Gibbs function and equilibrium constant

Gibbs energy change for a chemical reaction is given by

where $\Delta G^0$ is the standard Gibbs energy change — that is, the Gibbs energy change when the reactants and products in a reaction are in their standard states. $Q$ is called the reaction quotient. $Q$ is analogous to the equilibrium constant, and involves nonequilibrium concentrations — or partial pressures, in the case of a gaseous reaction.

Consider

or

(the stoichiometric coefficients appear as superscripts on the concentrations or partial pressures, and the species as subscripts, exactly as the book sets them).

When the reaction reaches equilibrium, $\Delta G = 0$, and $\mathrm{Q_c}$ and $\mathrm{Q_p}$ become $K_c$ and $K_p$, respectively. Thus,

$0 = \Delta G^0 + RT\,\ln K_c \quad \text{and} \quad 0 = \Delta G^0 + RT\,\ln K_p$ …