Kinetic consequences of the principle of microscopic reversibility

Abstract

The principle of microscopic reversibility was formulated for elementary reactions at equilibrium. It follows from the principle that for any system at equilibrium, and for any elementary reaction whether at equilibrium or not, the favoured reaction path in one direction must be the reverse of that in the opposite direction, and that the ratio of rate constants is the equilibrium constant. The same is true for non-chain reactions occurring under steady-state conditions provided that the alternative paths are equivalent as far as kinetic order is concerned. For reactions occurring under non-steady-state conditions, for chain reactions even in the steady state, and for non-chain reactions in which the alternative paths are not kinetically equivalent, the preferred reaction path in one direction may be different from that in the other; in such cases the ratio of the overall rate constants is not equal to the equilibrium constant. The principle of microscopic reversibility can never be used to prove the mechanism of a reaction when that for the reverse reaction has been established; it can only be used to eliminate kinetically equivalent mechanisms which are not the exact reverse of that for the reaction in the opposite direction.