Misinterpretation of Thermodynamic Parameters Evaluated from Activation Energy and Preexponential Factor Determined in Thermal Analysis Experiments

Abstract

Thermogravimetry (TGA) and differential scanning calorimetry (DSC) are used broadly to study the kinetics of thermally stimulated processes such as thermal decomposition (pyrolysis) or thermal polymerization. These studies typically yield the activation energy (E) and preexponential factor (A). The resulting experimental values of E and A are oftentimes used to determine the so-called “thermodynamic parameters”, i.e., the enthalpy, entropy, and Gibbs free energy. Attention is called to the persistent and mistaken trend to interpret the resulting quantities as the thermodynamic parameters of the conversion of reactants to products. In fact, these quantities are specific to the conversion of reactants to the activated complex and, as such, provide no insights into the thermodynamics of the conversion of reactants to products. The basics of the activated complex (transition state) theory are provided to explain the meaning of the equations used for evaluating the thermodynamic parameters from the experimental values of E and A. Typical examples of misinterpretation are highlighted and discussed briefly. The applicability of the theory to the systems studied by the thermal analysis kinetics is also discussed.