Many chemical reactions are reversible. This means they may proceed in both directions (from reactants to products, or from products to reactants). In such cases an equilibrium state occurs, in which the rates of the two reactions are identical. The following equation describes a general case in which reactants A & B are in equilibrium with products C & D. An equilibrium always favors the more thermodynamically stable side of the equilibrium. This is reflected by the equilibrium constant, K_eq , defined as the ratio of product concentrations to reactant concentrations. Thus, if K_eq is greater than 1, the products are favored over the reactants. Furthermore, the more K_eq differs from unity, the greater is the difference in stability between the sides of the equilibrium. Acid-base reactions are common examples of equilibria.