Barrons SAT Subject Test Chemistry, 13th Edition

Figure 16. A Possible Laboratory Preparation of Oxygen

TIP

A catalyst speeds up the rate of reaction by lowering the activation energy needed for the

reaction. A catalyst is not consumed.

In this preparation manganese dioxide (MnO 2 ) is often used. This compound

is not used up in the reaction and can be shown to have the same composition as it
had before the reaction occurred. The only effect it has is that it lowers the
temperature needed to decompose the KClO 3 , and thus speeds up the reaction.

Substances that behave in this manner are referred to as catalysts. The
mechanism by which a catalyst acts is not completely understood in all cases, but
it is known that in some reactions the catalyst does change its structure
temporarily. Its effect is shown graphically in the reaction graphs in Figure 17.

Figure 17. Effect of Catalyst on Reaction