Page 22 - Introduction to chemical reaction engineering and kinetics

4 Chapter 1: Introduction

“point” or “intrinsic” rate at the molecular level and is the more useful quantity. The
two rates are related as follows, with volume V as NQ:
For a uniform system, as in a well-stirred tank,

R, = rAV (1.4-3)

For a nonuniform system,

R, = t-A dV (1.4-4)
IV

The operational interpretation of rA, as opposed to this verbal definition, does de-
pend on the circumstances of the reacti0n.l This is considered further in Chapter 2 as a
consequence of the application of the conservation of mass to particular situations. Fur-
thermore, r, depends on several parameters, and these are considered in Section 1.4.2.
The rate with respect to any other species involved in the reacting system may be re-
lated to rA directly through reaction stoichiometry for a simple, single-phase system,
or it may require additional kinetics information a complex system. This aspect is
considered in Section 1.4.4, following a prelimi ry discussion of the measurement of
rate of reaction in Section 1.4.3.

1.4.2 Parameters Affecting Rate of Reaction: The Rate Law

Rate of reaction depends on a number of parameters, the most important of which are
usually

(1) The nature of the species involved in the reaction;
(2) Concentrations of species;
(3) Temperature;
(4) Catalytic activity;
(5) Nature of contact of reactants; and
(6) Wave-length of incident radiation.
These are considered briefly in turn.
(1) Many examples of types of very fast reactions involve ions in solution, such as the
neutralization of a strong acid by a strong base, and explosions. In the former case, the
rate of change may be dictated by the rate at which the reactants can be brought into
intimate contact. At the other extreme, very slow reactions may involve heterogeneous
reactions, such as the oxidation of carbon at room temperature. The reaction between
hydrogen and oxygen to form water can be used to illustrate both extremes. Subjected
to a spark, a mixture of hydrogen and oxygen can produce an explosion, but in the
absence of this, or of a catalyst such as finely divided platinum, the reaction is extremely

‘Attempts to define operationally the rate of reaction in terms of certain derivatives with respect to time (f)
are generally unnecessarily restrictive, since they relate primarily to closed static systems, and some relate to
reacting systems for which the stoichiometry must be explicitly known in the form of one chemical equation
in each case. For example, a IUPAC Commission (Mills, 1988) recommends that a species-independent rate
of reaction be defined by r = (l/v,V)(dnJdt), where vi and ni are, respectively, the stoichiometric coefficient
in the chemical equation corresponding to the reaction, and the number of moles of species i in volume V.
However, for a flow system at steady-state, this definition is inappropriate, and a corresponding expression
requires a particular application of the mass-balance equation (see Chapter 2). Similar points of view about rate
have been expressed by Dixon (1970) and by Cassano (1980).