Isokinetic behaviour

Compensation refers to the behaviour pattern in which a rise in ii, (which will decrease the rate of a reaction at any particular temperature) is partially or completely offset by an increase in A [28], From equation (4.6), a temperature, F, (=(f 7 ) ) exists [48], at which all reactions of the set expressed by equation (4.6) proceed at equal rates. Alternative descriptions for the phenomenon are isokinetic behaviour or the 6-rule, where 6= T, is the temperature of equal reaction rates. 

The existence of compensation behaviour can be accounted for as follows. All samples of calcite undergo dissociation within approximately the same temperature interval, many kinetic studies include the range 950 tolOOO K. The presence of COj (product) may decrease reactivity and a delay in heat flow into the reactant will decrease the reaction temperature. Thus, imder varied conditions, the reaction occurs close to a constant temperature. This is one of the conditions of isokinetic behaviour (groups of related reactions showing some variations of T within the set will nonetheless exhibit a well-defined compensation plot [61]). As already pointed out, values of A and E calculated for this reaction, studied under different conditions, show wide variation. This can be ascribed to temperature-dependent changes in the effective concentrations of reaction precursors, or in product removal [28] at the interface, and/or heat flow. The existence of the (close to) constant T, for the set of reactions, for which the Arrhenius parameters include wide variations, requires (by inversion of the argument presented above) that the magnitudes of A and E are related by equation (4.6). 

Reports of compensation behaviour appear to have been mainly empirical findings which have not contributed much to the theory of solid state reactions. Identification of the effect as a direct consequence of isokinetic behaviour [57,58] does, however, offer a theoretical foimdation for interpretation of the observed correlated changes in apparent Arrhenius parameters. The fit of Arrhenius parameters for crystolysis reactions to the expression ... 

It is worth noting that below the isokinetic point (T < T ) the reaction exhibits electrophobic behaviour, i.e. dr/dUwR > 0, while for T > T the reaction becomes electrophilic. At T = T the NEMCA effect disappears (see also the curve for T=370°C in Fig. 8.6). 

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