Page 135 - Introduction to chemical reaction engineering and kinetics

6.2 Description of Elementary Chemical Reactions 117

It is the combination of individual elementary reaction steps, each with its own rate
law, that determines the overall kinetics of a reaction. Elementary reactions have simple
rate laws of the form

r = k(T) ficYi (6.1-2)

where the temperature dependence of rate constant k is Arrhenius-like, and the reac-
tion orders ai are equal to the absolute values of the stoichiometric coefficients JVil of
the reactants (number NJ.
This chapter presents the underlying fundamentals of the rates of elementary chemi-
cal reaction steps. In doing so, we outline the essential concepts and results from physi-
cal chemistry necessary to provide a basic understanding of how reactions occur. These
concepts are then used to generate expressions for the rates of elementary reaction
steps. The following chapters use these building blocks to develop intrinsic rate laws
for a variety of chemical systems. Rather complicated, nonseparable rate laws for the
overall reaction can result, or simple ones as in equation 6.1-1 or -2.

6.2 DESCRIPTION OF ELEMENTARY CHEMICAL REACTIONS
An elementary step must necessarily be simple. The reactants are together with suffi-
cient energy for a very short time, and only simple rearrangements can be accomplished.
In addition, complex rearrangements tend to require more energy. Thus, almost all el-
ementary steps break and/or make one or two bonds. In the combustion of methane,
the following steps (among many others) occur as elementary reactions:

CH4 + O2 -+ CH; + HO;
OH*+CO-,CO,+H*

These two steps are simple rearrangements. The overall reaction
CH, + 20, -+ CO, + 2H,O

cannot occur in a single step; too much would have to transpire in a single encounter.

6.2.1 ‘Ijpes of Elementary Reactions

The following list of elementary reactions, divided into various categories, allows us to
understand and build rate laws for a wide variety of chemical systems.

6.2.1.1 Elementary Reactions Involving Neutral Species (Homogeneous Gas or
Liquid Phase)
This is the most common category of elementary reactions and can be illustrated by
unimolecular, bimolecular, and termolecular steps.

Unimolecular Steps:
l Fragmentation/dissociation- the molecule breaks into two or more fragments:

C,H,O-OH + C,H,O’ + OH*

l Rearrangements-the internal bonding of a molecule changes:
HCN -+ HNC