21/09/2026
One of the most common mistakes Cambridge A2 students make when solving questions on entropy and Gibbs free energy is forgetting to convert the enthalpy change (ΔH) from kJ to J.
Why?
Because the units must be consistent with those of entropy:
ΔH → J mol⁻¹
ΔS → J K⁻¹ mol⁻¹
It may seem like a very simple step, but surprisingly, many students forget it simply because they work too quickly and don’t pay enough attention to the units.
And once this conversion is missed, the calculation can go completely wrong, leading to an incorrect value for ΔG and, consequently, an incorrect conclusion about the feasibility of the reaction.
Another important point: don’t forget to convert the temperature from °C to K:
T(K) = T(°C) + 273
Finally, when deciding whether a reaction is feasible under the given conditions, we need to check the sign of the Gibbs free energy change (ΔG).
If ΔG is negative, the reaction is feasible under those conditions.
This is actually one of the most enjoyable topics in the syllabus, and there is a great opportunity to score very highly in these questions — provided that we pay close attention to the details and, most importantly, to the units. 🔥
