2SO₂(g) + O₂(g) ⇄ 2SO₃(g), ΔH = −395.7 kJ mol⁻¹. In the reaction above, the concentration of SO₃(g) can be increased by

Correct answer: Decreasing the temperature

Explanation: The forward reaction is exothermic (ΔH < 0). By Le Châtelier’s principle, lowering the temperature favours the exothermic direction, producing more SO₃ and increasing its equilibrium concentration.

• Decreasing the pressure — Incorrect. The reaction goes from 3 mol gas (reactants) to 2 mol gas (products); increasing pressure would favour SO₃, not decreasing it.
• Decreasing the temperature — Correct. Shifts equilibrium toward SO₃ for an exothermic reaction.
• Increasing the temperature — Incorrect. Favours the endothermic reverse reaction, reducing SO₃.
• Adding a catalyst — Incorrect. Changes the rate of reaching equilibrium, not the position (no change in equilibrium concentrations).