Leveraging De Donder relations for a thermodynamically rigorous analysis of reaction kinetics in liquid media

Abstract

This Mini-review considers the use of De Donder relations to facilitate a thermodynamically rigorous discussion of liquid-phase reaction kinetics through the analysis of elementary steps. In quantifying “solvent effects,” pure species reference states are convenient. Rate expressions developed with this convention capture all effects of thermodynamic non-ideality in solvent-specific activity coefficients or excess free energies, whereas commonly applied infinite dilution reference states lead to solvent- and composition-dependent standard-state rate and equilibrium constants. Two solvent effects are described: a “kinetic” effect, which comprises solvation of a transition state relative to reactants in an elementary step, and a “thermodynamic” effect, which comprises solvation of products relative to reactants in an elementary step. The former impacts the forward rate constant, and the latter impacts the reversibility. These effects are formally encoded in De Donder rate expressions; thus, they inherently account for solvation while maintaining thermodynamic consistency with respect to elementary rate and equilibrium constants. A series of case studies are presented. These demonstrate that solvent effects can lead to substantial deviations from anticipated behavior during routine analysis of liquid-phase reactions. Additionally, we find that surface reactions and degree of rate control in multi-reaction sequences can be impacted by bulk solvation, but such effects are difficult to predict a priori.