What is meant by the term 'variable activation energy' when applied in the kinetic analyses of solid state decompositions (crystolysis reactions)?

A recent review [1], 'Kinetic concepts of thermally stimulated reactions in solids: a view from a historical perspective' recommends 'acceptance of the concept of variable activation energy'. Such a modification to the accepted meaning of so important a fundamental kinetic parameter requires critical scrutiny of the consequences that this change might make to the theory of the subject, some aspects of which are discussed here. Section I distinguishes five possible alternative definitions and/or explanations for the variability of activation energy, E; the meaning of this term is not adequately addressed in [1]. Section 2 discusses the experimental evidence offered in [1] to justify the proposal that E values should be regarded as variable. It is concluded, for stated reasons, that the supporting information provided is insufficient and unsatisfactory. Some of the selected systems appear inappropriate because there is evidence that the initially solid reactant would have melted before the reaction of interest. For others the kinetic behaviour pattern has already been adequately explained by contributions from secondary or complex controls. The introduction of the concept E (variable) seems to be unnecessary to account for the patterns of rate characteristics presented. Section 3 discusses the aims and objectives of kinetic interpretations of thermoanalytical observations generally and the measurements of E values in particular. It is concluded that the long-term development of the chemical understanding of reactions proceeding in condensed phases is most satisfactorily approached by individually identifying and quantitatively determining each contribution from every factor that influences or controls the rate of any reaction of interest. Although, considerable theoretical problems currently beset this subject, attractive and optimistic prospects for future advances are however identified. This does not include the use of E (variable) because this parameter is regarded as being predominantly empirical in character with the consequence that this concept tends not to extend either the theory or the scientific foundations of behaviour observed. It appears that the new parameter is unlikely to contribute to the direct and reliable measurement of reactivities and of absolute reaction rates or in the formulation of reaction mechanisms. It is concluded here, therefore, that the proposed introduction of this term, E (variable), is a retrograde step, unlikely to advance science through the development of theory, and its use is not recommended for reasons that are explained. Appendix extends and develops, into a wider perspective, these kinetic and mechanistic themes by identifying some interim conclusions about the methods generally used to interpret rate data obtained from thermoanalytical measurements. (C) 2002 Elsevier Science B.V. All rights reserved.