PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 327

SubstanceXis an intermediate for which the Bodenstein principle applies [see9.5.5]

dcX

dτ

=k 1 cAcK−k 2 cX−k 3 cXcB= 0 =⇒ cX=

k 1

k 2 +k 3 cB

cAcK.

For the rate of formation of product P we have the kinetic equation

dcP

dτ

=k 3 cXcB=

k 1 k 3

k 2 +k 3 cB

cAcKcB, (9.171)

from which it follows that the rate of reaction is directly proportional to the catalyst concen-
tration.
Acid-base catalysisis a special case of homogeneous catalysis in which reactions in aque-
ous solutions are catalyzed by H 3 O+and OH−ions.

9.8.3 Heterogeneous catalysis

The reversible reaction
reactants ⇀↽ products,

during which the reactants are in the gas or liquid phase and the catalyst is in the solid phase,
can be divided into the following five consecutive processes:

1. Transport of the reactants toward the catalyst surface.


2. Adsorption of the reactants on the catalyst surface.


3. Chemical reaction on the catalyst surface.


4. Desorption of the products from the catalyst surface.


5. Transport of the product from the catalyst surface.
   The description of the whole process can be simplified by writing a kinetic equation solely
   for the rate-determining process and considering the other processes to be equilibrium. In
   consecutive processes, the slowest process is the rate-determining process [see9.5.4]. It is
   usually a chemical reaction.

9.8.3.1 Transport of reactants

Starting substances are transported toward the surface and products from the surface by way
ofdiffusion. If the diffusion is faster than adsorption (desorption) and chemical reaction, it is