# Explain why a mixture of SO₂(g), O₂(g) and SO₃(g) in which Qc > Kc is not at equilibrium, and describe the direction in which the reaction will proceed to reach equilibrium.

> IB DP Chemistry Standard Level (2023 syllabus) — R2.3 How far? The extent of chemical change · Explain · 4 marks

> The industrial production of sulfuric acid involves the reversible reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). At a given temperature, the equilibrium constant Kc has a fixed value.

## Mark scheme (4 marks)

1. Qc is the reaction quotient, calculated using the same expression as Kc but with the current (non-equilibrium) concentrations.
2. When Qc > Kc, the ratio of product concentrations to reactant concentrations is too large compared to the equilibrium position.
3. The reaction proceeds in the reverse/backward direction (to the left) to reach equilibrium.
4. As the reaction proceeds in reverse, product concentration(s) decrease and reactant concentration(s) increase until Qc equals Kc, at which point equilibrium is re-established.

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