Page 139 - Modeling of Chemical Kinetics and Reactor Design

Reaction Rate Expression 109

CHAPTER THREE

Reaction Rate Expression

INTRODUCTION

Chemical reactions are processes in which reactants are transformed
into products. In some processes, the change occurs directly and the
complete description of the mechanism of the reaction present can be
relatively obtained. In complex processes, the reactants undergo a
series of step-like changes and each step constitutes a reaction in its
own right. The overall mechanism is made up of contributions from
all the reactions that are sometimes too complex to determine from
the knowledge of the reactants and products alone. Chemical kinetics
and reaction mechanism as reviewed in Chapter 1 can often provide
a reasonable approach in determining the reaction rate equations.
Chemical kinetics is concerned with analyzing the dynamics of chemical
reactions. The raw data of chemical kinetics are the measurements of
the reactions rates. The end product explains these rates in terms of a
complete reaction mechanism. Because a measured rate shows a
statistical average state of the molecules taking part in the reaction,
chemical kinetics does not provide information on the energetic state
of the individual molecules.
Here, we shall examine a series of processes from the viewpoint
of their kinetics and develop model reactions for the appropriate rate
equations. The equations are used to arrive at an expression that relates
measurable parameters of the reactions to constants and to con-
centration terms. The rate constant or other parameters can then be
determined by graphical or numerical solutions from this relationship.
If the kinetics of a process are found to fit closely with the model
equation that is derived, then the model can be used as a basis for
the description of the process. Kinetics is concerned about the quantities
of the reactants and the products and their rates of change. Since
reactants disappear in reactions, their rate expressions are given a

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