A-Level Chemistry May/June 2025 Q3(d): Iron(II) oxide can also be reduced to iron by carbon monoxide, as shown. FeO(s) + CO(g)…

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  As the temperature increases, the reaction becomes less feasible.

  The Gibbs free energy equation is $\Delta G = \Delta H - T\Delta S$. For a reaction to be feasible, $\Delta G$ must be negative.

  In this reaction, the entropy change, $\Delta S$, is negative. Therefore, the term $-T\Delta S$ is positive. As temperature (T) increases, the magnitude of the positive $-T\Delta S$ term increases. This makes the overall value of $\Delta G$ less negative (more positive), thus making the reaction less feasible.

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  How the marks are awarded

  • M1 — Stating any two of the three required points: the reaction becomes less feasible; ΔG becomes less negative/more positive; the -TΔS term becomes more positive as T increases.
  • M2 — Stating all three of the required points, providing a complete explanation. The model answer states the reaction is less feasible, that ΔG becomes less negative, and correctly explains this is due to the -TΔS term becoming more positive with increasing temperature.

  Common mistakes

  • Stating that the reaction becomes more feasible, by incorrectly reasoning that because the reaction is exothermic, increasing temperature will favour it.
  • Confusing the sign of the entropy term, for example stating that since ΔS is negative, the -TΔS term becomes 'more negative' as temperature increases.
  • Applying Le Chatelier's principle to this Gibbs free energy question, which is not a sufficient explanation for marks.
  • Giving a vague answer such as 'ΔG increases' without explaining why by referring to the TΔS term.

  Examiner tip: Always break down the Gibbs free energy equation (ΔG = ΔH - TΔS) and analyse the sign of each term to predict the effect of temperature on reaction feasibility.

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  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

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