American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

In the context of thermodynamics, the relationship between Gibbs free energy (∆G), enthalpy (∆H), and entropy (∆S) is described by the equation:

∆G = ∆H - T∆S

When both ∆H and ∆S are positive, the sign and magnitude of ∆G will depend on the temperature (T). At low temperatures, the T∆S term becomes less significant relative to ∆H. Since ∆H is positive, this results in a positive value for ∆G, indicating that the reaction is non-spontaneous under these conditions.

As temperature increases, the influence of the T∆S term grows because it is a product of the temperature and the entropy change. If T is high enough, it can outweigh the positive ∆H, leading to a negative ∆G. This suggests that the reaction can become spontaneous at higher temperatures. Therefore, the reaction will only occur and be spontaneous when the temperature is sufficiently high to make ∆G negative.

In summary, under the given conditions where both enthalpy and entropy are positive, ∆G will definitely be positive at low temperatures, thus indicating that the reaction does not occur under those conditions.