Temperature dependence of rates of bimolecular elementary reactions

Abstract

There is now evidence that the Arrhenius plots of several elementary bimolecular reactions show substantial deviations from linearity when studied over a wide temperature range. If such deviations are widespread, they can cause serious difficulties in extrapolating low temperature kinetic data for use in theoretical studies of explosions. Accordingly, we have investigated the general behavior of the temperature dependence of elementary bimolecular reactions, considering both equilibrium and non-equilibrium effects on the kinetics. The activated complex theory model was used to explore equilibrium kinetic behavior. The essential factor in determining deviations from the Arrhenius law is shown to be the interconversion of translational, rotational, and vibrational degrees of freedom that takes place when two reactant molecules come together to form the activated complex. This leads to two effects, (i) in general the measured Arrhenius activation energy EA will be different from the barrier height E0 for the reaction, and (ii) the value of EA itself may be a function of T in which case the reaction will have a non-linear Arrhenius plot if studied over a large temperature range. Both positive and negative deviations from the Arrhenius law are shown to be possible, and the factors which influence this behavior are investigated.