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AS · Practice questions · Dynamic equilibrium and Le Chatelier's principle

Predict the shift.

Six original Cambridge-style questions on dynamic equilibrium and Le Chatelier's principle, including industrial compromise conditions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

The system opposes the change.

01
Recall
[2 marks]

State two features of a reaction at dynamic equilibrium.

  • The rates of the forward and reverse reactions are equal. ✓
  • The concentrations of reactants and products remain constant (in a closed system). ✓
02
Recall
[1 marks]

State Le Chatelier's principle.

  • If a change is made to a system at equilibrium, the position of equilibrium moves to minimise (oppose) that change. ✓
03
Explain
[3 marks]

For N2O4(g) ⇌ 2NO2(g), ΔH = +58 kJ mol−1, N2O4 is pale yellow and NO2 brown. Predict and explain what is seen when the sealed tube is placed in hot water.

  • The forward reaction is endothermic. ✓
  • Raising the temperature shifts the position to the right, absorbing heat. ✓
  • More NO2 forms, so the mixture turns darker brown. ✓
04
Explain
[3 marks]

Explain why the Haber process uses a pressure of about 200 atm and not a much higher pressure.

  • Higher pressure shifts the position right (4 mol of gas to 2 mol), increasing the yield, and increases the rate. ✓
  • But very high pressures need thick-walled equipment and a lot of energy for compression, which is expensive. ✓
  • They also increase safety risks, so 200 atm is a compromise. ✓
05
Explain
[2 marks]

Explain why the Contact process operates at only 1 to 2 atm even though higher pressure would shift the equilibrium towards SO3.

  • The position already lies far to the right under these conditions, giving a conversion of about 99%. ✓
  • The small extra yield would not justify the cost of high-pressure equipment. ✓
06
Explain
[2 marks]

An iron catalyst is used in the Haber process. State and explain its effect on the yield of ammonia and on the time taken to reach equilibrium.

  • No effect on the yield: it speeds up the forward and reverse reactions equally, so the position does not change. ✓
  • Equilibrium is reached more quickly because the activation energy is lowered. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.