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A-Level Chemistry May/June 2025 Q4(b): The formation of CaF2 at 298 K is shown. Ca(s) + F2(g) → CaF2(s) ΔΗ° = –1214 kJ mol-1 A…
A-Level Chemistry · Paper 9701/41 · May/June 2025 · Question 4(b) · [2 marks]
The formation of CaF2 at 298 K is shown. Ca(s) + F2(g) → CaF2(s) ΔΗ° = –1214 kJ mol-1 AG° = −1162 kJ mol-1 Calculate the entropy change, ∆Sº, in JK−1 mol¯¹, for this reaction.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The relationship between Gibbs free energy, enthalpy change, and entropy change is given by the equation:
This can be rearranged to solve for the standard entropy change, :
Substitute the values given in the question:
kJ mol⁻¹ kJ mol⁻¹ K
kJ K⁻¹ mol⁻¹
To convert this value to J K⁻¹ mol⁻¹, as required by the question, multiply by 1000:
J K⁻¹ mol⁻¹ (to 4 s.f.)
How the marks are awarded
- M1 — Awarded for correctly stating or using the Gibbs free energy equation, , and substituting the temperature T = 298 K.
- M2 — Awarded for the final correct answer of -174.5, which requires correct rearrangement, calculation, and unit conversion from kJ to J.
Common mistakes
- Forgetting to convert the final answer from kJ K⁻¹ mol⁻¹ to J K⁻¹ mol⁻¹, resulting in an answer of -0.174 or -0.175.
- Making a sign error when substituting the value for ΔG°, for example calculating (-1214 - 1162) instead of (-1214 - (-1162)).
- Incorrectly rearranging the Gibbs equation, for example to , which results in an answer with the wrong sign (+174.5).
- Leaving the answer to an inappropriate number of significant figures, such as -170 or -174.4966.
Examiner tip: Always ensure the units of your energy terms (ΔG and ΔH) are consistent before substituting them into an equation, and then convert your final answer to the units specified in the question.
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