Equilibrium constants

For a general reaction like

The equilibrium constant is

where the words [A], [B],...strictly stand for activities but are often approximated as concentrations or partial pressures. (this presumes ideal thermodynamic behaviour and is a much better approximation for gases than in solution.) Pure solids and liquids are not included in an equilibrium constant as they are present in their standard state.

A very large value (» 1) of K indicates a strong thermodynamic tendency to react, so that very little of the reactants (A and B) will remain at equilibrium. Conversely, a very small value (<<1) indicates very little tendency to react: in this case the reverse reaction (C and D going to A and B) will be very favorable. For any reaction K may be associated to the standard Gibbs free energy change (ΔG^Θ) according to

where R stand for gas constant (=8.314 J K^-1 mol^-1) and T is the absolute temperature (in K). So equilibrium constants can be estimated from tabulated values of  and trends may frequently be interpreted in terms of changes in ΔH^Θ and ΔS^Θ.

Equilibrium constants change by the temperature in a way that relies on ΔHΘ for the reaction. Within accordance with Le Chatelier's principle, K increases with increase in temperature for an endothermic reaction and decreases for the exothermic one.