The change in Gibbs free energy ($\Delta G$) provides a criterion for the spontaneity of a process at constant temperature and pressure. If $\Delta G_{\text{system}} < 0$, the process is spontaneous. If $\Delta G_{\text{system}} > 0$, the process is non-spontaneous. If $\Delta G_{\text{system}} = 0$, the system has attained a state of equilibrium, meaning the rate of the forward process equals the rate of the backward process, and there is no net movement in any particular direction .