Page 64 - Modeling of Chemical Kinetics and Reactor Design

34 Modeling of Chemical Kinetics and Reactor Design

A decrease in reaction order with respect to hydrogen ion
concentration with increasing acidity has frequently been observed
for reactions involving weak acids.

TESTING KINETIC MODELS

Testing kinetic models involves the following procedures:

1. Propose possible mechanisms involving elementary steps.
2. Assume that the rate equations for each elementary step can be
written by inspection of the stoichiometric equation:

A + B * → AB * (1-140)

or

*
− ( r A ) =− ( r B ) = kC C * B (1-141)
A
3. Assume that after a short initial period that the rates of con-
centration change of all active centers are zero. For example:

r
A + B  → AB * (1-142)

*
k 1

AB + A  → A + B * (1-143)
*
k 2
2 r
( −r B * ) = k C C B * (1-144)
A
1
( +r B * ) = k C C B * (1-145)
2
A
The net rate of disappearance is

( −r B * ) net =− dC B = kC C B − k C C AB * (1-146)
1
A
2
A
dt
From the above assumption, (–r ) = 0, steady state approximation.
B* net
This assumption derives rate equations from which terms
involving the concentrations of active centers can be eliminated.