Page 172 - Introduction to chemical reaction engineering and kinetics

Chapter 7

Homogeneous Reaction

Mechanisms and Rate Laws

This chapter provides an introduction to several types of homogeneous (single-phase)
reaction mechanisms and the rate laws which result from them. The concept of a re-
action mechanism as a sequence of elementary processes involving both analytically
detectable species (normal reactants and products) and transient reactive intermedi-
ates is introduced in Section 6.1.2. In constructing the rate laws, we use the fact that the
elementary steps which make up the mechanism have individual rate laws predicted by
the simple theories discussed in Chapter 6. The resulting rate law for an overall reaction
often differs significantly from the type discussed in Chapters 3 and 4.
There are several benefits which arise from knowledge of the reaction mecha-
nism. The first benefit of practical value is that the functional form of the rate law
derived from the correct mechanism is more precise, enabling better reactor mod-
eling and optimization, and more confident extrapolation to conditions outside the
database. The second benefit is that a better understanding of the mechanism reveals
the steps in the mechanism which limit the overall rate or selectivity in the reac-
tion, and thus provides guidance to improve the process. Important examples where
knowledge of the reaction mechanisms is critical can be found (1) in atmospheric-
chemistry models, including the stratospheric ozone problem, air pollution, and ni-
trogen oxide formation in combustion, and (2) in an industrial process like ethane
dehydrogenation, where detailed molecular models of the free-radical chemistry are re-
quired to predict the influence of feed composition and reactor parameters on product
selectivity.
Constructing a reaction mechanism is a way of modeling a chemical reaction. There
is no fixed set of rules to follow, but a proposed mechanism must be consistent with
the overall stoichiometry and observed rate law. It is difficult to verify the mechanism
of a given reaction. Testing the predicted rate laws against observations is a key step
in gaining confidence in a proposed mechanism, but proof requires identifying the re-
action intermediates (often in very small concentrations) under reaction conditions,
or measurements of the kinetics of all the individual elementary reactions involving all
the intermediates. Other techniques used to provide information about reaction mecha-
nisms include isotope-substitution and stereochemical studies. Rate constants for many
elementary chemical reactions have been measured. Despite the difficulty, an incom-
plete or imprecise mechanism which contains the essence of the reaction pathways is
often more valuable than a purely empirical kinetics rate law.
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