Once chemical equilibrium is reached, the numerical value of the mass action expression remains constant. This is true regardless of the original starting situation before reaction occurred. It is the numerical value of the expression, but not necessarily that of any of its components, that remains constant.

When the numerical value of the mass action expression is less than K_eq, reaction will proceed from left to right (net change from reactants to products). When the numerical value of the mass action expression is greater than K_eq, reaction will proceed from right to left (net change from products to reactants).

The Law of Mass Action relates the concentrations of reactants and products at equilibrium to a quantity that is called the equilibrium constant.

The equilibrium constant K_eq (or K_c) is the ratio of product concentrations to reactant concentrations, with each concentration raised to the power by the corresponding coefficient in the balanced chemical equation. In the reaction aA + bB cD + dD the equilibrium constant expression is written as: