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

When both the change in enthalpy (∆H) and the change in entropy (∆S) are negative, the Gibbs free energy change (∆G) can be understood using the Gibbs free energy equation:

\[

\Delta G = \Delta H - T \Delta S

\]

In this equation, T represents the temperature in Kelvin. Since both ∆H and ∆S are negative, we can analyze their contributions to ∆G at different temperature ranges.

With ∆H being negative, this part of the equation indicates that the reaction is exothermic, releasing energy which favors the spontaneity of the reaction. Meanwhile, ∆S being negative suggests that the disorder of the system is decreasing, which does not favor spontaneity on its own.

At low temperatures, the term \(T \Delta S\) becomes less significant because T is a smaller value, meaning that the negative effect of the negative entropy (since ∆S is negative) is minimized relative to the negative enthalpy. Therefore, the overall value of ∆G remains negative, leading to spontaneity.

As the temperature increases, the \(T \Delta S\) term becomes larger (more negative), which can eventually outweigh the negative