Physical Chemistry , 1st ed.

The individual steps in the mechanism are proposed to be

Cl 2 →2 Cl

ClCH 4 →HCl CH 3 

CH 3 Cl 2 →CH 3 Cl Cl

ClCH 4 →HCl CH 3 

CH 3 Cl 2 →CH 3 Cl Cl

and so on

Again, you can see that some of the products of each individual elementary
process violate our normal rules of valence, but these are meant to be short-
lived, intermediate chemical species. Some of the product species are free
radicals, and others are the ultimate products of the reaction. Overall, the
chemical reaction is

CH 4 Cl 2 →CH 3 Cl HCl

Again, as it should be.
Once a reaction has been broken down into its hypothetical individual
steps, a normal question to ask is how fast these steps can go. Presumably, the
overall reaction’s rate will be dependent on the rates of the individual elemen-
tary processes. This is certainly the case, and modern chemical experimenta-
tion has actually gotten to the point that rates of certain elementary processes
can be probed individually (for example, using ultrafast laser experiments).
Knowing the rates of these individual steps is enormously helpful in under-
standing the rates of the net reaction.
There is a useful point, however: an overall reaction can go only as fast as its
slowest step. The elementary process that has the slowest rate is the one that con-
trols the rate of the overall chemical reaction. Steps before it get backed up, and
steps after it go faster and deplete their reactants as fast as they are made. It’s like
a slow driver on a one-lane road. Cars behind the driver (like the steps before the
slowest step) are backed up, and cars in front of that driver (like the steps after
the slowest step) can speed away. It is the same idea with individual elementary
processes. Because of this, the elementary process that controls the rate of the
overall reaction is called the rate-determining step(or RDS for short).
In some cases, the RDS is the initial elementary process in a mechanism. If
that’s the case, then the rate law of the entire reaction is simple: it is just the
rate law as dictated by the stoichiometry of the first elementary process.
(Remember, although rate laws of overall reactions aren’t necessarily related to
the stoichiometry of the reaction,they are for elementary processes.) Suppose,
for example, that the formation of water from H 2 and O 2 has an RDS that is
the first elementary process in the mechanism. That process is

H 2 O 2 →2OH

In Example 20.10, we showed that the rate of this elementary process is given by

rate k[H 2 ][O 2 ]

where kis some rate constant. However, if we can show that this step is the
rate-determiningstep, then the rate for the entire reactionis

rate k[H 2 ][O 2 ]

which is the same rate law.

20.7 Mechanisms and Elementary Processes 709