Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 216

ference is supported by the fact that the application of isoconversional methods to
the thermal decomposition of calcium carbonate tends to produce rather consistent
values of the activation energy. Figure 4.42 presents five E ʱ dependencies estimated
form four different data sets obtained under nonisothermal conditions in the atmo-
sphere of flowing nitrogen [ 110 , 114 - 116 ]. Except the data of Elder [ 114 ], the E ʱ
values do not show any significant variation with ʱ and practically all of them fit
within 10 % of 190 kJ mol − 1 , which is obviously a much narrower range than the
one seen in Fig. 4.41 .
The application of isoconversional methods to the isothermal decomposition of
calcium carbonate under nitrogen flow also yields the E ʱ values that do not practi-
cally vary with ʱ and that average to 186 kJ mol − 1 [ 117 ]. The value is certainly with-
in the limits for the nonisothermal values. Isoconversional treatment of the constant
rate thermal analysis data (temperature is varied to maintain the process rate con-
stant) on decomposition of calcium carbonate performed under vacuum (4⇅10 − 3 Pa)
has also yielded nearly constant E ʱ values [ 118 ]. The latter average to 224 kJ mol − 1 .
Keeping in mind that a confidence interval for an individual E ʱ value is rarely less
than 10 % of the value, the difference between the activation energies respectively
obtained under vacuum and nitrogen (Fig. 4.42 ) is hardly significant.
Considering that the thermal decomposition of calcium carbonate under nitro-
gen flow does not demonstrate consistently descending E ʱ dependencies, we can
suppose that the pressure term has a negligible contribution to the effective acti-
vation energy of the process under such conditions. Apparently, it is easy for this
process to reach the temperature range when the rate of the forward reaction be-
comes much faster than that of the reverse reaction. As discussed earlier, this is
expected for reversible processes having larger reaction enthalpy. For the thermal
decomposition of calcium carbonate, this value is 178 kJ mol − 1 [ 93 ]. Even larger
enthalpies are reported [ 93 ] for decomposition of strontium (235 kJ mol − 1 ) and
barium (269 kJ mol − 1 ) carbonates. Incidentally, isoconversional analysis of the ther-
Fig. 4.42 Isoconversional
activation energies for the
nonisothermal decomposition
of calcium carbonate under
nitrogen. (The data are from
Elder [ 114 ] ( diamonds ), Tan
et al. [ 116 ] ( pentagons , data
are courtesy of Guanglei
Tan), Gao et al. [ 115 ] ( stars ).
Half-filled symbols cor-
respond to the same data
set [ 110 ] processed by the
differential ( squares, data are
courtesy of Alan Burnham)
and integral method ( circles,
data of Sergey Vyazovkin))
α