ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

The equation for calculating Gibbs free energy is represented as ΔG = ΔH - TΔS. This relationship is fundamental in thermodynamics and determines the spontaneity of a process at constant temperature and pressure.

In this equation:

- ΔG represents the change in Gibbs free energy.

- ΔH refers to the change in enthalpy (heat content) of the system.

- T is the absolute temperature measured in Kelvin.

- ΔS is the change in entropy, which quantifies the disorder or randomness of a system.

The equation illustrates that the change in Gibbs free energy is influenced by the balance between the enthalpy change and the entropy change of a system, adjusted for temperature. A negative value of ΔG indicates that a reaction is spontaneous, while a positive value suggests that the reaction is non-spontaneous under the specified conditions.

Choosing the correct sign for the equation is crucial; the subtraction of the product of temperature and entropy from the enthalpy change allows us to account for the energy that is dispersed or spread out as a system moves toward equilibrium. Thus, this formulation helps chemists predict whether a reaction will proceed forward under given conditions.