MASS ACTION, LAW OF

fundamental law of chemical kinetics, formulated in the years 1864 to 1879 by the Norwegian scientists Cato M. Guldberg and Peter Waage. The law states that the rate, or velocity, of any simple chemical reaction is proportional to the product of the masses of the reacting substances, each raised to a certain power. The magnitude of the exponent of each mass is equal to the corresponding number of molecules taking part in the reaction. Thus, the reaction for the reacting compounds X, Y, and Z, which form the product compound P, in which a, b, and g represent the numbers of reacting molecules, is represented by the balanced chemical equation aX + bY + gZ P; the reaction rate (r) is given by r = k(CX)a(CY)b(CZ)g. In the equation, k represents a proportionality constant called the specific reaction rate or the velocity constant, and the C 's represent concentrationse.g., moles per litreof X, Y, and Z, respectively. All quantitative expressions of the rates of chemical reactions are based on the law of mass action.