 Thermochemical equilibrium

At fixed temperature and pressure, an interior reacting equilibrium minimizes Gibbs free energy under the constraints imposed by stoichiometry. Each independent reversible reaction then has zero actual Gibbs energy of reaction,This is not a requirement that the standard Gibbs energy of reaction vanish.

Using dimensionless thermodynamic activities $a_i$, define the reaction quotient $Q={\prod }_i{a}_i^{{\nu }_i}$. The relationtherefore givesThe equilibrium constant $K$ is dimensionless because each activity is measured relative to its declared standard state. In general $K\ne 1$ even though the actual equilibrium Gibbs energy of reaction is zero. ^[ref]

These equations determine an equilibrium composition within the model. They do not supply the reaction rate or guarantee rapid equilibration; that requires the chemical kinetics.