Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 213

~ 615 ﾰC, i.e., when P 0 rises above 400 Pa. These conclusions have direct relevance
for isoconversional analysis of reversible thermal decompositions studied under
nonisothermal conditions. They suggest that the E ʱ dependencies estimated for this
type of processes can be expected to have unusually large values at low conversions
(i.e., ʱ ₒ 0). However, as ʱ increases, the E ʱ values should be expected to decrease
until reaching some plateau value.
The aforementioned discussion explains the conditions under which one can
eliminate the pressure dependence of the effective activation energy of reversible
thermal decomposition. Let us note that even if such conditions are accomplished,
the effective activation energy would still be a composite value that involves the ac-
tivation energy of the reverse reaction as well as the reaction and adsorption enthal-
pies (Eqs. 4.86 and 4.87). Nevertheless, the effective activation energy of revers-
ible decomposition studied under vacuum is frequently interpreted as the activation
energy of the forward reaction. For such an interpretation to be reasonable, the rate
of the forward reaction should exceed significantly the rate of the reverse reaction.
Note that in Eq. 4.87 the sum of the activation energy of the reverse reaction ( E 2 )
and the reaction enthalpy (∆ H r ) gives an estimate for the activation energy of the
forward reaction ( E 1 ; Fig. 4.39 ). The adsorption enthalpy enters Eqs. 4.86 and 4.87
only because the rates of adsorption and the reverse reaction are not considered to
be significantly slower than the rate of the forward reaction. The rates of adsorption
and the reverse reaction can be significantly decelerated by effectively removing
the gaseous product. This is accomplished by applying vacuum or fast purge with
an inert gas at ambient pressure.
Another important factor in slowing down the rates of adsorption and the reverse
reaction is increasing temperature. First, increasing temperature depresses adsorp-
tion. Adsorption is an exothermic process and, thus, disfavored by increasing tem-
perature. The effect is especially strong in the case of chemisorption that is char-
acterized by large negative values of the adsorption enthalpy [ 1 ]. For example, the
enthalpy of adsorption of carbon dioxide on calcium oxide is about − 200 kJ mol − 1
Fig. 4.39 Activation energy
diagram for a reversible
endothermic process
(
(
∆ + U
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