Enthalpy-Entropy Compensation (EEC) Effect: Decisive Role of Free Energy

The "enthalpy-entropy compensation" (EEC) effect has been a long-standing fascinating yet unresolved phenomenon in chemical thermodynamics. The reasons for the observation of EEC are not clear. Various views such as empirical, extrathermodynamic, error-related, solvation, and so forth as reasons for the H/S linear correlation are floating. Statistical reasons and a hidden Carnot's cycle (involving microscopic "heating and cooling" machines) have also been proposed recently for the observation of EEC. In this work, we have attempted a different line of approach to understand and explain the phenomenon. In the EEC treatment, the enthalpy (ΔH) and entropy (ΔS) values of "similar processes" are considered keeping aside the role of the other important thermodynamic parameter, that is, the free energy (ΔG). Considering ΔG along with ΔH and ΔS, it is established that the conventional EEC plot is not appropriate and mathematically sound. Consideration of ΔG may account for correlations of different kinds, linear, nonlinear, and so forth. Reports of non- or anticompensation phenomenon also prevail in the literature. A realistic account of the role of ΔG along with ΔH and ΔS in the understanding of such EEC correlations using authentic literature data is presented and discussed herein. EEC has several facets. Planned studies on similar systems with a wide range of ΔG values are required for realistic evaluation of the EEC and antienthalpy entropy compensation manifestations.