Page 62 - Modeling of Chemical Kinetics and Reactor Design

32 Modeling of Chemical Kinetics and Reactor Design

GUIDELINES TO FORMULATING
REACTION MECHANISM

Guidelines have been proposed within which the kineticist works
in developing a reaction mechanism. Edward et. al [9] have collated
the most extensive collection of guidelines (see Chapter 3), which
enable a kineticist to judge the feasibility of a proposed mechanism.
They also allow the elimination of a proposed sequence of reactions
as being unreasonable from the experimental data. The following are
the guidelines to formulating reaction mechanism.

1. The most fundamental basis for mechanistic speculation is a
complete analysis of the reaction products. It is essential to
obtain a complete quantitative and qualitative analysis for all
products of the reaction. Such analyses can provide essential
clues as to the identity of the reaction intermediates.
2. The atomic and electronic structure of the reactants and products
may point to the nature of possible intermediate species. The
structural arrangement of atoms in the molecules that react must
correspond at the instant of reaction to interatomic distances
appropriate for the formation of new species.
3. All of the elementary reactions involved in a mechanistic
sequence must be feasible with respect to bond energies. Bond
energy considerations also may conclude that highly endothermic
elementary reactions will be slow processes because of the large
activation energies normally associated with these reactions.
4. The number of elementary reactions employed must be suf-
ficient to provide a complete path for the formation of all
observed products.
5. All of the intermediates produced by the elementary reactions
must be consumed by other elementary reactions so that there
will be no net production of intermediate species.
6. The majority of known elementary steps are bimolecular, the
remainder being unimolecular or termolecular. The ammonia
synthesis reaction is known to occur by a number of steps
rather than as N + 3H → 2NH .
2
2
3
7. A postulated mechanism for a reaction in the forward direction
must also hold true for the reverse reaction. Three corollaries
of this guideline should be borne in mind when postulating a
reaction mechanism. First, the rate-limiting step for the reverse