Page 134 - Introduction to chemical reaction engineering and kinetics

116 Chapter 6: Fundamentals of Reaction Rates

calculations-not in any remaining scientific mystery. However, the current level of the-
oretical understanding has improved our ability to estimate many kinetics parameters,
and has sharpened our intuition in the search for improved chemical processes.
In many cases, reaction rates cannot be adequately represented by equation 6.1-1,
but are more complex functions of temperature and composition. Theories of reaction
kinetics should also explain the underlying basis for this phenomenon.

6.1.2 Relating to Reaction Mechanisms and Elementary Reactions
Even a “simple” reaction usually takes place in a “complex” manner involving multiple
steps making up a reaction mechanism. For example, the formation of ammonia, rep-
resented by the simple reaction N, + 3H, + 2NH,, does not take place in the manner
implied by this chemical statement, that is, by the simultaneous union of one molecule
of N, and three molecules of H, to form two of NH,. Similarly, the formation of ethy-
lene, represented by C,H, + GH, + HZ, does not occur by the disintegration of one
molecule of C,H, to form one of C,H, and one of H, directly.
The original reaction mechanism (Rice and Herzfeld, 1934) proposed for the forma-
tion of %H, from GH, consists of the following five steps?

C,H, -+ 2CHj
CH; + C,H, + CH, + GH;
GH; -+ C2H, + Ho
Ho + C,H, + H, + C,H;
Ho + GH; + C,H,

where the “dot” denotes a free-radical species.
We use this example to illustrate and define several terms relating to reaction funda-
mentals:
Elementary reaction: a chemical reaction step that takes place in a single molecular en-
counter (each of the five steps above is an elementary reaction); it involves one, two,
or (rarely) three molecular entities (atoms, molecules, ions, radicals, etc.). Only a small
number of chemical bonds is rearranged.
Reaction mechanism: a postulated sequence of elementary reactions that is consistent with
the observed stoichiometry and rate law; these are necessary but not sufficient conditions
for the correctness of a mechanism, and are illustrated in Chapter 7.
Reactive intermediate: a transient species introduced into the mechanism but not appearing
in the stoichiometric equation or the rate law; the free atomic and free radical species
Ho, CH:, and C,H: are reactive intermediates in the mechanism above. Such species
must ultimately be identified experimentally to justify their inclusion.
Molecularity of a reaction: the number of reacting partners in an elementary reaction: uni-
molecular (one), bimolecular (two), or termolecular (three); in the mechanism above,
the first and third steps are unimolecular as written, and the remainder are bimolecu-
lar. Molecularity (a mechanistic concept) is to be distinguished from order (algebraic).
Molecularity must be integral, but order need not be; there is no necessary connec-
tion between molecularity and order, except for an elementary reaction: the numbers
describing molecularity, order, and stoichiometry of an elementary reaction are all the
same.

‘In the dehydrogenation of &He to produce CzH4, CH4 is a minor coproduct; this is also reflected in the second
step of the mechanism; hence, both the overall reaction and the proposed mechanism do not strictly represent
a simple system.