1. 1
   A sealed gas syringe contains an equilibrium mixture of colourless dinitrogen tetroxide and brown nitrogen dioxide at 25 °C: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +58 kJ mol⁻¹.

   (a)

   What is the equilibrium constant expression, Kc, for this reaction?

   [1 mark]

   • A[NO₂] ÷ [N₂O₄]
   • B[N₂O₄] ÷ [NO₂]²
   • C2[NO₂] ÷ [N₂O₄]
   • D[NO₂]² ÷ [N₂O₄]

   (b)

   The plunger is pushed in quickly and held, at constant temperature. Which statement describes the new equilibrium compared with the original one?

   [1 mark]

   • AThe position of equilibrium shifts to the right and Kc is unchanged.
   • BThe position of equilibrium shifts to the left and Kc decreases.
   • CThe position of equilibrium shifts to the left and Kc is unchanged.
   • DThe position of equilibrium does not change because both species are gases.

   (c)

   The syringe is placed in a beaker of hot water. Predict and explain the effect on the colour of the mixture and on the value of Kc.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethanoic acid and ethanol are mixed with a few drops of concentrated sulfuric acid in a sealed flask at 25 °C and left to reach equilibrium: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). Under these conditions Kc = 4.0.

   (a)

   What is the value of Kc for the hydrolysis of ethyl ethanoate, CH₃COOC₂H₅(l) + H₂O(l) ⇌ CH₃COOH(l) + C₂H₅OH(l), at 25 °C?

   [1 mark]

   • A−4.0
   • B0.25
   • C2.0
   • D4.0

   (b)

   Which statement is correct once this mixture has reached equilibrium?

   [1 mark]

   • AThe rates of the forward and backward reactions are equal and all concentrations stay constant.
   • BThe forward and backward reactions have both stopped.
   • CThe concentrations of reactants and products are equal.
   • DThe rate of the forward reaction is greater than that of the backward reaction.

   (c)

   A drying agent that absorbs water, but none of the other substances, is added to the equilibrium mixture. Predict and explain the effect on the amount of ethyl ethanoate and on the value of Kc.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ammonia is made by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Starting from a 1 : 3 mixture of nitrogen and hydrogen, the percentage of ammonia in the equilibrium mixture was found to be: at 200 atm, 62.8% at 300 °C, 36.3% at 400 °C and 17.6% at 500 °C; at 400 °C, 25.1% at 100 atm, 36.3% at 200 atm and 47.0% at 300 atm. Industrial plants typically operate at about 450 °C and 200 atm with an iron catalyst.

   (a)

   Deduce, using the data, whether the forward reaction is exothermic or endothermic. Explain your reasoning with reference to the equilibrium constant.

   [3 marks]

   (b)

   Explain the effect of pressure shown by the data, and evaluate why industry operates at about 450 °C and 200 atm rather than at 300 °C or at 300 atm.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the Contact process, sulfur dioxide is oxidised to sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The equilibrium constant is very much greater than 1 at the operating temperature. The gases are passed over a vanadium(V) oxide catalyst at about 450 °C and 1–2 atm, using air in excess.

   (a)

   Deduce the Kc expression for this reaction and discuss how the magnitude of K and Le Châtelier's principle explain the choice of temperature, pressure and excess air.

   [6 marks]

   (b)

   Evaluate each of the following student statements. (1) 'The vanadium(V) oxide catalyst increases K, so more SO₃ is produced at equilibrium.' (2) 'Because K is so large, the reaction 2SO₃(g) → 2SO₂(g) + O₂(g) does not happen at 450 °C.' (3) 'Removing SO₃ from the mixture between catalyst beds increases the value of K.'

   [6 marks]

   Total for question 4: 12 marks

End of questions