DIFFUSION AND DISEQUILIBRIUM FOR RAPID BIMOLECULAR REACTIONS IN SOLUTION - INFLUENCE OF THE BACK-REACTION

The influence of reversibility and diffusion on the rate constant of rapid bimolecular reactions in solution is explored with statistical nonequilibrium thermodynamics. For reactions with stoichiometry A + B ⇄ C, we find that in dilute solution the Smoluchowski-Collins-Kimball steady-state expression for the rate constant is valid only when the steady state is sufficiently far from equilibrium (as determined by the criterion ΔG ≪ -RT In [k°/4πD′R]). Comparable results hold for bimolecular reactions with other stoichiometries. On the basis of these considerations, we put forward two caveats in the interpretation of experimental data: (1) for measurements in an equilibrium ensemble (e.g., equilibrium perturbation techniques), the bimolecular rate constant equals the intrinsic rate constant and is not diffusion dependent; (2) while the Smoluchowski-Collins-Kimball expression is valid at steady states sufficiently far from equilibrium, the ratio of the forward to the reverse rate constants does not equal the equilibrium constant. The implications of these caveats for the interpretation of experimental data, including acid-base neutralization reactions, are discussed. © 1990, American Chemical Society. All rights reserved.