Some Basics En Route to Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 13

not represent accurately the actual multistep kinetics. In either case, the result of
analysis is a set of kinetic triplets that are effective because the actual kinetics gets
averaged through the use of approximate models. At any rate, when dealing with
condensed-phase and heterogeneous kinetics one needs to understand the meaning
of effective kinetic parameters. Applied to isoconversional methods, this means to
understand the concept of variable activation energy.

1.2 Understanding Variable Activation Energy

Then I wanted to know the meaning of the fourth beast, which
was different from all the others and most terrifying.
Daniel 7:19

Variable activation energy (i.e., the activation energy that varies with temperature
and/or conversion) would be much easier to appreciate, if it were not for that eye-
catching graph found in undergraduate general or physical chemistry texts in sec-
tions dealing with kinetics or, more specifically, with the activated-complex theory.
An example of such graph is shown in Fig. 1.9 borrowed from a classical paper by
Eyring [ 26 ]. This graph perfectly captures the essence of a reaction act, in which re-
actants must overcome some energy barrier in order to turn into products. The graph
is so self-explanatory and the idea appears so straightforward that the constancy

Fig. 1.9 Energy barrier in the theory of the activated complex. (Reproduced from Eyring [ 26 ]
with permission ACS)