Page 368 - Elements of Chemical Reaction Engineering Ebook

Nonelementary 7

Reaction

Kinetics

The next best thing to knowing something is knowing
where to find it.
Samuel Johnson (1709--1784)

Until now, we have been discussing homogeneous reaction rate laws in which
the concentration is raised to some power n, which is an integer. That is, the
rate law (i.e., kinetic rate expression) is

We said that if y1 = 1, the reaction was first-order with respect to A; if n = 2,
the reaction was second-order with respect to A; and so on. However, a large
number of homogeneous reactions involve the formation and subsequent reac-
tion of an intermediate species. When this is the case it is not uncommon to
find a reaction order that is not an integer. For example, the rate law for the
decornposition of acetaldehyde,

at approximately 500°C is

Another common form of the rate law resulting from reactions involving active
intermediates is one in which the rate is directly proportional to the reactant
concentration and inversely proportional to the sum of a constant and the reac-
tant concentration. An example of this type of kinetic expression is observed
for the formation of hydrogen iodide,

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