inside this reactor?

   [1 mark]

   • AN2 + 3H2 ⇌ 2NH3
   • BN2 + H2 ⇌ NH3
   • C2N2 + 3H2 ⇌ 2NH3
   • DN2 + 3H2 ⇌ NH3

   (c)

   State what is meant by the term 'dynamic equilibrium' and explain why the amounts of nitrogen, hydrogen and ammonia in the sealed reactor stop changing once equilibrium is reached.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A plant engineer running the Haber process increases the pressure inside the reactor while keeping the temperature and catalyst unchanged.

   (a)

   Compared with the original conditions, what effect does this have on how quickly equilibrium is reached?

   [1 mark]

   • AEquilibrium is reached more slowly, because higher pressure reduces the frequency of collisions
   • BEquilibrium is no longer reached, because the reaction becomes irreversible
   • CEquilibrium is reached at the same rate, because pressure has no effect on gas reactions
   • DEquilibrium is reached more quickly, because higher pressure increases the frequency of collisions between particles

   (b)

   The engineer then also adds an iron catalyst to the same reactor, without changing the temperature, pressure or concentrations of reactants. What effect does this have on the position of equilibrium?

   [1 mark]

   • AIt shifts the equilibrium position further towards ammonia
   • BIt shifts the equilibrium position further towards nitrogen and hydrogen
   • CIt has no effect on the position of equilibrium, only on the rate of attainment of equilibrium
   • DIt removes the equilibrium entirely, making the reaction go to completion

   (c)

   Explain how decreasing the temperature of the reactor would affect the rate at which equilibrium is reached.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An industrial chemist chooses to run the Haber process at a compromise temperature of about 450°C, rather than at a much lower temperature that would give a higher percentage yield of ammonia.

   (a)

   Explain why a compromise temperature is chosen instead of the lower temperature that maximises yield.

   [3 marks]

   (b)

   The same chemist also chooses to run the process at a pressure of about 200 atmospheres rather than a much higher pressure, even though a higher pressure would increase the yield of ammonia further. Explain, in terms of cost and safety, why a very high pressure is not used, and describe how the availability of raw materials also influences the conditions chosen for this reaction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school laboratory prepares a small sample of ammonium sulfate fertiliser by reacting ammonia solution with dilute sulfuric acid, evaporating the resulting solution and allowing crystals to form. A fertiliser company produces ammonium sulfate on a much larger scale using the same underlying reaction.

   (a)

   Compare the laboratory preparation with the industrial production of ammonium sulfate, describing the method used in each case and the reasons for any differences between them.

   [6 marks]

   (b)

   Explain why ammonium sulfate is useful as a fertiliser, referring to the elements it supplies to plants, and evaluate why manufacturing it on an industrial scale is considered important for agriculture.

   [6 marks]

   Total for question 4: 12 marks

End of questions