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AQA · GCSE · Chemistry · Higher

Pressure effect on equilibrium yield

Q07.3 2 marks

The equation for the reaction is:

2SO2(g)+O2(g)⇌2SO3(g)\mathrm{2 SO_2(g) + O_2(g) \rightleftharpoons 2 SO_3(g)}

Explain why the percentage yield of sulfur trioxide in this reaction is greater if the pressure is higher.

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Marking points

  1. 1 Compares the gaseous amounts correctly: three moles/molecules of gas on the left and two on the right, or valid converse.
  2. 2 Explains that higher pressure shifts equilibrium to the right, increasing sulfur trioxide yield.

Full-mark answer

The left side has three moles of gas, while the right side has two. Increasing pressure shifts the equilibrium to the side with fewer gas molecules, so it shifts right and produces a greater yield of sulfur trioxide.

Why this answer loses marks

Higher pressure raises the yield only because particles collide more often.

Collision frequency explains rate, not the equilibrium position; compare gas moles and state the resulting shift.

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