Physical Chemistry , 1st ed.

Both pathways react bromate ion to products, but path I has Brions as a re-

actant whereas path II has Brions as a product. Thus, when enough Bris

available, path I dominates; when [Br] is small, path II is favored. Path II then

generates Brions, so as [Br] increases, path I becomes favored,...and the

cycle goes on. Ce^4 is reduced to Ce^3 by some steps in path II, then reoxi-

dized to Ce^4 by other steps in the same pathway. As such, the reaction serves

as a Ce(III)/Ce(IV) half-reaction, and the oscillations can be followed electro-

chemically by using the BZ reaction itself as a half cell. Various indicators and

spectroscopic techniques can also be used to follow the changing concentra-

tions of the intermediates. Figures 20.22 and 20.23 show diagrams of how con-

centrations change during the course of a BZ reaction.

Why are chemists interested in oscillating reactions? Not only are they in-

teresting from a kinetic perspective, but some important chemical reactions are

oscillatory in nature. Among the most important ones are the reactions that

cause a heart to beat. Specific chemical processes promote electrochemical re-

sponses that cause the heart to contract, pump blood—and keep us alive. An

understanding of oscillating reactions thus yields a better understanding of the

biochemistry of complex living systems.

20.10 Transition-State Theory

Not all kinetics is phenomenological. In recent years there have been advances

in understanding the kinetics of reactions from a theoretical perspective. In

this section, we will review the basics of some theoretical kinetics.

Collision theory is a simple description of reacting molecules that treats

them as hard spheres. Some of the basic concepts of collision theory were con-

sidered at the end of section 20.6. Although this model does predict some nu-

merical reaction parameters having about the right order of magnitude, its de-

scription of molecules as hard spheres and use of steric factors as “fudge

factors” ignores the complex nature of even simple molecular reactions. A

more realistic approach is necessary.

Transition-state theory(sometimes called activated-complex theory) is a

more realistic model of a bimolecular elementary step in a reaction. It takes

20.10 Transition-State Theory 719

1  10 ^4

t (s)

750

[Br

] M

1  10 ^7

150

ln[CeIII]

[CeIV]

300 450 600

1  10 ^5

1  10 ^6

Figure 20.22 The oscillating nature of the Belousov-Zhabotinsky reaction can be illustrated

by the varying concentrations of some of the species involved. Many of these concentrations are

measured electrochemically.Source:Reprinted with permission from the Journal of the American

Chemical Society,Vol. 94, No. 25, p. 8651.

Richard Magna/Fundamental Photographs
Figure 20.23 Under certain conditions, oscil-
lating reactions can produce different colors over
time, leading to some fascinating visual displays.