ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

When the change in enthalpy (∆H) is positive and the change in entropy (∆S) is negative, the Gibbs free energy change (∆G) can be analyzed using the equation:

∆G = ∆H - T∆S

In this scenario, since ∆H is positive, it signifies that the process is endothermic, absorbing energy from the surroundings. On the other hand, a negative ∆S indicates that the disorder of the system decreases, meaning that the system is becoming more ordered.

The term T∆S will be negative as well because it is the product of a positive temperature (in Kelvin) and negative entropy change. When this term is subtracted from a positive ∆H, it results in ∆G being positive overall. This means that the process is non-spontaneous at any temperature, as a positive ∆G indicates that the forward reaction is not favored under standard conditions.

The dependence of ∆G on temperature does not apply here because with both ∆H positive and ∆S negative, regardless of temperature, ∆G will remain positive, thus reaffirming that the process cannot occur spontaneously.