Page 60 - Introduction to chemical reaction engineering and kinetics

Chapter 3

Experimental Methods in

Kinetics: Measurement

of Rate of Reaction

The primary use of chemical kinetics in CRE is the development of a rate law (for a
simple system), or a set of rate laws (for a kinetics scheme in a complex system). This
requires experimental measurement of rate of reaction and its dependence on concen-
tration, temperature, etc. In this chapter, we focus on experimental methods themselves,
including various strategies for obtaining appropriate data by means of both batch and
flow reactors, and on methods to determine values of rate parameters. (For the most
part, we defer to Chapter 4 the use of experimental data to obtain values of parameters
in particular forms of rate laws.) We restrict attention to single-phase, simple systems,
and the dependence of rate on concentration and temperature. It is useful at this stage,
however, to consider some features of a rate law and introduce some terminology to
illustrate the experimental methods.

3.1 FEATURES OF A RATE LAW: INTRODUCTION

3.1.1 Separation of Effects

In the general form of equation 1.4-5 (for species A in a reaction), we first assume that
the effects of various factors can be separated as:

rA = r~(conc.)r~(temp.)r~(cut. activity). . . (3.1-1)

This separation is not always possible or necessary, but here it means that we can focus
on individual factors explicitly in turn. In this chapter, we consider only the first two
factors (concentration and temperature), and introduce others in subsequent chapters.

3.1.2 Effect of Concentration: Order of Reaction

For the effect of concentration on r,, we introduce the concept of “order of reaction.”
The origin of this lies in early investigations in which it was recognized that, in many
cases, the rate at a given temperature is proportional to the concentration of a reactant
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