Explain why the reaction 2SO₂(g) + O₂(g) → 2SO₃(g) is spontaneous at low temperatures but becomes non-spontaneous at high temperatures.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The reaction is exothermic (ΔH < 0), giving a negative contribution to ΔG and favouring spontaneity.
The entropy change ΔS is negative because 3 mol of gas are converted to 2 mol of gas, reducing disorder.
Using ΔG = ΔH − TΔS, at low temperatures the −TΔS term is small, so the negative ΔH dominates and ΔG < 0 – the reaction is spontaneous.
At high temperatures the −TΔS term (which is positive, since ΔS < 0) becomes large enough to make ΔG > 0, so the reaction is no longer spontaneous.
The entropy change ΔS is negative because 3 mol of gas are converted to 2 mol of gas, reducing disorder.
Using ΔG = ΔH − TΔS, at low temperatures the −TΔS term is small, so the negative ΔH dominates and ΔG < 0 – the reaction is spontaneous.
At high temperatures the −TΔS term (which is positive, since ΔS < 0) becomes large enough to make ΔG > 0, so the reaction is no longer spontaneous.
Examiner tips
- Show the sign of ΔH and ΔS clearly; use ΔG = ΔH − TΔS to explain the temperature dependence.
- Include the reasoning that a negative ΔS makes the TΔS term positive, which opposes the negative ΔH at high T.
Common mistakes
- Confusing the sign of ΔS or forgetting that ΔS is negative for this reaction.
- Failing to mention that the −TΔS term becomes positive and dominates at high temperatures.
- Using the wrong equation (e.g., ΔG = ΔH + TΔS) or mis‑labeling the contribution of each term.
Mark scheme (4 marks)
- The reaction is exothermic (ΔH is negative), which gives a negative contribution to ΔG and favours spontaneity.
- The entropy change ΔS is negative because three moles of gas are converted to two moles of gas, reducing disorder.
- Using ΔG = ΔH − TΔS: at low temperatures the −TΔS term is small, so the negative ΔH dominates, giving ΔG < 0 (spontaneous).
- 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.
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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