Page 370 - Elements of Chemical Reaction Engineering Ebook

Sec. 7.1 Fundamentals 34 1

kl
(CH3)2N2 + (CH3)2N2 (CH3)2N2 -k [(CH3)2N21* (7-5)
This activation can occur when translational kinetic energy is transferredl into
energy stored in internal degrees of freedom, particularly vibrational de,grees
of freed~m.~ An unstable molecule (Le., active intermediate) is not formed
solely as a consequence of the molecule moving at a high velocity (high trans-
lational kinetic energy). The energy must be absorbed into the chemical blonds
where high-amplitude oscillations will lead to bond ruptures, molecular rear-
rangement, and decomposition. In the absence of photochemical effects or
similar phenomena, the transfer of translational energy to vibrational energy to
Properties of an produce an active intermediate can occur only as a consequence of molelcular
active intermediate collision or interaction. Other types of active intermediates that can be foimed
A*
arefree radicals (one or more unpaired electrons, e.g., H.), ionic intermediates
(e.g., carbonium ion), and enzyme-substrate complexes, to mention a few.
In Lindemann's theory of active intermediates, decomposition of' the
intermediate does not occur instantaneously after internal activation of the
molecule; rather, there is a time lag, although infinitesimally small, during
which the species remains activated. For the azomethane reaction, the active
intermediate is formed by the reaction

Because the reaction is elementary, the rate of formation of the active inteime-
diate in Equation (7-5) is

Nonelementary rAZ0*(7-S) = klCkO (7-6)
reaction is seen as
a sequence of where
elementary
reactions AZO [(CH3)2N2]
There are two reaction paths that the active intermediate (activated complex)
may follow after being formed. In one path the activated molecule may
become deactivated through collision with another molecule,

k2
[(CH3)12N21* + (CH3)2N2 --+ (CH3)2N2 + (CH3)2N2 ('7-7)
with

r~zoq-7) = -k2CmoC~za* ('7 - 8)
This reaction is, of course, just the reverse reaction of that given by Equation
(7-5). In the alternative path the active intermediate decomposes spontaneously
to form ethane and nitrogen:
k3
[(CH3)2N21" > C2H6 + N2 (7-9)
(7-10)

W. J. Moore, PhysicuE Chemistry, 5th ed., Prentice Hall, Upper Saddle River, N.J., 1972.