Dynamic equilibrium and Le Chatelier's principleEdexcel International A Level Chemistry: Subtopic test

1. 1
   Dinitrogen tetroxide is a colourless gas that is in equilibrium with brown nitrogen dioxide: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. A sample of the equilibrium mixture is sealed in a glass tube so that the depth of brown colour can be observed.

   (a)

   Which statement correctly describes this system at dynamic equilibrium?

   [1 mark]

   • AThe forward reaction has stopped and only brown NO₂ is present
   • BThe concentrations of N₂O₄ and NO₂ are equal and the reactions have stopped
   • CThe reactions only occur when the temperature is changed
   • DThe rate of the forward reaction equals the rate of the reverse reaction, and the concentrations stay constant

   (b)

   The tube is heated at constant volume. What is seen, and why?

   [1 mark]

   • AThe colour becomes paler, because the equilibrium shifts left, the exothermic direction
   • BThe colour becomes darker brown, because the equilibrium shifts right, the endothermic direction
   • CThe colour is unchanged, because temperature does not affect the position
   • DThe colour becomes darker brown, because the equilibrium shifts left

   (c)

   The tube is compressed at constant temperature so that the pressure increases. Predict the change in the position of equilibrium and justify your answer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethanol is manufactured by the reversible reaction of ethene with steam: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g), ΔH = −45 kJ mol⁻¹. The reaction is carried out in a closed reactor at about 570 K and 65 atm using a solid acid catalyst.

   (a)

   What is the effect of increasing the pressure on the equilibrium yield of ethanol?

   [1 mark]

   • AThe yield decreases, because the equilibrium shifts to the side with more gas molecules
   • BThe yield is unchanged, because the reaction is exothermic
   • CThe yield increases, because there are fewer gas molecules on the right
   • DThe yield increases, because the enthalpy change becomes more negative

   (b)

   Ethanol is condensed and removed as it forms, and unreacted ethene and steam are recycled. What is the effect on the position of equilibrium?

   [1 mark]

   • AIt shifts to the right to replace the ethanol removed
   • BIt shifts to the left because ethanol is a product
   • CIt does not change, because ethanol is removed as a liquid
   • DIt shifts to the left because steam is recycled

   (c)

   The reactor uses a solid acid catalyst. State and explain the effect of the catalyst on the position of equilibrium and on the time taken to reach equilibrium.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Sulfur trioxide is made in the Contact process by the reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The plant uses a vanadium(V) oxide catalyst at about 720 K and a pressure of 1 to 2 atm.

   (a)

   Explain why a temperature of about 720 K is a compromise for this process.

   [3 marks]

   (b)

   At 720 K the equilibrium yield of sulfur trioxide is about 97% at 1 atm and about 99% at 10 atm. Use these data to evaluate whether the plant should operate at a much higher pressure.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methanol is made from synthesis gas in a closed reactor: CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹. A copper-based catalyst is used at about 500 K and a high pressure of about 80 atm. Unreacted gases are separated from the methanol and recycled.

   (a)

   Predict and justify the effect on the position of equilibrium and the yield of methanol of increasing the temperature, increasing the pressure and removing methanol as it forms. Your answer should refer to dynamic equilibrium and Le Chatelier's principle, and state the effect of the catalyst.

   [6 marks]

   (b)

   The equilibrium yield of methanol at 80 atm is about 95% at 400 K, 75% at 500 K and 40% at 600 K, but the catalysed reaction is too slow to be economic at 400 K. Evaluate the conditions of 500 K and 80 atm used by the plant.

   [6 marks]

   Total for question 4: 12 marks

End of questions