Chemistry Reference
In-Depth Information
where
C
is the concentration and
n
is the reaction order. For a reaction to be homo-
geneous, the reactants should be miscible at the molecular level and be present in
the form of single phase. Then, the reaction would occur throughout the whole reac-
tion volume and its rate would be proportional to the amount of reactants in this vol-
ume or, in other words, to their concentrations. For instance, in a gas-phase reaction:
HI
+→
2
HI
()
,
2
()
g
2
()
g
g
the two gases are mixed in a reaction vessel and the reaction zone is the whole vol-
ume of the vessel. Increasing the amounts of gases in the reaction vessel increases
the number molecules in the reaction zone and, thus, the reaction rate.
However, this does not hold if the reactants cannot be mixed at the molecular
level and thus have to be present as individual phases. Such reactions can occur only
at the interface of the reactant phases. The reactions of this type are called heteroge-
neous. For example, in a reaction:
2
Cu
+→
S uS
()
,
()
s
()
s
2
s
two solid reactants are brought into a contact so that the reaction zone is limited to
the interfacial contact area. If the copper and sulfur reactants are prepared in the
form of cylinders (Fig.
1.1
) with the circular bases of equal size and made to react
by bringing the bases of the solid cylinders into a contact, the reaction rate will be
proportional to the area of the circular bases. Nevertheless, it will be independent
of the total amount of copper and sulfur outside the interfacial reaction zone, i.e.,
independent of the cylinder heights. For this reason, the total amounts of the re-
actants that can be expressed as the concentration do not carry much meaning for
heterogeneous kinetics.
Note that the processes that include a condensed phase generally tend to dem-
onstrate heterogeneous kinetics. Reactions between solids, or solids and liquids,
Fig. 1.1
Schematic represen-
tation of a reaction between
two solids,
Cu
(s) and
S
(s)
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