PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 326

9.8 Catalysis.

9.8.1 Basic terms

Acatalystis a substance whose presence changes the mechanism and consequently also the
rate of a chemical reaction. It is not consumed by the reaction but it leaves it unchanged, i.e. in
the same state of matter, or in the same crystalline modification in the case of a solid catalyst.
The catalyst does not change the composition of an equilibrium mixture.

Note:The last assertion is not entirely true. A catalyst in a larger amount may act as an

inert substance [see chapter Chemical equilibrium], and its presence changes the activities

of the reactants.

Autocatalysis is a process during which the products of a reaction catalyze its course.
An example of autocatalysis is the oxidation of oxalic acid by permanganate ions in an acidic
environment

5 (COOH) 2 + 2 MnO 4 −+ 6 H+ → 8 H 2 O + 10 CO 2 + 2 Mn2+,

catalyzed by manganous ions. The initial course of the reaction is very slow, but as soon as a
certain amount of Mn2+ions is formed, the rate of reaction increases many times.
Catalysis may be homogeneous, in which case the catalyst is in the same phase (gas
or liquid) as the reactants (substances participating in the reaction), andheterogeneous, in
which case the catalyst is usually in the solid phase and the reactants are either gaseous or liquid.
Enzyme catalysisis a special case in which the catalyst is an enzyme, a (macromolecular)
protein whose dimensions are 10 to 100 nm.

9.8.2 Homogeneous catalysis

The reaction A + B→P proceeds in the presence of catalyst K by the following mechanism:

A + K

→k^1

←

k 2

X ,

X+ B

k 3

→ K + P.