Chemistry Reference
In-Depth Information
The rate of nucleation of Mn(IV) was dependent on [Mn
2+
] relative to
[Mn(III)] and also increased with an increase in pH. At pH > 3 in HClO
4
solu-
tions, reactions (6.46) and (6.47) were not kinetically separable. The activation
energy of reaction (6.46) was determined as 39.5 kJ/mol [178].
The rate of the reaction between Mn(III) and H
2
O
2
has also been studied
in the acidic medium (pH 0-2) [178], which followed mixed first- and second-
order kinetics. The mechanism involved the formation of the Mn(II)-superox-
ide complex (reaction 6.51) as well as the reaction between Mn(IV) and H
2
O
2
(reaction 6.52). Mn(IV) was from the disproportionation of Mn(III) (reaction
6.49):
+
+
2
+
+
Mn III H O
(
)
+
→
(
MnO
+
2
H
)
→
Mn
+
HO H
+
(6.51)
2
2
2
2
2
+
+
Mn IV H O
(
)
+
→
Mn
+
O H
+
.
(6.52)
2
2
2
The rate constants of reaction (6.51) varied from 0.25-11 × 10
4
/M/s in the
pH range of 0-5.2 [178]. The estimated rate constant for reaction (6.52) was
≥10
6
/M/s.
The formation of Mn(III) has also been observed in the reaction of Mn(II)
with the
SO
−
radical. This reaction was studied in the presence of excess sulfite
at pH 3.0 with a 0.01 M ionic strength [180]. under these conditions, [Mn(II)]
total
existed as Mn
2+
(aq), [Mn(HSO
3
)]
+
, and [Mn(SO
3
)Mn]
2+
, represented in Equa-
tions (6.53)-(6.55):
(6.53)
Mn
2
+
+
SO H
−
+ →
+
Mn
3
+
+
HSO
−
5
5
[
Mn HSO
(
)]
+
+
SO H
−
+ →
+
[
Mn HSO
(
)]
2
+
+
HSO
−
(6.54)
3
5
3
5
[
Mn SO Mn
(
)
]
2
+
+
SO H
−
+ →
+
[
Mn SO Mn
(
)
]
3
+
+
HSO
−
.
(6.55)
3
5
3
5
The overall second-order rate constant for the reaction of Mn(II) and the
SO
−
radical ranged from 2 × 10
8
/M/s to 2 × 10
10
/M/s, which was dependent on
the kind of Mn(II) species under the experimental conditions.
The reaction of Mn
2+
(aq) with acylperoxyl radicals (CH
3
OO
•
) and alkylp-
eroxyl radicals (ROO
•
) in acidic aqueous solutions and in 95% acetic acid also
produced Mn(III) [181]. The proposed scheme is represented by Equations
(6.56) and (6.57):
•
Mn II ROO
(
)
+
Mn III OOR
(
)
(6.56)
+
Mn III OOR H
(
)
+ →
Mn III ROOH
(
)
+
.
(6.57)
The initial formation of Mn(III)OOR was first order for each reactant. The
second-order rate constants were (0.5-1.6) × 10
6
/M/s and (0.5-5.0) × 10
5
/M/s
for acylperoxyl radicals and alkylperoxyl radicals, respectively. Mn(II) cata-
lyzed the dissociation of Mn(III)OOR back to Mn(II) and ROO
•
(reaction
6.58) [181]:
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