Abstract

Kinetic measurements measure the rates of reactions. The information obtained is useful for fundamental understanding of chemical mechanism and for a variety of applications. The overall rate law for a reaction and the definition of associated rate constants must be consistent with the stoichiometric chemical equation for the reaction, but cannot be predicted from the stoichiometric equation alone. A detailed reaction scheme (or an even more descriptive reaction mechanism), which specifies any intermediates that occur in the reaction and describes exactly which atoms or molecules react in each elementary step, is needed to predict a rate law. The goal of kinetic measurements is to verify the functional form of the rate law and determine any constants in the rate law. The constants are quantities that do not vary when concentrations of reactants change; but they do vary with temperature, solvent, and other reaction conditions. Kinetic measurements involve preparing reactants at concentrations different from equilibrium values and then monitoring concentrations as they approach equilibrium. Simple manual mixing may suffice. More rapid reactions may be studied by flow-mixing reactants in a moving stream or by injecting and mixing rapidly in a stopped-flow device. Even faster reactions require perturbation techniques that start with a homogeneous mixture and perturb conditions away from equilibrium. This can be accomplished by changing external thermodynamic conditions such as temperature or pressure. Altematively, a reagent precursor may be switched suddenly to an active form by a flash of light (flash photolysis) or a pulse of electrons (pulse radiolysis). A host of reaction schemes are possible, but the Michaelis-Menten scheme applicable to much enzyme catalysis receives special attention. The temperature dependence of rate constants is discussed in terms of an activation energy, and the dependence on pressure is characterized by an activation volume. Brief mention is made of recent progress in reaction dynamic studies, which provide in a few cases both experimental and theoretical treatments at the full quantum mechanical level.