Physical Chemistry , 1st ed.

atoms are not arranged properly as they collide. In the other case, orientation

factors are more favorable, and the reaction might proceed (if all other factors

are favorable). Steric factors include not just the size of the collisional cross sec-

tion (discussed in the previous chapter) but also considerations of reactant

structure and geometry. A lot of current research, using molecular beams, is

trying to determine the exact orientational relationships that promote a reac-

tion that forms products. This description of simple chemical reactions is

called collision theoryand is a basic theoretical model of the kinetics of chem-

ical reactions.

20.7 Mechanisms and Elementary Processes

Consider the following gas-phase reaction:

2H 2 (g) O 2 (g) →2H 2 O (g) (20.57)

Does the reaction really proceed like this at the molecular level? No, it does not:

we recognize the above chemical reaction as simply the overall balanced chem-

ical reaction. At the molecular level the individual reactant molecules are in-

teracting in completely different ways; it’s just that overallthey react to yield

the above balanced reaction.

The individual steps in any general chemical reaction are called elementary

processes.The overall combination of sequential elementary processes, which

collectively yields the balanced chemical reaction, is called the mechanismof

the reaction. Although balanced chemical reactions are usually easy to deter-

mine, mechanisms of chemical reactions are much more difficult because the

elementary processes are usually very quick and involve unstable species—

transition states, for example—whose existences are difficult to determine,

much less measure.

Because we cannot follow individual molecules from beginning (reactants)

to end (products), it is very difficult to provea mechanism for a chemical

706 CHAPTER 20 Kinetics

(a)

(b)

H

Cl

Na

H

Cl

Na

H

Cl

Na

H

Cl

Na

H

Cl

Na

H

Cl

Na

Figure 20.17 A simple example of how steric factors influence the probability of reactions

occurring. (a) A sodium atom approaches an HCl molecule, but the orientations are not con-

ducive for a reaction to occur, so after colliding they simply go on their way. (b) Here the orien-

tation of the HCl is more conducive to reaction, so upon collision the Cl atom bonds with the

Na atom and new product species are formed.