Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 6

Big Idea #4: Chemical Reactions and their Rates


As we mentioned earlier in the book, a catalyst increases the rate of a chemical reaction without being consumed in the process; catalysts do not appear in the balanced equation. In some cases, a catalyst is a necessary part of a reaction because in its absence, the reaction would proceed at too slow of a rate to be at all useful.

When looking at elementary steps, a catalyst is present both before and after the overall reaction. Catalysts cancel out of the overall reaction but are present in elementary steps.

Example: A + B → C

  I. A + X → Y

II. B + Y → C + X

A catalyst increases the
rate of a chemical reaction
by providing an alternative
reaction pathway with a
lower activation energy.

In the above example, X is a catalyst and Y is a reaction intermediate.

When a catalyst is introduced to a reaction, the ensuing reaction is said to undergo catalysis. There are many types of catalysis. One of the most common is surface catalysis, in which a reaction intermediate is formed as in the example above. Another is enzyme catalysis, in which the catalyst binds to the reactants in a way to reduce the overall activation energy of the reaction. Enzymes are very common in biological applications. Finally, in acid-base catalysis, reactants will gain or lose protons in order to change the rate of reaction. Acids and bases will be studied in more depth in Big Idea #6.