Page 23 - Introduction to chemical reaction engineering and kinetics

1.4 Aspects of Kinetics 5
slow. In such a case, it may be wrongly supposed that the system is at equilibrium, since
there may be no detectable change even after a very long time.
(2) Rate of reaction usually depends on concentration of reactants (and sometimes
of products), and usually increases as concentration of reactants increases. Thus, many
combustion reactions occur faster in pure oxygen than in air at the same total pressure.
(3) Rate of reaction depends on temperature and usually increases nearly exponen-
tially as temperature increases. An important exception is the oxidation of nitric oxide,
which is involved in the manufacture of nitric acid; in this case, the rate decreases as T
increases.
(4) Many reactions proceed much faster in the presence of a substance which is itself
not a product of the reaction. This is the phenomenon of catalysis, and many life pro-
cesses and industrial processes depend on it. Thus, the oxidation of SO, to SO3 is greatly
accelerated in the presence of V,O, as a catalyst, and the commercial manufacture of
sulfuric acid depends on this fact.
(5) The nature or intimacy of contact of reactants can greatly affect the rate of re-
action. Thus, finely divided coal burns much faster than lump coal. The titration of an
acid with a base occurs much faster if the acid and base are stirred together than if the
base is simply allowed to “dribble” into the acid solution. For a heterogeneous, catalytic
reaction, the effect may show up in a more subtle way as the dependence of rate on the
size of catalyst particle used.
(6) Some reactions occur much faster if the reacting system is exposed to incident
radiation of an appropriate frequenc$?%us, a mixture of hydrogen and chlorine can be
kept in the dark, and the reaction to form hydrogen chloride is very slow; however, if
the mixture is exposed to ordinary light, reaction occurs with explosive rapidity. Such
reactions are generally called photochemical reactions.

The way in which the rate of reaction depends on these parameters is expressed math-
ematically in the form of a rate law; that is, for species A in a given reaction, the rate
law takes the general form

r, = r,(conc., temp., cat. activity, etc.) (1.4-5)

The form of the rate law must be established by experiment, and the complete expres-
sion may be very complex and, in many cases, very difficult, if not impossible, to formu-
late explicitly.

1.4.3 Measurement of Rate of Reaction-Preliminary

The rate of chemical reaction must be measured and cannot be predicted from prop-
erties of chemical species. A thorough discussion of experimental methods cannot be
given at this point, since it requires knowledge of types of chemical reactors that can be
used, and the ways in which rate of reaction can be represented. However, it is useful to
consider the problem of experimental determination even in a preliminary way, since
it provides a better understanding of the methods of chemical kinetics from the outset.
We require a means to follow the progress of reaction, most commonly with respect
to changing composition at fixed values of other parameters, such as T and catalytic
activity. The method may involve intermittent removal of a sample for analysis or con-
tinuous monitoring of an appropriate variable measuring the extent of reaction, without
removal of a sample. The rate itself may or may not be measured directly, depending on
the type of reactor used. This may be a nonflow reactor, or a continuous-flow reactor,
or one combining both of these characteristics.