Eradicating erroneous Arrhenius arithmetic

A general and critical analysis is made of theories used to interpret those thermochemical rate measurements that are directed towards investigations of the mechanisms of chemical changes that result from the heating of initially solid reactants. It is concluded that the concepts and reaction models in current usage are inadequate and that the assumptions conventionally made in the interpretation of this type of experimental data are in urgent need of radical reappraisal. The present article specifically identifies several shortcomings in results obtained from the Coats-Redfern equation, which is widely used to calculate Arrhenius parameters for reactions that are regarded as capable of representation by rate equations characteristic of decompositions of solids. The view that activation energy values can,vary with the kinetic model used in their calculation is regarded as unacceptable for reasons that are discussed in detail. Introduction. An appraisal is made of the limitations of the various theories available for the interpretation of rate data obtained from thermal analytical measurements. Literature survey. From an examination of research reports, mainly published in the last decade, a common pattern of variation of magnitudes of Arrhenius parameters with rate expression (kinetic model), calculated using the Coats-Redfern equation, is identified as a set of Trends. This pattern of rate parameters is shown to occur widely. Reappraisal of methods of analysis of thermochemical rate data. It is shown that the Trends identified in calculated Arrhenius parameters are mathematical artefacts that arise through identified inadequacies in the assumptions underlying the approximate computational method. Discussion. It is concluded that the concept of the 'variable activation energy', which has recently become approved through its use in considering nonisothermal kinetic data, is unacceptable. If the theory of thermal analysis is to remain consistent with other branches of chemistry, and if scientific order is to be developed within the thermoanalytical literature, the definitions of all terms must remain common and constant. The shortcomings, which have been identified here in the approximate mathematical methods of thermal rate data interpretation, introduce doubt into the significance of many published Arrhenius parameters. Some consequences of these uncertainties for the future development of the theory of the subject and our progress towards understanding the controls and mechanisms of crystolysis reactions are examined. Meaningful kinetic data must be obtained if thermal analysis results are to be considered in the context of the recent general theoretical models developed by L'vov, which are based on a physical approach to thermal reactions. This theory offers insights into the controls and mechanisms of thermal reactions involving solids and introduces the possibility of promoting scientific order in this literature, which is not currently obvious. (C) 2002 Elsevier Science B.V. All rights reserved.