Deviations from the linear mixture rule in nonequilibrium chemical kinetics

Abstract

In experimental studies of vibrational relaxation, dissociation, or isomerization of molecular gases, it is common to use mixtures of the reacting gas with inert diluents. Rate constants for the reaction in pure reactant gas or pure diluent gas are then evaluated by extrapolation using the linear mixture rule (LMR): kLMR=∑ixiki, where xi is the mole fraction of gas i and ki is the rate constant for the reaction in pure component i. However, this rule is only obeyed rigorously for first order or pseudo-first-order processes having a single rate-determining step or which proceed at thermal equilibrium. We demonstrate theoretically that deviations from the linear mixture rule are always positive or zero (ktrue≥kLMR) and show with model calculations that the neglect of these deviations resulting from the use of the linear mixture rule may lead to large overestimates of the true pure component rate constants.