Page 31 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 1

CHAPTER ONE

Reaction Mechanisms

and Rate Expressions

INTRODUCTION

The field of chemical kinetics and reaction engineering has grown
over the years. New experimental techniques have been developed to
follow the progress of chemical reactions and these have aided study
of the fundamentals and mechanisms of chemical reactions. The
availability of personal computers has enhanced the simulation of
complex chemical reactions and reactor stability analysis. These
activities have resulted in improved designs of industrial reactors. An
increased number of industrial patents now relate to new catalysts and
catalytic processes, synthetic polymers, and novel reactor designs. Lin
[1] has given a comprehensive review of chemical reactions involving
kinetics and mechanisms.
Conventional stoichiometric equations show the reactants that take
part and the products formed in a chemical reaction. However, there
is no indication about what takes place during this change. A detailed
description of a chemical reaction outlining each separate stage is
referred to as the mechanism. Mechanisms of reactions are based on
experimental data, which are seldom complete, concerning transition
states and unstable intermediates. Therefore, they must to be con-
tinually audited and modified as more information is obtained.
Reaction steps are sometimes too complex to follow in detail. In
such cases, studying the energy of a particular reaction may elucidate
whether or not any intermediate reaction is produced. Energy in the
form of heat must be provided to the reactants to enable the necessary
bonds to be broken. The reactant molecules become activated because
of their greater energy content. This change can be referred to as the
activated complex or transition state, and can be represented by the
curve of Figure 1-1. The complex is the least stable state through

1