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

1. 1
   A sealed glass tube contains a mixture of colourless dinitrogen tetroxide and brown nitrogen dioxide at a constant temperature of 298 K: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. At the start the tube contained only N₂O₄. After some minutes the brown colour stops getting darker and then stays the same.

   (a)

   Which condition must be met for this mixture to reach dynamic equilibrium?

   [1 mark]

   • AA catalyst must be present
   • BThe system is closed, so no reactant or product can escape
   • CThe initial concentrations of N₂O₄ and NO₂ must be equal
   • DThe forward reaction must be exothermic

   (b)

   The sealed tube is warmed to 350 K and the mixture becomes darker brown. Which statement is correct once a new equilibrium is established?

   [1 mark]

   • AThe position of equilibrium has shifted to the right because the forward reaction is endothermic
   • BThe position of equilibrium has shifted to the left because the forward reaction is endothermic
   • CThe position of equilibrium has not changed because the rates of both reactions increase
   • DThe forward reaction has stopped and only the reverse reaction takes place

   (c)

   Explain what is meant by dynamic equilibrium in this tube.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethanoic acid reacts with ethanol in a sealed flask at 60 °C in the presence of a few drops of concentrated sulfuric acid: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). The enthalpy change for the reaction is close to zero. Equal amounts of the two reactants are mixed and the mixture is left until its composition stops changing.

   (a)

   Extra ethanol is added to the equilibrium mixture at constant temperature. Which statement is correct once equilibrium is re-established?

   [1 mark]

   • AThe equilibrium shifts to the left and the amount of ethyl ethanoate decreases
   • BThe position of equilibrium is unchanged because changing a concentration has no effect
   • CThe equilibrium shifts to the right and the amount of ethyl ethanoate increases
   • DThe equilibrium shifts to the right and the concentration of ethanol returns to its original value

   (b)

   Assume the sulfuric acid acts only as a catalyst. What is its effect on this equilibrium?

   [1 mark]

   • AIt shifts the position of equilibrium to the right, increasing the yield of ester
   • BIt increases the rate of the forward reaction only
   • CIt lowers the enthalpy change so the equilibrium moves to the right
   • DIt increases the rates of the forward and reverse reactions equally, so equilibrium is reached sooner with no change in the yield of ester

   (c)

   Water is removed from the mixture as it forms, using a dehydrating agent. Use Le Chatelier's principle to explain the effect on the yield of ethyl ethanoate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the Haber process nitrogen and hydrogen react in a 1 : 3 mole ratio over an iron catalyst: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. In a pilot study at 200 atm the equilibrium mixture contained about 62% ammonia at 300 °C and about 18% ammonia at 500 °C. The iron catalyst works too slowly below about 400 °C. The industrial plant operates at about 450 °C and 200 atm, and unreacted gases are recycled.

   (a)

   Use the data to explain why the plant operates at about 450 °C rather than 300 °C.

   [3 marks]

   (b)

   Explain why the plant uses 200 atm rather than 1000 atm, and why the unreacted gases are recycled.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the contact process sulfur dioxide is oxidised to sulfur trioxide in a reactor containing several beds of vanadium(V) oxide catalyst: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The reactor runs at about 450 °C and 1–2 atm. The gas is cooled between the beds. After the third bed the gas is passed through concentrated sulfuric acid, which absorbs the SO₃, and the remaining gas then enters a final catalyst bed.

   (a)

   Evaluate the use of a vanadium(V) oxide catalyst and a pressure of only 1–2 atm in this process.

   [6 marks]

   (b)

   Explain, using Le Chatelier's principle, why the gas is cooled between the catalyst beds and why SO₃ is absorbed before the final bed.

   [6 marks]

   Total for question 4: 12 marks

End of questions