The objective of this chapter is to obtain a theoretical description of the rates of chemical reactions on a macroscopic level and to relate corresponding laws to mechanisms for reactions on a microscopic level. The rate of a reaction depends on a variety of factors: on the temperature, pressure; concentrations of the reactants and products; and on whether or not a catalyst is present. The rate law for a reaction is defined as the change in the concentration of one of the reactants or products with respect to time. In general, the rate of change of the chosen species will be a function of the concentrations of the reactant and product species as well as the external parameters such as temperature. The stochiometry of the reaction determines its proportionality constant. Consider the general reaction a*A + b*B = c*C + d*D, where a; b; c; d are the stochiometric coefficients and the rate of concentration [C] change is defined as: rate r = [(1/c)]d[C]/dt. This rate varies with time and is equal to some functions of the concentrations: [(1/c)]d[C]dt = f([A],[B],[C],[D]). Of course, the rates of change for the concentrations of the other species in the reaction are related to that of the first species, by the stochiometry of the reaction. For the example presented above, it is as follows: d[C] cdt = dD ddt = - d[A] adt = - d[B] bdt $$ \frac{{{\text{d[C]}}}}{{{\text{cdt}}}}{\text{ = }}\frac{{{\text{dD}}}}{{{\text{ddt}}}}{\text{ = - }}\frac{{{\text{d[A]}}}}{{{\text{adt}}}}{\text{ = - }}\frac{{{\text{d[B]}}}}{{{\text{bdt}}}} $$ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315146768/f622be42-fd92-47fe-9885-b5f3ab4faa47/content/math5_1.tif"/>