Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 223

Fig. 4.48 Isoconversional
activation energies evaluated
for isothermal decomposi-
tion of ammonium nitrate in
the solid and liquid states.
(Adapted from Vyazovkin
et al. [ 143 ] with permission
of ACS)

VROLG
OLTXLG

α

Fig. 4.49 Single Arrhenius
plot fits both solid- and
liquid-state data on the
thermal decomposition of
ammonium nitrate. (Adapted
from Vyazovkin et al. [ 143 ]
with permission of ACS)

VROLG
OLTXLG

7 .

nitrate are described by the same kinetic triplet. Note that under nonisothermal con-
ditions the process starts in the solid but ends in the liquid state. However, melting
that occurs in the initial stages of the process does not affect the isoconversional
values of the activation energy. They remain practically constant throughout the
process and average to 93 1 kJ mol − 1 [ 143 ].
The absence of a difference in the kinetic triplets for the liquid- and solid-state
decomposition of ammonium nitrate appears to be due to the fact that the process
follows the mechanism of dissociative sublimation/vaporization [ 143 ]. In the solid
state, the rate of this process is determined by the surface to volume ratio of solid
particles. Melting of the particles turns them into droplets but the surface to volume
ratio remains practically the same. Therefore, the process rate remains unaffected
by melting.