Effect of concentration on the rate of chemical reactions

Abstract

A method that has recently been used to examine the rate of molecular transport across a phase boundary—statistical rate theory— is extended to obtain the expression for the rate of a single step chemical reaction. The concentration dependence of the newly derived rate expression leads to the prediction of a higher reaction rate in the period immediately after the reaction has begun than any of the previous theories and gives a different physical interpretation of the ‘‘rate constant’’. When ions are reacting in solution under near-equilibrium conditions, the concentration dependence of the newly derived rate expression reduces to the conventional expression; further under these conditions, the new rate expression leads to an expression for the conventional rate constant that is equivalent to the Bro/nsted primary salt effect for the change in the rate constant with ionic strength. To examine the predicted behavior in the initial period, the predictions are compared with the measurements of ion concentration during electron exchange reactions. When these experiments were originally conducted, using radioactive labeling techniques, substantial disagreement was found between the conventional theory and the measurements. To bring them together the concept of ‘‘zero-time exchange’’ was introduced which amounts to adding a constant to the predictions of the conventional theory. The physical basis for this concept has remained undefined. When the statistical rate theory expression is compared with the data from six experiments available in the literature, it is found for five of the six that there is no measurable disagreement between the data and the theory and only small disagreement for the other case. From the point of view of statistical rate theory it is unnecessary to introduce the concept of zero-time exchange.