Chemical Equilibria: Redefining Equilibrium Constant

Recall that the solubility of a sparingly soluble salt increases with the addition of an inert salt in a phenomenon known as the salt effect.

The extent of the salt effect depends on the ionic strength of the solution.

To account for the deviation from ideality, the equilibrium constant must relate the activity—which incorporates the partial molar Gibbs energy, or chemical potential—of each species.

This equilibrium constant, called the thermodynamic or standard equilibrium constant, expresses the Gibbs energy change of the process and incorporates the effects of ionic strength.

At negligible ionic strength, the activity coefficient is approximately unity, and the concentration equilibrium constant is approximately equal to the thermodynamic equilibrium constant.

This approximation is valid for solutions of singly charged ions or non-dissociating species with ionic strengths lower than 0.01 moles per liter.

However, for solutions with ionic strengths greater than 0.01 moles per liter or multiply charged ions, activity coefficient corrections are needed to avoid significant errors.