Free energy is a state function, so its value depends only on the conditions of the initial and final states of the system. A convenient and common approach to the calculation of free energy changes for physical and chemical reactions is by use of widely available compilations of standard state thermodynamic data.

One method involves the use of standard enthalpies and entropies to compute standard free energy changes, ΔG°, according to the following relation.

ΔG° = ΔH° − TΔS°