Factors affecting equilibrium and KEdexcel International A Level Chemistry: Subtopic test

1. 1
   Ammonia is manufactured by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. The reaction is carried out at about 450 °C and 200 atm using an iron catalyst.

   (a)

   What is the effect of increasing the pressure at constant temperature on the equilibrium amount of ammonia and on the value of Kp?

   [1 mark]

   • AMore ammonia at equilibrium and Kp increases
   • BMore ammonia at equilibrium and Kp unchanged
   • CLess ammonia at equilibrium and Kp unchanged
   • DNo change in the amount of ammonia and Kp unchanged

   (b)

   The temperature is raised from 450 °C to 550 °C at constant pressure. Which statement is correct?

   [1 mark]

   • AKp decreases and the equilibrium moves towards the reactants
   • BKp increases because the rate of the forward reaction increases
   • CKp is unchanged but the equilibrium moves towards the products
   • DKp decreases but the equilibrium composition is unchanged

   (c)

   Explain why using the iron catalyst does not change the equilibrium yield of ammonia.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Limestone is heated in a sealed kiln in which this equilibrium is established: CaCO₃(s) ⇌ CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹. For this equilibrium Kp = p(CO₂).

   (a)

   What is the effect of raising the temperature of the kiln on the equilibrium pressure of carbon dioxide and on Kp?

   [1 mark]

   • ABoth decrease
   • BThe equilibrium pressure increases but Kp is unchanged
   • CThe equilibrium pressure is unchanged but Kp increases
   • DBoth increase

   (b)

   More solid calcium carbonate is added to the kiln at constant temperature and volume once equilibrium has been reached. What happens to the equilibrium pressure of carbon dioxide?

   [1 mark]

   • AIt increases because more reactant is present
   • BIt decreases because the equilibrium moves to the left
   • CIt is unchanged
   • DIt doubles when the amount of CaCO₃ doubles

   (c)

   The volume of the sealed kiln is halved at constant temperature. Explain why the equilibrium pressure of carbon dioxide returns to its original value.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The Contact process makes sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The entropy change of the system is −188 J K⁻¹ mol⁻¹, and both this and ΔH may be assumed constant with temperature. Use R = 8.31 J K⁻¹ mol⁻¹.

   (a)

   Calculate ΔStotal at 700 K and hence the value of K at 700 K.

   [3 marks]

   (b)

   Calculate K at 900 K. Use your values to explain the effect of increasing the temperature on the equilibrium position and on K for this reaction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Ethanol is manufactured by the hydration of ethene: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g), ΔH = −46 kJ mol⁻¹. The industrial process uses a phosphoric acid catalyst at about 570 K and 70 atm, and unreacted ethene is recycled.

   (a)

   Explain the effect on the equilibrium yield of ethanol, and on the value of Kp, of (i) increasing the temperature, (ii) increasing the pressure and (iii) adding more catalyst.

   [6 marks]

   (b)

   Evaluate why a temperature of 570 K and a pressure of 70 atm are used, rather than a lower temperature and a much higher pressure.

   [6 marks]

   Total for question 4: 12 marks

End of questions