The Foundations of Chemistry

We can describe two categories of catalysts: (1) homogeneous catalysts and (2) hetero-
geneous catalysts.

Homogeneous Catalysis

A homogeneous catalystexists in the same phase as the reactants. Ceric ion, Ce^4
, at
one time was an important laboratory oxidizing agent that was used in many redox
titrations (Section 11-8). For example, Ce^4
oxidizes thallium(I) ions in solution; this reac-
tion is catalyzed by the addition of a very small amount of a soluble salt containing
manganese(II) ions, Mn^2
. The Mn^2
acts as a homogeneous catalyst.

Mn^2 


2Ce^4 

Tl
8888n2Ce^3 

Tl^3 

This reaction is thought to proceed by the following sequence of elementary steps.

Ce^4 

Mn^2 
88nCe^3 

Mn^3 
 step 1

Ce^4 

Mn^3 
88nCe^3 

Mn^4 
 step 2

Mn^4 

Tl
 88nMn^2 

Tl^3 
 step 3

2Ce^4 

Tl
 88n2Ce^3 

Tl^3 
 overall

Some of the Mn^2
catalyst reacts in step 1, but an equal amount is regenerated in step 3
and is thus available to react again. The two ions shown in blue, Mn^3
and Mn^4
, are
reaction intermediates.Mn^3
ions are formed in step 1 and consumed in an equal amount
in step 2; similarly, Mn^4
ions are formed in step 2 and consumed in an equal amount in
step 3.
We can now summarize the species that can appear in a reaction mechanism.

1.Reactant:More is consumed than is formed.

2.Product:More is formed than is consumed.

3.Reaction intermediate:Formed in earlier steps, then consumed in an equal amount

in later steps.

4.Catalyst:Consumed in earlier steps, then regenerated in an equal amount in later

steps.

Strong acids function as homogeneous catalysts in the acid-catalyzed hydrolysis of esters
(a class of organic compounds—Section 27-14). Using ethyl acetate (a component of nail
polish removers) as an example of an ester, we can write the overall reaction as follows.

This is a thermodynamically favored reaction, but because of its high energy of activa-
tion, it occurs only very, very slowly when no catalyst is present. In the presence of strong
acids, however, the reaction occurs more rapidly. In this acid-catalyzed hydrolysis, different
intermediates with lower activation energies are formed. The sequence of steps in the
postulatedmechanism follows.

CH 3 C OCH 2 CH 3 (aq)

O

H 2 O

H



 CH 3 C OH(aq) CH 3 CH 2 OH(aq)

O

ethyl acetate acetic acid ethanol

“Hydrolysis” means reaction with

water.

16-9 Catalysts 689