An abbreviated description of the rate law follows. If you're unfamiliar with rates of chemical
reactions, you may want to visit the Kinetics SparkNote for a full explanation.
The Rate-Limiting Step
Almost all reactions consist of discrete steps. Consider the reaction of A to B. The reaction must go
through intermediates B and C in order to get to D. Notice that the rate of steps A to B and C
to D are much greater than that of B to C. The reaction will bottleneck at B to C, and thus the overall
rate of the reaction can never be greater than the rate of B to C. Thus B to C is the rate-limiting
step. When you measure the rate of a reaction, you are in fact measuring the rate-limiting step.
The rate law is a mathematical equation that describes the rate of the overall reaction and, by
correspondence, the rate-limiting step. The rate law has great power because it describes what molecules
are present in the rate-limiting step.
The Rate Equation
The rate law of the above reaction is:
is a constant determined by the reaction and conditions. The values of a and b are determined by
varying the concentrations of X and Y. For example, if the concentration of X is doubled while the
concentration of Y is constant, and the rate quadrupl es, then a must equal two. Likewise, if in a
separate experiment the concentration of Y doubles and the concentration of X stays the same, and the
rate does not change, than b must equal zero. Thus it appears that two molecules of X and no molecules
of Y are involved in the rate-limiting step.
For substitution and elimination reactions, the values of a and b are zero or one. The sum of a and b is
the reaction order. Substitution and elimination reactions have orders of one and two.
An Energetic Approach
Let's take the formation of C from A through the intermediate B:
Here's a hypothetical plot of reaction coordinate vs. energy of the reaction:
The activation energy (
) of A to B is much greater than the activation energy of B to C.
Fewer molecules of A will gain enough energy to surmount the hump to B than molecules of B to C per unit
time. This indicates that under most circumstan ces the rate of A to B is less than the rate of B to C.