Dynamic equilibrium and Le Chatelier's principleAQA A-Level Chemistry: Subtopic test

1. 1
   Cobalt(II) ions in aqueous solution take part in the equilibrium [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l). The [Co(H₂O)₆]²⁺ ion is pink and the [CoCl₄]²⁻ ion is blue, and the forward reaction is endothermic. A student prepares a purple mixture of the two ions in a sealed tube and leaves it at constant temperature.

   (a)

   Which statement describes the mixture when it is at equilibrium?

   [1 mark]

   • AThe forward reaction has stopped but the reverse reaction continues
   • BThe concentrations of the pink and blue ions are equal
   • CThe rate of the forward reaction is greater than the rate of the reverse reaction
   • DThe forward and reverse reactions occur at equal rates and the concentrations stay constant

   (b)

   A few drops of concentrated hydrochloric acid are added to the purple mixture at constant temperature. What is observed?

   [1 mark]

   • AThe position of equilibrium shifts to the right and the mixture becomes bluer
   • BThe position of equilibrium shifts to the left and the mixture becomes pinker
   • CThe position of equilibrium does not change, because the temperature is constant
   • DThe mixture becomes bluer because the reverse reaction stops

   (c)

   The purple mixture turns blue when it is heated. Explain this observation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ammonia is manufactured by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. The process uses an iron catalyst at a temperature of about 450 °C and a pressure of about 20 MPa.

   (a)

   What is the effect of increasing the pressure on the position of equilibrium?

   [1 mark]

   • AIt shifts to the left, because the forward reaction is exothermic
   • BIt shifts to the right, because there are fewer moles of gas on the right
   • CIt does not change, because an iron catalyst is used
   • DIt shifts to the left, because the reverse reaction is endothermic

   (b)

   What is the effect of the iron catalyst on the position of equilibrium?

   [1 mark]

   • AIt shifts to the right, so more ammonia is formed
   • BIt shifts to the left, so less ammonia is formed
   • CIt has no effect, but equilibrium is reached more quickly
   • DIt shifts to the right, because the forward reaction is speeded up more than the reverse reaction

   (c)

   Explain why a compromise temperature of about 450 °C is used rather than a much lower temperature.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the Contact process, sulfur dioxide is oxidised to sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. A vanadium(V) oxide catalyst is used at about 450 °C and a pressure of about 200 kPa. Under these conditions about 99% of the sulfur dioxide is converted into sulfur trioxide.

   (a)

   Predict and explain the effect of increasing the temperature on the equilibrium yield of sulfur trioxide.

   [3 marks]

   (b)

   Explain why the Contact process is operated at a pressure of only about 200 kPa, even though a higher pressure would increase the equilibrium yield.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methanol is made from carbon monoxide and hydrogen: CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹. The process uses a copper-based catalyst at about 250 °C and a pressure of 5 to 10 MPa. The gases leaving the reactor are cooled so that methanol condenses and is removed, and the unreacted carbon monoxide and hydrogen are recycled to the reactor.

   (a)

   Predict and explain the effects of (i) increasing the temperature, (ii) increasing the pressure and (iii) adding more catalyst on the position of equilibrium and the rate of reaction.

   [6 marks]

   (b)

   Evaluate the conditions and the method of operating the process described, in terms of the yield of methanol and the cost of production.

   [6 marks]

   Total for question 4: 12 marks

End of questions