Equilibrium Constant

Equilibrium Constant

For a general chemical equilibrium

the thermodynamic equilibrium constant can be defined such that, at equilibrium,

where {A} is the activity of the chemical species A, etc. This expression can be derived by considering the Gibbs free energy change for the reaction. It is common practice to use concentrations, rather than activities, defining a concentration quotient, K_c.

K_c is equal to the thermodynamic equilibrium constant divided by a quotient of activity coefficients, which, in certain circumstances, can have a value of 1; in those cases K_c = K. (K_c appears here to have units of concentration raised to some power while K is dimensionless; however as discussed below under Definitions, the concentration factors in K_c are properly divided by a standard concentration so that K_c is dimensionless also.)

A knowledge of equilibrium constants is essential for the understanding of many chemical systems, as well as biochemical processes such as oxygen transport by hemoglobin in blood and acid-base homeostasis in the human body.

Stability constants, formation constants, binding constants, association constants and dissociation constants are all types of equilibrium constant. See also Determination of equilibrium constants for experimental and computational methods.