Gibbs energy

As pointed out in the preceding section, it is necessary to determine $\Delta S_{sys}$ and $\Delta S_{surr}$ for predicting the spontaneity of a reaction. We are more interested in the system (the reaction mixture); it is therefore convenient to consider the criterion of spontaneity in terms of the thermodynamic properties of the system. This problem was solved by the American theoretician J. W. Gibbs, who introduced a new thermodynamic property called Gibbs energy, usually denoted by $G$.

The Gibbs energy is defined as

where $H$ is the enthalpy and $S$ the entropy of the system. Since $H$, $T$ and $S$ are state functions, $G$ is a state function. A change in Gibbs energy depends on the initial and final states of the system, and not on a path connecting the two states. …