1. 1
   A student studies a reversible reaction at equilibrium in a sealed container, where the forward reaction is exothermic and the reverse reaction is endothermic, and investigates the effect of raising the temperature of the system on the position of equilibrium.

   (a)

   A reversible reaction has an exothermic forward reaction and an endothermic reverse reaction. If the temperature of this system at equilibrium is raised, what happens to the yield from the endothermic reaction?

   [1 mark]

   • AIt increases
   • BIt decreases
   • CIt stays exactly the same
   • DThe reaction stops entirely

   (b)

   When the temperature is raised for this system, what happens to the yield from the exothermic reaction?

   [1 mark]

   • AThe reaction becomes irreversible
   • BIt increases
   • CIt stays exactly the same
   • DIt decreases

   (c)

   Explain what would happen to the position of equilibrium (and to the relative amounts of product) if the temperature of this system were instead lowered.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student researches the Haber process, in which nitrogen and hydrogen react reversibly to form ammonia: N2 + 3H2 is in equilibrium with 2NH3, with the forward reaction (forming ammonia) known to be exothermic.

   (a)

   In the Haber process, N2 + 3H2 <=> 2NH3, the forward reaction (forming ammonia) is exothermic. If the temperature of the Haber process reactor is increased, what happens to the equilibrium yield of ammonia?

   [1 mark]

   • AIt stays exactly the same
   • BIt increases
   • CIt decreases
   • DAmmonia is no longer formed at all

   (b)

   Based only on maximising the equilibrium yield of ammonia, what temperature would theoretically be best to use?

   [1 mark]

   • AAs high a temperature as possible
   • BAs low a temperature as possible
   • CExactly 0 degrees Celsius, with no exceptions
   • DTemperature makes no difference to the yield

   (c)

   Despite the answer to the previous part, industry actually uses a moderately high temperature (around 450 degrees Celsius) for the Haber process rather than a very low temperature. Explain why this is done, referring to rate of reaction as well as equilibrium yield.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemical process engineer is designing a reactor for a reversible industrial reaction in which the forward reaction (forming the valuable product) is endothermic, and must decide whether raising or lowering the reactor temperature would increase the equilibrium yield of this product.

   (a)

   State the general rule describing how the yield of an endothermic reaction and an exothermic reaction each change when the temperature of a reversible reaction at equilibrium is raised.

   [3 marks]

   (b)

   Given that the forward reaction (forming the valuable product) is endothermic in this reactor, explain whether the engineer should raise or lower the temperature to increase the equilibrium yield of the product, and explain the reasoning behind your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is comparing the theoretical (equilibrium yield only) and practical (yield combined with rate) considerations for choosing an operating temperature in two industrial reversible reactions: the Haber process (exothermic forward reaction) and a hypothetical process with an endothermic forward reaction.

   (a)

   Explain fully the rule that governs how changing the temperature of a reversible reaction at equilibrium affects the yield of the endothermic and exothermic reactions, and apply this rule to explain the choice of approximately 450 degrees Celsius as the operating temperature in the Haber process.

   [6 marks]

   (b)

   For the hypothetical industrial process in which the forward reaction (forming the desired product) is endothermic, explain what temperature would give the maximum theoretical equilibrium yield, and discuss whether using this temperature would necessarily be the best practical choice for the industrial process, giving reasons.

   [6 marks]

   Total for question 4: 12 marks

End of questions