From microscopic to macroscopic descriptions

Abstract This chapter discusses bimolecular reactions from both a microscopic and macroscopic point of view. The outcome of an isolated reactive scattering event can be specified in terms of an intrinsic fundamental quantity, the reaction cross-section that can be measured in a molecular beam experiment. It depends on the quantum states of the molecules as well as the relative velocity of reactants and products. The relation between the cross-section and the macroscopic rate constant is derived. The rate constant is a weighted average of the product between the relative speed of the reactants and the reaction cross-section. The chapter concludes with the special case of thermal equilibrium, where the velocity distributions for the molecules are the Maxwell-Boltzmann distribution. The expression for the rate constant at temperature T is reduced to a one-dimensional integral over the relative speed of the reactants.