The Foundations of Chemistry

the appropriate orientation could allow the new IXC bond to form at the same time that

the CXCl bond is breaking. This collection of atoms, which we represent as

is what we call the transition state of this reaction (Figure 16-11a). From this state, either

of two things could happen: (1) the IXC bond could finish forming and the CXCl bond

could finish breaking with Clleaving, leading to products, or (2) the IXC bond could

fall apart with Ileaving and the CXCl bond re-forming, leading back to reactants.

REACTION MECHANISMS AND THE

RATE-LAW EXPRESSION

The step-by-step pathway by which a reaction occurs is called its mechanism.Some reac-

tions take place in a single step, but most reactions occur in a series of elementary steps.

The reaction orders for any single elementary stepare equal to the coefficients for that

step.

In many mechanisms, however, one step is much slower than the others.

A reaction can never occur faster than its slowest step.

This slow step is called the rate-determining step.The speed at which the slow step

occurs limits the rate at which the overall reaction occurs.

As an analogy, suppose you often drive a distance of 120 miles at the speed limit of 60

mi/h, requiring 2 hours. But one day there is an accident along the route, causing a slow-

down for several hours. After passing the accident scene, you resume the posted speed of

60 mi/h. If the total time for this trip was 4 hours, then the averagespeed would be only

120 miles/4 hours, or 30 mi/h. Even though you drove for many miles at the same high

speed, 60 mi/h, the overall rate was limited by the slow step, passing the accident scene.

The balanced equation for the overall reaction is equal to the sum of allthe individual

steps, including any steps that might follow the rate-determining step. We emphasize again

that the rate-law exponents do not necessarily matchthe coefficients of the overallbalanced

equation.

For the general overall reaction

aA
bB88ncC
dD

the experimentally determined rate-law expression has the form

ratek[A]x[B]y

The values of xand yare related to the coefficients of the reactants in the slowest

step, influenced in some cases by earlier steps.

16-7

I C

H H

H

Cl

We can view this transition state as
though carbon is only partially bonded
to I and only partially bonded to Cl.

680 CHAPTER 16: Chemical Kinetics

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 15.12, Reaction Mechanisms.