Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 217

mal decomposition of strontium carbonate measured under a flow of nitrogen also
demonstrates a practically constant value of E ʱ (~ 220 kJ mol − 1 ) at ʱ < 0.5 [ 119 ].
The situation appears quite different for another class of reversible processes: the
thermal dehydration of crystal hydrates. The enthalpies for dehydration are quite
small, mostly under 60 kJ mol − 1 [ 93 ]. This is markedly less than the enthalpies for
decomposition of carbonates (from 70 to 270 kJ mol − 1 ) and sulfates (from 190 to
790 kJ mol − 1 ) [ 93 ]. The most studied thermal dehydration is that of calcium oxalate
monohydrate. The enthalpy of this process is only 38 kJ mol − 1 [ 120 ]. It obviously
means that the difference in the activation energies of the forward and reverse pro-
cesses is quite small. Therefore, an increase in temperature of dehydration should
not be very efficient in accomplishing the situation when the rate of the forward
reaction becomes significantly faster than that of the reverse one. In other words,
unless the process is run in high vacuum, the contribution of the pressure term is not
likely to be negligible.
Figure 4.43 demonstrates the results of application of isoconversional methods
to four different samples [ 22 , 114 , 121 , 122 ] of calcium oxalate monohydrate de-
hydrated nonisothermally under a flow of nitrogen. The resulting E ʱ dependencies
are consistently descending. This agrees well with the afore-discussed effect of the
pressure term on the effective activation energy that should reveal itself in large
E ʱ values at low conversions that should gradually drop to some plateau value as
temperature and conversion increase. Descending E ʱ dependencies have also been
reported [ 123 , 124 ] for the nonisothermal dehydration of lithium sulfate monohy-
drate in the atmosphere of flowing nitrogen. Depending on the sample type and
other conditions, the drop in E ʱ with increasing ʱ has been as much as 140 and as
little as 20 kJ mol − 1 .
Although pressure and temperature are the key factors that control the kinetics
of reversible decompositions, there are a number of other factors that may play an
important role. For example, attaining global equilibrium can be complicated by
Fig. 4.43 E ʱ dependencies
for the thermal dehydration
of calcium oxalate monohy-
drate under nitrogen. (Data
from Masuda et al. [ 122 ],
squares ; Urbanovici and
Segal [ 121 ], stars ; Vyazovkin
[ 22 ], circles ; and Elder [ 114 ],
pentagons )
α