Sulfur dioxide and oxygen react together in a closed container to form sulfur trioxide. The equation for the reaction is: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). At a certain temperature, the equilibrium concentrations are: [SO₂] = 0.20 mol/dm³, [O₂] = 0.10 mol/dm³, and [SO₃] = 0.40 mol/dm³. Write the expression for the equilibrium constant, Kc, for this reaction and explain what it tells us about the position of equilibrium at this temperature.

WJEC A-Level Chemistry (Wales) — 3.8 Equilibrium constants · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The contact process uses this reversible reaction to produce sulfur trioxide, which is a key step in making sulfuric acid. The reaction is carried out in a closed system and reaches dynamic equilibrium.

Model answer (5 marks)

Kc = \dfrac{[\text{SO}_3]^2}{[\text{SO}_2]^2[\text{O}_2]}\n\nSubstituting the given concentrations:
Kc = \dfrac{(0.40)^2}{(0.20)^2(0.10)} = \dfrac{0.16}{0.004} = 40\,\text{mol}^{-1}\text{dm}^3

Kc is a large number (≫1), so the equilibrium lies to the right and the reaction favours the formation of SO₃. The value of Kc is fixed by the temperature; changing concentrations does not change Kc.

Examiner tips

  • Write the full expression with the correct exponents; remember the product side is squared.
  • Show the substitution step and give the numerical result with units.
  • Explain that a large Kc means the equilibrium lies to the right and that Kc is temperature‑dependent, not concentration‑dependent.

Common mistakes

  • Using the wrong exponent for SO₃ in the numerator (e.g. not squaring it).
  • Omitting the units or giving an incorrect numerical value for Kc.
  • Failing to state that Kc is temperature dependent and not affected by concentration changes.

Mark scheme (5 marks)

  1. Correct Kc expression with SO₃ squared in the numerator
  2. Correct calculation or identification that Kc = 40 mol⁻¹dm³ (or equivalent large value shown by substitution)
  3. Kc is a large number / greater than 1
  4. A large Kc means the equilibrium lies to the right / favours the products
  5. Kc only changes with temperature / concentrations do not change Kc

Key terms in this question

equilibrium constant · Kc · position of equilibrium

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