Keq

Keq, or the equilibrium constant, is a quantity that expresses the ratio of the activities of products to reactants at chemical equilibrium for a given reaction and temperature. For a balanced equation aA + bB ⇌ cC + dD, the equilibrium constant is Keq = (a_C)^c (a_D)^d / [(a_A)^a (a_B)^b], where a_i denotes the activities of the species. In dilute solutions or ideal gas conditions, activities can be approximated by concentrations [A], [B], [C], [D] or by partial pressures P, but pure solids and liquids have activities taken as 1 and do not appear in the expression.

Keq is temperature dependent and remains constant only at a fixed temperature. It relates to the standard Gibbs free energy change via ΔG° = -RT ln Keq. The temperature dependence is described by the van’t Hoff relationship d ln Keq/dT = ΔH°/(R T^2). For gas-phase reactions, the relation between Kp and Kc is Kp = Kc (RT)^{Δn}, where Δn is the change in the number of moles of gas.

The reaction quotient Q uses current conditions and has the same form as Keq. If Q < Keq, the reaction tends to proceed toward products; if Q > Keq, it shifts toward reactants until equilibrium is reached at the given temperature.

Special considerations include heterogeneous equilibria, where pure solids and liquids are omitted from the expression, and non-ideal behavior, where activities must be used. In practice, Keq is a dimensionless quantity when defined in terms of activities; when concentrations are used without standard-state conventions, the numerical value can reflect units.