Page 32 - Elements of Chemical Reaction Engineering Ebook

Sec. 1.1 Defini.tion of the Rate of Reaction, -r, 3

lose their chemical identity per unit time per unit volume through the brealung
and subsequent re-forming of chemical bonds during the course of the reac-
tion. In order for a particular species to “appear” in the system, some pre-
scribeld fraction of another species must lose its chemical identity.
There are three basic ways a species may lose its chemical identity. One
way is by decomposition, in which a molecule is broken down into snnaller
molecules, atoms, or atom fragments. For example, if benzene and propylene
are formed from a cumene molecule,

cumene benzene propylene
the cumene molecule has lost itshdentity (i.e., disappeared) by brealung its
bonds to form these molecules. A second way that a. molecule may lose its spe-
cies identity is through combination with another molecule or atom. In the
example above, the propylene molecule would lose its species identity if the
reaction were carried out in the reverse direction so that it combiiied with ben-
zene to form cumene.
The third way a species may lose its identity is through isomerization,
such as the reaction

(343 CH3
A species can lose I + I
its identity by CH*=C-lCH,CH, CH3C=CHCH3
decomposition,
combination, Here, although the molecule neither adds other molecules to itself nor brleaks
or isomerization
into smaller molecules, it still loses its identity through a change in configura-
tion.
To summarize this point, we say that a given number of molecules (e.g.,
mole) of a particular chemical species have reacted or disappeared when the
molecules have Lost their chemical identity.
The rate at which a given chemical reaction proceeds can be expressed in
several ways. It can be expressed either as the rate of disappearance of reac-
tants or the rate of formation of products. For example, the insecticide DDT
(dichlorodiphenyltrichloroethane) is produced from chlorobenzene and chloral
in the presence of fuming sulfuric acid.
2C6HSC1+ CC1,CHO __j (C6H4Cl)ZCHCC13 + HZO

Letting the symblol A represent the chemical chloral, the numerical value of the
What is -rA? rate of reaction, --rA, is defined as the number of moles of chloral reacting
ri?
(disappearing) per unit time per unit volume (mol/dm3 s). In the next chapter
we delineate the prescribed relationship between the rate of formation of one