Isokinetic and Compensation Temperature in the Analysis of Thermal Dissociation of the Solid Phase under Dynamic Conditions

Abstract

Sets of Arrhenius parameters, determined according to known different equations for dynamic conditions, in the vast majority form the Kinetic Compensation Effect (KCE). Converting these data to the simplified components of the Eyring equation comes down to Enthalpy–Entropy Compensation (EEC), which is consistent with the second law of thermodynamics. It has been proved that the impact of the generally known Coats−Redfern solution on the equation in differential form results in an isokinetic form of the equations and a very important coordinate T0;α0 (initial temperature and conversion degree), depending on the heating rate. This makes it possible to determine the parameters of Arrhenius’ law for both in silico and experimental data. An analytical method for determining this coordinate has been proposed. These considerations have given rise to an analysis of the relationship between two temperatures: initial and isokinetic. The sense of isokinetic temperature has been verified by the parameters CQF and K. Going further, it was found that the effects of EEC can be transformed into KCE and vice versa, which means that the two temperatures should be identical, i.e., Tiso=Tc. However, the experimental data indicate that the analyzed temperatures form a sequence T0↔Tiso↔Tc≤Teq, where Teq is the equilibrium temperature.

Elements of Transition-State Theory in Relation to the Thermal Dissociation of Selected Solid Compounds

“…The geometric interpretation of Equations ( 55) and ( 56) is presented in Figure 6, which, as suggested by [68], shows the identity of the isokinetic/compensatory temperature for KCE/EEC, thanks to the constancy of activation entropy changes at this temperature. These equations can be used both for 'step by step' data and extrapolating for kinetics when 𝐸 = 0, 𝐴 = 𝑘 .…”

“…Finally, for ln 𝐴 = 15.4 (𝐴 in s −1 ), assuming in accordance with [68,69] (see Figure A5 in [68]) instead of the unknown isokinetic temperature, the measured harmonic mean temperature 𝑇 = 1023.07 K, then according to Equation ( 14) one obtains ∆𝑆 = −135.4 J•mol −1 •K −1 (see a3 in Table 1).…”