Page 63 - Modeling of Chemical Kinetics and Reactor Design

Reaction Mechanisms and Rate Expressions 33

reaction must be the same as that for the forward reaction.
Second, the reverse reaction cannot have a molecularity greater
than three, just as the forward reaction is so limited. As a
consequence, the ammonia decomposition reaction 2NH → N 2
3
+ 3H cannot occur as a simple bimolecular process. Third, if
2
the reaction rate expression for the forward reaction consists
of two or more independent terms corresponding to parallel
reaction paths, there will be the same number of independent
terms in the rate expression for the reverse reaction. At equili-
brium, both the total rate of the forward reaction is equal to
the total rate of the reverse reaction, and the forward rate of
each path is equal to the reverse rate of that particular path.
8. Transitory intermediates involving highly reactive species do
not react preferentially with one another to the exclusion of
their reaction with stable species.
9. When the overall order of a reaction is greater than three, the
mechanism probably has one or more equilibria and inter-
mediates prior to the rate-determining step.
10. Inverse orders arise from rapid equilibria prior to the rate-
determining step.
11. Whenever a rate law contains non-integers orders, there are
intermediates present in the reaction sequence. When a frac-
tional order is observed in an empirical rate expression for a
homogeneous reaction, it is often an indication that an important
part of the mechanism is the splitting of a molecule into free
radicals or ions.
12. If the magnitude of the stoichiometric coefficient of a reactant
exceeds the order of the reaction with respect to that species,
there are one or more intermediates and reactions after the rate-
determining step. Before applying this rule, the stoichiometric
equation must be formulated for the reaction such that all
coefficients are integers.
13. If the order of a reaction with respect to one or more species
increases as the concentration of those species increases, it is
an indication that the reaction may be proceeding by two or
more parallel paths.
14. If there is a decrease in the order of a reaction with respect to
a particular substance as the concentration of that species
increases, the dominant form of that species in solution may
be undergoing a change caused by the change in concentration.