Page 46 - Modeling of Chemical Kinetics and Reactor Design

16 Modeling of Chemical Kinetics and Reactor Design

decompose into products. This is represented by the following reac-
tion steps:

A + B → P (1-64)
A + [ AB → (1-65)
*
B P
Transition state intermediate

The overall rate of reaction depends on the rate of decomposition of
AB* to product P.
The collision theory considers the rate to be governed by the
number of energetic collisions between the reactants. The transition
state theory considers the reaction rate to be governed by the rate of
the decomposition of intermediate. The formation rate of the intermediate
is assumed to be rapid because it is present in equilibrium concentrations.

CHAIN REACTIONS

Atoms and free radicals are highly reactive intermediates in the
reaction mechanism and therefore play active roles. They are highly
reactive because of their incomplete electron shells and are often able
to react with stable molecules at ordinary temperatures. They produce
new atoms and radicals that result in other reactions. As a consequence
of their high reactivity, atoms and free radicals are present in reaction
systems only at very low concentrations. They are often involved in
reactions known as chain reactions. The reaction mechanisms involv-
ing the conversion of reactants to products can be a sequence of
elementary steps. The intermediate steps disappear and only stable
product molecules remain once these sequences are completed. These
types of reactions are referred to as open sequence reactions because
an active center is not reproduced in any other step of the sequence.
There are no closed reaction cycles where a product of one elementary
reaction is fed back to react with another species. Reversible reactions
of the type A + B [ C + D are known as open sequence mechanisms.
The chain reactions are classified as a closed sequence in which an
active center is reproduced so that a cyclic reaction pattern is set up.
In chain reaction mechanisms, one of the reaction intermediates is
regenerated during one step of the reaction. This is then fed back to
an earlier stage to react with other species so that a closed loop or