REACTION RATE

the speed, or velocity, at which a chemical reaction proceeds, expressed in terms of the amount of a product formed per unit time or the amount of a reactant used per unit time. Thus, for the reaction of two compounds X and Y that form a product Z, the equation is X + Y Z, and the reaction rate may be given by the rate of increase of the concentration of Z or by the rate of decrease of the concentration of X or Y. Mathematically, the reaction rate is given by dCZ/dt, -dCX/dt, or -dCY/dt, in which C represents the concentration (e.g., moles per litre) of the species denoted by the subscript, and the symbol d/dt is the mathematical expression for the rate of change of some quantity with respect to time (the derivative with respect to time). Chemical reactions proceed at different speeds depending on the nature of the reacting substances and the type of chemical transformation. In general, reactions in which ions (electrically charged particles) combine or separate occur very rapidly, while those in which covalent bonds are formed or broken are much slower. For a given set of reactants, the speed of the reaction will vary with the temperature or pressure imposed on the reacting system and the amounts of reactants used. Ordinarily the reaction will gradually slow down as the reactants become depleted. In some cases the addition of a substance not itself a reactant, called a catalyst, accelerates a reaction that normally takes place at a very low rate. The reaction-rate constant, or the specific rate constant, is the proportionality constant in the equation that describes the relationship between the rate of a chemical reaction and the concentrations of the reacting substances. If r represents reaction rate, k is the symbol customarily used for the reaction-rate constant, and f(C) is an expression for the concentrations of the reactants, then the equation for these values is r = kf(C). If the reaction rate, or velocity, is visualized as being determined by two factors, one representing the amount of molecules present and the other the type and the condition of those molecules, then the rate constant is a quantity that represents the latter. The prediction, measurement, and interpretation of reaction rates are subjects of the branch of chemistry known as chemical kinetics.