Compensation effect resulting from the use of mechanistic equations, and its physical significance

Abstract

A compensation effect is observed in the activation parameters resulting from various mechanistic equations; the T computed from the slope of the E* vs In A plot matches the observed DTG peak.In the Arrhenius equation(1) the activation parameters A and E*, instead of being independent, have sometimes been found to exhibit a correlation, A increasing with E* through a series of reactions. Such ~ compensation effect was first reported in connection with catalytic studies [1]. Evidence for such a kinetic compensation has since been reported in other fields as well [2][3][4][5][6][7][8][9][10][11], even though many of the reports have subsequently been disputed. Several theories and explanations that account for such compensation behaviour have also been put forward [2,4,7,[12][13][14][15][16].The kinetic parameters reported for solid-state reactions differ widely; for example, the estimates of the activation energy for the decomposition of calcium carbonate vary between 26 and 377 kcal/mol, while those of the pre-exponential factor vary between 102 and 1069. Procedural factors such as the heating rate, sample size, sample pretreatment, container shape and the atmosphere are implicated in the widely differing calculated parameters. Zsak6 has suggested [17] that thermal decomposition processes can be better characterized by means of compensation parameters, since the shapes and positions of the TG curves can be greatly influenced by the working conditions, whereas the compensation parameters are independent of them. We have thus been able to identify the mechanisms of thermal decomposition of some copolymers using a compensation parameter "Sp = E*/log A [18], in spite of the wide scatter in the computed kinetic parameters.John Wilev& Sons, Limited, Chichester Akad~miai Kiad6, Budapest