Free Energy and Equilibrium: The Basis of AG" = -RT In K for Reactions in Solution Gordon M. Barrow Royal Roads Military College, FMO Victoria, B.C., VOS 1B0, Canada For the chemist, the thermodynamic relation AGO = -RT In K is the dramatic, useful culmination of the main development of thermodynamics. When this result is obtained in a physical chemistry or thermodynamics course, the student finds that a chemically useful quantity, the value of the equilibrium constant, can be deduced from free energies. The seemingly irrelevant treatment of calorimeters and engines that leads to the free energy function can then he seen to be appropriate and worthwhile. The AGO = -RT In K relation is so important in chemical thermodynamics that a clear, revealing derivation, or at least a description of the principal features of such a derivation, should be presented. Furthermore, this treatment should be applicable to equilibria established in solution as well as to those established in the gas phase. Here it will be shown that the usually presented argument for the applicability of the AGO = -RT in K to reactions in solution is not adequate. When a satisfactory treatment is presented, it is seen that the important, usually overlooked, role of the solvent must be included if the AGO = -RT in K relation is to be derived. This solvent contribution is often important, even in the limit of infinite dilution and in the absence of specific solute-solvent interactions. The Importance and Basis of A G O = -RT in K The relation AGO = -RT In K has two principal uses. I t provides a basis for accepting the general validity of the constancv of the thermodvnamic eauilibrium constant exoression. I n 5,. cl
