Gibbs energy of reaction (${\Delta }_{r}G$)

The derivative of Gibbs energy with respect to reaction extent at fixed temperature and pressure: ${\Delta }_{r}G={\sum }_i{\nu }_i{\mu }_i$. For a reversible electrochemical cell reaction, ${\Delta }_{r}G=-nFE$.

At fixed temperature, $dG=VdP$ gives the pressure correction for the participating solids. Constant reaction volume reduces the integral to $\Delta V_{\mathrm{solids}}(P-P_0)$. A gas product treated by chemical fugacity instead contributes $\nu RT\ln (f/P_0)$ relative to its reference state. Adding a separate gas-volume correction to that term would double count the same pressure dependence.

The Gibbs energy of reaction for specified standard states. It determines the dimensionless equilibrium constant through ${\Delta }_r{G}^{\circ }=-RT\ln K$.