Recent Developments in Dispersive Kinetics

Abstract

In general, chemical reactions proceeding on time scales comparable to, or shorter than, those of internal rearrangements in a reaction system renewing the environment of the reactants (mixing), are dispersive. For dispersive kinetics, as for dispersive transport and dispersive relaxation, many time scales coexist. The rate coefficients for dispersive processes depend on time. For a time-dependent specific reaction rate, using the concept of energy profile along the reaction path, one finds the potential energy barrier separating reactants from products to evolve in time during the course of reaction. The evolution of the energy barrier during the course of reaction is described in terms of energy distribution functions related directly to the distribution function of logarithms of lifetimes calculable from kinetic equations with a time-dependent specific reaction rate. This phenomenological approach is compared with that in which the kinetic equations with time-dependent specific reaction rates are interpreted in terms of the superposition of classical reaction patterns. Special attention is paid to renor-malization of rate coefficients following from the stochastic theory of renewals (structural relaxation) in the reaction system. This phenomenological approach to kinetics is taken as a convenient basis to present a number of comprehensive models of dispersive kinetics developed in the 1990s and to discuss some recently published experimental data to show what one derives directly from experimental data and what the detailed mechanistic models have to account for to be acceptable.