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

In this situation, the Gibbs free energy change, ΔG, determines the spontaneity of a reaction and is given by the equation:

ΔG = ΔH - TΔS

Where:

- ΔG is the Gibbs free energy change,

- ΔH is the enthalpy change,

- T is the temperature in Kelvin,

- ΔS is the entropy change.

When ΔH is positive, it means that the reaction is endothermic, which typically favors non-spontaneity. A negative ΔS indicates that the randomness or disorder of the system decreases, which further discourages spontaneity because spontaneous processes tend to increase entropy.

For spontaneity, ΔG needs to be negative. Given that you have both a positive ΔH and a negative ΔS, as the temperature (T) increases, the term TΔS becomes more negative, but since ΔH is already positive, the equation suggests that ΔG will likely stay positive or remain less negative at all temperatures. This combined effect indicates that the reaction is not favorable at any temperature, leading us to conclude that the reaction will never occur.