Page 133 - Introduction to chemical reaction engineering and kinetics

Chapter 6

Fundamentals of

Reaction Rates

In the preceding chapters, we are primarily concerned with an empirical macroscopic
description of reaction rates, as summarized by rate laws. This is without regard for any
description of reactions at the molecular or microscopic level. In this chapter and the
next, we focus on the fundamental basis of rate laws in terms of theories of reaction
rates and reaction “mechanisms.”
We first introduce the idea of a reaction mechanism in terms of elementary reaction
steps, together with some examples of the latter. We then consider various aspects of
molecular energy, particularly in relation to energy requirements in reaction. This is
followed by the introduction of simple forms of two theories of reaction rates, the col-
lision theory and the transition state theory, primarily as applied to gas-phase reactions.
We conclude this chapter with brief considerations of reactions in condensed phases,
surface phenomena, and photochemical reactions.

6.1 PRELIMINARY CONSIDERATIONS

6.1.1 Relating to Reaction-Rate Theories
As a model of real behavior, the role of a theory is twofold: (1) to account for ob-
served phenomena in relatively simple terms (hindsight), and (2) to predict hitherto
unobserved phenomena (foresight).
What do we wish to account for and predict? Consider the form of the rate law used
for the model reaction A + . . . + products (from equations 3.1-8 and 4.1-3):

(-rA) = A exp(-E,lRT) fi$’ (6.1-1)
i = l

We wish to account for (i.e., interpret) the Arrhenius parameters A and EA, and the
form of the concentration dependence as a product of the factors cp’ (the order of re-
action). We would also like to predict values of the various parameters, from as simple
and general a basis as possible, without having to measure them for every case. The
first of these two tasks is the easier one. The second is still not achieved despite more
than a century of study of reaction kinetics; the difficulty lies in quantum mechanical
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