Chemistry Reference
In-Depth Information
12.5
12.0
11.5
11.0
10.5
10.0
9.5
9.0
3.2
3.3
3.4
3.5
3.6
3.7
10
3
/
T
(K)
Figure 6.26.
Arrhenius plot of the rate constant
k
71
for the reaction of Fe
2+
with FeO
2+
.
Ionic strength adjusted with NaClO
4
. (Δ) pH 0.0 (µ = 1.0); (
) pH 1.0 (µ = 0.1); (
)
pH 1.0 (µ = 1.0); (
) pH 2.0 (µ = 0.01); (
) pH 2.0 (µ = 1.0) (adapted from Jacobsen
et al. [326] with the permission of Wiley Inc.).
2
+
2
+
3
+
−
4
(6.95a)
FeO
+
Fe
+
H O Fe
→
+
2
OH
k
=
7 2 10
.
×
/M/s
2
105
a
2
+
2
+
4
+
4
FeO
+
Fe
+
H O Fe OH Fe
→
(
)
k
=
1 8 10
.
×
/M/s
.
(6.95b)
2
2
105
b
The branching of reaction (6.95) was also supported by the curvature in the
Arrhenius plot (Fig. 6.26). The yield of the dimer (reaction 6.95b) was depen-
dent on the temperature, which may be related to a larger activation energy
(
E
a
(6.95a) = 7.0 kJ/mol and
E
a
(6.95b) = 42.0 kJ/mol). The reactivity of the
ferryl ion with H
2
O
2
is about 200 times slower than with HO
2
[326].
The decay of the dimer followed first-order kinetics, which was independent
of the initial Fe
2+
and ozone concentrations. However, the decay was strongly
dependent on the acidity (Fig. 6.27). The linear relationship of the first-order
rate constant (
k
106
) with the proton concentration can be explained by reac-
tions (6.96a) and (6.96b):
4
+
3
+
−
Fe OH Fe
(
)
→
2
Fe
+
2
OH
k
=
0 63
.
/s ionic strength
(
=
0 85
.
)
2
106
a
(6.96a)
4
+
+
3
+
Fe OH Fe
(
)
+
2
H
→
2
Fe
+
2
H O
k
=
1 95
.
/M/s ionic strength
(
=
85).
(6.96b)
0
.
2
2
106
b
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