Physical Chemistry Third Edition

524 12 Chemical Reaction Mechanisms I: Rate Laws and Mechanisms

12.1 Reaction Mechanisms and Elementary

Processes in Gases

Most chemical reactions occur through mechanisms that involve at least two steps. In

many cases we have asequential mechanism, with one step being completed before

the next step occurs. There are alsoconcerted mechanisms, in which two steps occur

simultaneously. The steps in a mechanism are assumed to beelementary processes.An

elementary process is one that cannot be broken down into simpler steps. We classify

an elementary process by itsmolecularity, which is the number of reactant particles

(molecules, atoms, ions, or radicals) that are involved in it. If more than one substance is

involved in an elementary process, we define the molecularity of each substance as the

number of particles of that substances involved in the step and the overall molecularity

as the sum of these molecularities.

One of the gas-phase reactions that endanger the ozone layer in the earth’s upper

atmosphere is

2NO 2 +O 3 −→ N 2 O 5 +O 2 (12.1-1)

The reaction mechanism of the forward reaction is thought to consist of the following

steps:^1

(1) NO 2 +O 3 −→NO 3 +O 2

(2) NO 3 +NO 2 −→N 2 O 5

(12.1-2)

There is onereactive intermediate,NO 3 , which is produced in one step and consumed

in the other step. Addition of the steps of this mechanism gives the stoichiometric

equation, with cancellation of NO 3. Both steps in the mechanism of Eq. (12.1-2) are

bimolecular. That is, they involve two reactant particles.Unimolecularsteps involve

a single particle.Termolecularsteps involve three particles. Termolecular processes

are relatively slow because of the small probability that three molecules will collide or

diffuse together at once, and these processes occur less frequently in mechanics than do

bimolecular processes. Elementary processes involving four or more reactant particles

probably do not occur in chemical reaction mechanisms. We now make an important

assertion concerning the rate law of any elementary process:In an elementary process,

the order of any substance is equal to the molecularity of that substance.We now justify

this assertion for bimolecular elementary processes in the gas phase.

The Collision Theory of Bimolecular Elementary

Processes in Gases

Bimolecular elementary processes involve the collisions of two molecules, which we

discussed in Chapter 9. We now show that such a process obeys a second-order rate law.

The collision rate in a gas is very large, typically several billion collisions per second

for each molecule. If every collision in a reactive mixture led to chemical reaction,

gas-phase reactions would be complete in nanoseconds. Since gas-phase reactions are

almost never this rapid, it is apparent that only a small fraction of all collisions lead to

chemical reaction. We make the important assumption:The fraction of binary collisions

(^1) H. S. Johnston and D. M. Yost,J. Chem. Phys., 17 , 386 (1949).