Journal of the American Chemical Society, Vol.117, No.21, 5687-5692, 1995
Solvent Reorganization and Thermodynamic Enthalpy-Entropy Compensation
This paper develops a formalism for treating the solvent reorganization that accompanies all chemical reactions and physical processes in liquid solutions, In particular, cage environments are shown explicitly, and overall equations are separated ito a nominal equation (which is essentially the conventional chemical equation) and an environmental (env) equation. The separation is useful because it follows from the Second Law that in dilute solution Delta G(env) (same as Delta G degrees(env)) for solvent reorganization is generally zero, so that the nominal equation accounts for the observed standard free energy change Delta G degrees associated with the process. On the other hand, Delta H-env and Delta S-env can be substantial, especially when the solvation involves hydrogen bonding. And since Delta G(env) = 0, there is enthalpy-entropy compensation to the extent that Delta H-env = T Delta S-env. Conventional thermodynamic accounting requires that Delta H-env and Delta S-env are added to, and become part of, Delta H degrees and Delta S degrees for the overall process. Thus, when Delta H-env >> Delta G degrees, the plot of Delta H degrees vs Delta S degrees is nearly a straight line, with a slope close to the experimental temperature T. Two examples approaching this situation are presented and discussed.