# Explain why the reaction 2SO₂(g) + O₂(g) → 2SO₃(g) is spontaneous at low temperatures but becomes non-spontaneous at high temperatures.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.4 Entropy and spontaneity (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. The reaction is exothermic (ΔH is negative), which gives a negative contribution to ΔG and favours spontaneity.
2. The entropy change ΔS is negative because three moles of gas are converted to two moles of gas, reducing disorder.
3. Using ΔG = ΔH − TΔS: at low temperatures the −TΔS term is small, so the negative ΔH dominates, giving ΔG < 0 (spontaneous).
4. At high temperatures the −TΔS term (which is positive, since ΔS is negative) becomes large enough to make ΔG positive, so the reaction is no longer spontaneous.

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