Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 161

Chapter 4
Chemical Processes

4.1 Introduction

Why do I like chemistry? Why do you like apples?—by virtue
of our sensations. It's all the same thing. Deeper than that
men will never penetrate.

Ivan Turgenev, Fathers and Children.

The processes of transitions between the phases considered in Chap. 3 are both
diverse and complex. Nevertheless, they cannot compare in the diversity and com-
plexity with chemical reactions. Generally, all phase transitions boil down to spa-
tial rearrangement of molecules, atoms, or ions that remain essentially unchanged.
Chemical reactions go much further; they destroy the chemical bonds between the
atoms, changing the reactant molecules to the molecules of reaction products.
When a chemical reaction is stimulated thermally, it rarely occurs in a single
step, yielding a single set of reaction products. This is because heat is not a very
selective reaction stimulus. Rising temperature increases the amplitude of vibra-
tion of all chemical bonds in a molecule. Once the amplitude reaches a critical size
for a bond of a particular strength, the bond breaks. However, the respective level
of thermal energy would also be sufficient to break a somewhat stronger bond,
although with a somewhat lower probability. For example, when heated to suffi-
ciently high temperature, an ethanol molecule would break its carbon-carbon bond
first because it has the lowest (~ 370 kJ mol −1 [ 1 ]) dissociation enthalpy of all bonds
in the molecule. However, the same temperature would be sufficient to break the
carbon-oxygen bond whose dissociation enthalpy is about 380 kJ mol − 1 [ 1 ]. There-
fore, the thermal decomposition of ethanol would occur via at least two steps that
compete with each other: