9

Rates of Chemical Reactions

These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to . . .

• Explain how each of the following factors affects the rate of a chemical reaction: nature of the reactants, surface area exposed, concentrations, temperature, and the presence of a catalyst.
• Draw reaction diagrams with and without a catalyst.
• Explain the Law of Mass Action.
• Describe the relationship between reaction mechanisms and rates of reaction.

This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.

The measurement of reaction rate is based on the rate of appearance of a product or disappearance of a reactant. It is usually expressed in terms of a change in concentration of one of the participants per unit time.

Experiments have shown that for most reactions the concentrations of all participants change most rapidly at the beginning of the reaction; that is, the concentration of the products shows the greatest rate of increase, and the concentrations of the reactants the highest rate of decrease, at this point. This means that the rate of a reaction changes with time. Therefore a rate must be identified with a specific time.