Formulation of Enthalpy, Entropy, and Volume Changes Accompanying Association Reaction in Solution in Terms of Molecular Distribution Functions

The equilibrium constant of the chemical reaction in solution is formulated by means of the distribution function at the semiclassical level. The equivalence of the present expression and the traditional one that is based on the standard chemical affinity is proved. The changes of the thermodynamic quantities (enthalpy, entropy, and volume) at constasnt temperature and pressure are expressed on the basis of the present formulation. They are composed of the following three terms: (i) the internal change, (ii) the translational change, and (iii) the solvation change. Each term is evaluated for the dimerization of nitrogen dioxide. It is shown that the volume change is a good measure for the solvation change because it does not contain the internal change. Since the solvation change is explicitly expressed by means of the radial distribution function and its temperature or pressure derivative in an explicit manner, it is useful for discussing the solvent effect on the chemical reaction from the point of view of the liquid structure.