TEMPERATURE-DEPENDENCE OF THE RATE OF REACTION IN THERMAL-ANALYSIS - THE ARRHENIUS EQUATION IN CONDENSED PHASE KINETICS

Special problems are encountered in modeling the temperature dependence of the kinetics of heterogeneous, condensed phase systems. In the division of the model for the reaction rate into two parts, a) f(a) which is physical (translational) and b) k(T) which is chemical (vibrational), complications arise in defining the temperature dependence of part a) which may take various mathematical forms and then in coupling it with the Arrhenius temperature dependence of part b). The role of f(a) in thermal analysis systems is discussed. The concept of rate-controlling step is applied to the simplification of the temperature dependent term. The significance of the compensation effect in these systems is described and an heuristic rationalization for it is suggested. Maximum practical temperature ranges for thermal analysis experiments and the effect of temperature measurement imprecision on obtaining meaningful Arrhenius parameters are discussed. The WLF and other equations used to describe the temperature dependence of f(a) are not found to couple compatibly with the Arrhenius equation.