Page 185 - Modeling of Chemical Kinetics and Reactor Design

Reaction Rate Expression 155

If there is excessively large aldehyde, its concentration remains
constant and the reaction can be referred to as pseudo-first order
kinetics. Equation 3-192 can be rewritten as

dC
− Br 2 = kC (3-193)
dt 1 Br 2

where

k = k C CH CHO (3-194)
2
1
3
The second order rate constant k is k = k C , and can be
2 2 1 CH CHO
3
determined by measuring k at known aldehyde concentrations. The
1
use of excess reagent concentrations has important practical conse-
quences where physical methods, such as light absorption or con-
ductance, are used to monitor reactions.
PRACTICAL MEASUREMENTS OF REACTION RATES

Many techniques have been employed in kinetic studies to deter-
mine reaction rate constants and reaction orders from either reactant
or product concentrations at known times. The most desirable analytical
methods allow continuous and rapid measurement of the concentra-
tion of a particular component. Any of the methods used to monitor
the course of reaction must satisfy the following criteria:

• The method should not interfere with the system by affecting the
kinetic processes occurring during the investigation.
• It must give an exact measure of the extent of the reaction.
• The measurement should be representative of the system at the
time it is made or at the time the sample analyzed was taken.

Analyses of kinetic data are based on identifying the constants of
a rate equation involving the law of mass action and some transfer
phenomena. The law of mass action is expressed in terms of con-
centrations of the species. Therefore, the chemical composition is
required as a function of time. Laboratory techniques are used to
determine the chemical composition using an instrument that is suitably
calibrated to give the required data. The techniques used are classified
into two categories, namely chemical and physical methods.