Some Basics En Route to Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 18

The effect is illustrated in Fig. 1.13 . It appears that oftentimes revealing the vari-
ability is a matter of time as well as of having the proper tools. Undoubtedly, the
isoconversional method is one of such tools.
A frequently asked question is what is a meaning of variable activation energy?
Some people believe that the variability of the effective activation energy deprives
it of a physical meaning. To be fair, one should first state what the expected physical
meaning is. Oftentimes, the expected meaning is the height of a single and constant
energy barrier. However, as we showed earlier, this is not a very realistic expecta-
tion, and this is the meaning that variable activation energy can only deliver when it
is found to be invariable, which is a possible experimental outcome. In our opinion,
variable activation energy is physically meaningful as long as it can be explained
in terms of the activation energies for individual steps of the overall process (cf.,
Eq. 1.21 or 1.23).
To reinforce the point, we can draw an analogy with another widely used ef-
fective physical property: the atomic masses of elements. From theory we know
that an atom contains a certain number of protons and neutrons whose total mass
constitutes the mass of an atom. Since to a high degree of accuracy the mass of
either proton or neutron is 1 Da, the atomic masses of elements should be practi-
cally integer numbers. However, the actual atomic masses that we find in a periodic
table rarely agree with this theoretical expectation because they are effective masses
averaged over the natural abundances of isotopes encountered on Earth. Although
these effective masses cannot be interpreted within a concept of a single atom, they
are by no means physically meaningless parameters. Rather, they are the masses
that matter practically.
Fig. 1.13 Effect of tempera-
ture on the activation energy
of sucrose inversion, not
catalyzed ( I ) and catalyzed
by hydrochloric acid of dif-
ferent concentrations ( II 1M,
III 2M, IV 3M). (Reproduced
from Leininger and Kilpat-
rick [ 41 ] with permission of
ACS)