5 Steps to a 5 AP Chemistry 2019

202 ❯ STEP 4. Review the Knowledge You Need to Score High

Catalysts

A catalyst is a substance that speeds up the rate of reaction without being consumed in the

reaction. A catalyst may take part in the reaction and even be changed during the reaction,

but at the end of the reaction, it is at least theoretically recoverable in its original form. It

will not produce more of the product, but it allows the reaction to proceed more quickly.

In equilibrium reactions (see the chapter on Equilibrium), the catalyst speeds up both the

forward and reverse reactions. Catalysts speed up the rates of reaction by providing a dif-

ferent mechanism that has a lower activation energy. The higher the activation energy of

a reaction, the slower the reaction will proceed. Catalysts provide an alternative pathway

that has a lower activation energy and thus will be faster. In general, there are two distinct

types of catalyst.

Homogeneous Catalysts

Homogeneous catalysts are catalysts that are in the same phase or state of matter as the

reactants. They provide an alternative reaction pathway (mechanism) with a lower activa-

tion energy.

The decomposition of hydrogen peroxide is a slow, one-step reaction, especially if the

solution is kept cool and in a dark bottle:

2 H 2 O 2 → 2 H 2 O + O 2

However, if ferric ion is added, the reaction speeds up tremendously. The proposed

reaction sequence for this new reaction is:

2 Fe^3 + + H 2 O 2 → 2 Fe^2 + + O 2 + 2 H+

2 Fe^2 + + H 2 O 2 + 2 H+ → 2 Fe^3 + + 2 H 2 O

Notice that in the reaction the catalyst, Fe^3 +, was reduced to the ferrous ion, Fe^2 +,

in the first step of the mechanism, but in the second step it was oxidized back to the ferric

ion. Overall, the catalyst remained unchanged. Notice also that although the catalyzed reac-

tion is a two-step reaction, it is significantly faster than the original uncatalyzed one-step

reaction.

Heterogeneous Catalysts

A heterogeneous catalyst is in a different phase or state of matter from the reactants.

Most commonly, the catalyst is a solid and the reactants are liquids or gases. These catalysts

lower the activation energy for the reaction by providing a surface for the reaction, and

also by providing a better orientation of one reactant so its reactive site is more easily hit

by the other reactant. Many times these heterogeneous catalysts are finely divided metals.

The Haber process, by which nitrogen and hydrogen gases are converted into ammonia,

depends upon an iron catalyst, while the hydrogenation of vegetable oil to margarine uses

a nickel catalyst.

Experiments

Unlike other experiments, a means of measuring time is essential to all kinetics experi-

ments. This may be done with a clock or a timer. The initial concentration of each reac-

tant must be determined. Often this is done through a simple dilution of a stock solution.

STRATEGY