Untitled - Kelly Walsh High School

198 CHEMISTRY FOR THE UTTERLY CONFUSED
Each individual reaction in the mechanism is an elementary step or reaction.
Each of these reaction steps has its own individual rate of reaction. One of the
reaction steps is slower than the rest and is the rate-determining step. This ratedetermining
step limits how fast the overall reaction can occur. Therefore, the
rate law of the rate-determining step is the rate law of the overall reaction.
It is possible to determine the rate equation for an elementary step directly
from the stoichiometry. This will not work for the overall reaction. The reactant
coefficients in an elementary step become the reaction orders in the rate equation
for that elementary step.
Many times a study of the kinetics of a reaction gives clues to the reaction
mechanism. For example, consider the following reaction:
NO2(g) CO(g) l NO(g) CO2(g) It has been determined experimentally that the rate law for this reaction is: Rate
k[NO 2] 2
The rate law indicates that the reaction does not occur with a simple collision
between NO 2 and CO. The reaction might follow this mechanism:
NO2(g) NO2(g) l NO3(g) NO(g)
NO3(g) CO(g) l NO2(g) CO2(g) Notice that if you add these two steps together, you get the overall reaction.
We have determined that the first step is the slow step in the mechanism, the
rate-determining step. If we write the rate law for this elementary step it is:
Rate k[NO 2] 2 , identical to the experimentally determined rate law for the
overall reaction.
Note that both of the steps in the mechanism are bimolecular reactions, reactions
that involve the collision of two chemical species. Unimolecular reactions
are reactions in which a single chemical species decomposes or rearranges. Both
bimolecular and unimolecular reactions are common, but the collision of three
or more chemical species (termolecular) is quite rare. Thus, in developing or
assessing a mechanism, it is best to consider only unimolecular or bimolecular
elementary steps.
13-7 Utterly Confused About Rate
Law and Half-Life (t 1/2)
In order to work problems of this type it is important to know what the order
of the reaction is. This may come directly from the problem, for example, “the