Page 373 - Elements of Chemical Reaction Engineering Ebook

344 Nonelementary Reaction Kinetics Chap. 7

in which case the rate expression follows first-order kinetics,

- kl k3
'N2 - - cAZO = kCAZO (7-20)
k2
Apparent reaction In describing reaction orders for this equation one would say the reaction
orders is apparent Brst-order at high azomethane concentrations and apparent sec-
ond-order at low azomethane concentrations.

7.2 Searching for a Mechanism

In many instances the rate data are correlated before a mechanism is found. It
is a normal procedure to reduce the additive constant in the denominator to 1.
We therefore divide the numerator and denominator of Equation (7-18) by k3
to obtain

(7-21)

7:2.1 General Considerations

The rules of thumb listed in Table 7-1 may be of some help in the devel-
opment of a mechanism that is consistent with the experimental rate law. Upon
application of Table 7-1 to the azomethane example just discussed, we see
from rate equation (7- 18) that:
1. The active intermediate, AZO*, collides with azomethane, AZO
[Equation (7-7)], resulting in the appearance of the concentration of
AZO in the denominator.
2. AZO* decomposes spontaneously [Equation (7-9)], resulting in a con-
stant in the denominator of the rate expression.
3. The appearance of AZO in the numerator suggests that the active
intermediate AZO* is formed from AZO. Referring to Equation (7-5),
we see that this case is indeed true.

TABLE 7-1, RULES OF THUMB FOR DEVELOPMENT OF A MECHANISM
1. Species having the concentration(s) appearing in the denominator.of the rate law probably col-
lide with the active intermediate, e.g.,
A + A' ---+ [collision products]
2. If a constant appears in the denominator, one of the reaction steps is probably the spontaneous
decomposition of the active intermediate, e.g.,
A* ---+ [decomposition products]
3. Species having the concentration(s) appearing in the numerator of the rate law probably pro-
duce the active intermediate in one of the reaction steps, e.g.,
[reactant] ---+ A" + [other products]