Page 40 - Modeling of Chemical Kinetics and Reactor Design

10 Modeling of Chemical Kinetics and Reactor Design

1
− ( r ) =− ( r ) = ( +r ) (1-39)
2
H 2 Br 2 HBr
and, if the reaction took place in a single step, then the rate expression
may be represented as:

+ ( r HBr ) = k C H 2 C Br 2 (1-40)

The slope of a concentration-time curve to define the rate expression
can be determined. However, experimental studies have shown the reac-
tion cannot be described by simple kinetics, but by the relationship:

kC C 05 .
1
+ ( r )= H 2 Br 2 (1-41)
HBr C
k 2 + HBr
C
Br 2
where k and k are the rate constants. The reaction between hydrogen
1
2
and bromine is an example of a non-elementary reaction. The follow-
ing steps account for the rate expression:
Br → 2 Br * (1-42)
2

Br + H → HBr H * (1-43)
+
*
2
H + HBr → Br + H (1-44)
*
*
2
*
*
Br and H being highly reactive intermediates.
TYPES OF INTERMEDIATE

STABLE INTERMEDIATES

Stable intermediates are those where concentration and lifespan are
comparable to those of stable reactants and products. An example is
the reaction between methane and oxygen in the gas phase at 700 K
and 1 atmosphere. The overall reaction is:

CH + 2 O → CO + 2 H O (1-45)
4 2 2 2