Chemistry Reference
In-Depth Information
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
[H
+
]
Figure 6.27.
Apparent rate constant for the decay of the dimer Fe(OH)
2
Fe
4+
measured
at 320 nm as a function of [H
+
] at different temperatures, 5-40°C. Constant ionic strength
at 1.0 M
ClO
−
. (
) 5°C, (
) 10°C, (
) 15°C, (
) 20°C, (
) 25°C, (
) 30°C, (
) 35°C, (
)
40°C (adapted from Jacobsen et al. [326] with the permission of Wiley Inc.).
The activation energies for reaction (6.96a,b) were determined as 73.0 and
47.0 kJ/mol. The reaction of FeO
2+
with H
2
O
2
produced HO
2
by a one-electron
transfer process (reaction 6.107). HO
2
further reacted with
FeO
+
(reaction
6.98):
FeO
2
+
+
H O
→
Fe
3
+
+
HO OH
+
−
k
=
1 0 10
.
×
4
/M/s
(6.97)
2
2
2
107
FeO
2
+
+
HO
→
Fe
3
+
+
O OH
+
−
k
= ×
2 10
4
/M/s
.
(6.98)
2
2
108
The rate constants for the
FeO
+
reactions with H
2
O
2
also followed a linear
relationship shown in Figure 6.25, which also suggests the reactions with inor-
ganic substrates proceed by a single electron transfer [326].
The reaction kinetics of the ferryl ion with phenol, nitrobenzene, o- and
p-nitrophenol in 1 M HClO
4
was also studied [327]. The values of the second-
order rate constants (
k
, /M/s) increased with an increase in the dissociation
energy of the O-H bond of the studied molecules. A negative slope between
log
k
and σ (NO
2
) was determined. The rate constants decreased with an
increase in the one-electron reduction potentials of the corresponding radical
cations of the substrates. The mechanism involving the electrophilic addition
and the electron transfer explained the observed trend in the kinetics of the
reactions.
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