KEY2CHEM

Types of Catalysts

The function of a catalyst is to increase the rate of the reaction by lowering the activation energy. This can take a variety of forms. An acid-base catalyst functions by allowing a reactant to either gain or lose a proton, which increases the rate of the reaction. A surface catalyst either allows the formation of a new intermediate or increases the collision frequency between reacting particles. An enzyme catalyst binds reactant particles in a specific way that lowers the activation energy (potentially via a reaction intermediate), which increases the reaction rate.

Example 1.

In the reaction below for the dehydration of an alcohol to an alkene, $$\require{mhchem}\ce{H+}$$ functions as an _______.

A. acid-base catalyst

B. surface catalyst

C. enzyme catalyst

Solution

A. acid-base catalyst

$$\require{mhchem}\ce{H+}$$ donates a proton to catalyze the dehydration reaction. The reaction is very slow in the absence of the acid.

Example 2.

Hydrogenation (addition of $$\require{mhchem}\ce{H2}$$) to carbon-carbon double bonds in alkenes results in the formation of carbon-carbon single bonds (alkanes). If the reaction is catalyzed on solid platinum metal, what type of catalyst is the platinum?

A. acid-base catalyst

B. surface catalyst

C. enzyme catalyst

Solution

B. surface catalyst

The platinum metal provides an alternate reaction mechanism for the hydrogenation reaction to proceed.

Example 3.

Sucrose (table sugar) can be hydrolyzed into its monosaccharide components: glucose and fructose. The reaction in aqueous solutions is very slow, but adding invertase (derived from yeast) increases the rate of the reaction. Invertase is an example of a(n) _______.

A. acid-base catalyst

B. surface catalyst

C. enzyme catalyst

Solution

C. enzyme catalyst

An enzyme catalyst functions by binding the reactants in a specific way to increase the rate of the reaction by lowering the activation energy (often by formation of a different reaction intermediate).