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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 4: Reaction Mechanisms

Most chemical reactions actually occur in a sequence of simple reactions instead of all at once. These intermediate reactions are called "elementary steps." Consider the following reaction:

CO + NO2 → CO2 + NO (net equation)

This reaction actually takes place in two elementary steps:

NO2 + NO2 → NO3 + NO

CO + NO3 → NO2 + CO2

The first elementary step produces NO3 and NO, while the second combines the NO3 with CO to make NO2 and CO2. Note that NO3 is produced in one step and immediately consumed by the next. We therefore call NO3 an "intermediate"—it is produced and consumed in the course of a reaction, but it does not appear in the overall equation, which is called the "net equation."

The elementary steps in a mechanism do not happen at the same rate. In the example above, the first step happens slowly and the second quickly. The slowest step is called the "rate-determining step" because it limits the rate of the overall reaction. It's similar to a production line in a factory; the slowest worker determines the overall rate of production. No matter how fast the other workers are, the slowest person will determine the rate. (Figure 12-6)

Rate-Determining Step

Figure 12-6. Rate-Determining Step

© U.S. National Archives and Records Administration.

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Rate-Determining Step

Figure 12-6. Rate-Determining Step

Chemical reactions that take place in multiple steps have their rates determined by their slowest, rate-determining step. This is the same thing that happens in an assembly line if one phase of the processes is slow. Or during rush hour, one traffic jam might have the greatest effect on the total amount of time it takes to get home.

Knowing which step is the slowest is important if we want to speed up the reaction. In the reaction above, recall that the first step—in which two NO2 molecules react—is the slow, rate-determining step. Because this step controls the overall rate of the reaction, we should focus on it, rather than on the second step, if we wanted to speed up this reaction. One way to speed up this step (and therefore the entire reaction) would be to increase the concentration of NO2.

Looking only at the reactants in the overall reaction, NO2 and CO, would be misleading; it might appear that increasing both the reactant concentrations would speed things up. But with a thorough understanding of the underlying reaction mechanism, it is clear that increasing the CO concentration will not help speed up the first slow step. Adding more CO would speed up the second step in the mechanism, which was already fast.

To put this in more technical terms, the overall chemical equation does not determine the rate law. If we took the chemical equation at face value and assumed that the reaction simply occurred when a molecule of NO2 collides with a molecule of CO, we would conclude that the rate law would be:

Rate = k[NO2][CO]

But experimentation would show this to be wrong; the concentration of CO has no impact on the rate.

Glossary

Elementary steps

A sequence of simple chemical reactions that make up the mechanism of an overall reaction.

Intermediate

A chemical species produced by an elementary step of a chemical reaction, and then consumed by another. Intermediates do not appear in the overall chemical equation.

Net equation

A chemical equation for a reaction that lists only the reactants and products participating in the reaction.

Rate-determining step

The slowest elementary step in a reaction mechanism.

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