Chemistry Reference
In-Depth Information
(
)
=
2
()
()
(
)
k
E
q
2
1
-
q
k
E
q
1
-
q
[4.23]
4
-
-
4
-
-
-
O
O
O
O
2
2
(
)
()
()
k
E
q
=
k
E c
2
1
-
q
[4.24]
5
-
-
5
O
-
O
O
2
2
In a similar way as in both previous parts, the following rate equation for
mechanism 1 with stage 3 as RDS is obtained:
12
12
()
()
(
)
(
)
kk
E
k
c
c
1
-
q
1
-
q
3
4
5
-
-
•
-
OH
OH
O
HO
v
=
KK
2
3
1
2
12
12
12
()
()
k
E
k
c
-4
-5
O
2
12
12
12
()
()
(
)
kk
E
k
E c
EE
1
-
q
--
3
4
-
5
O
-
[4.35]
-
2
O
12
12
()
()
k
k
4
5
Even though the O
-
particle is formed in the stage preceding the RDS, it
reacts further in a stage that is faster than the RDS (stage 4 of the
sequence). It is expected that the surface coverage of O
-
will be extremely
small so that the (1 -q) term is equal to 1. OH
•
, which is also formed in
stage 2, reacts further by means of the RDS so that the (1 -q) term cannot
be neglected as such. To do this anyway, an additional condition k
-2
>> k
3
is needed, where the formed OH
•
which does not immediately react by
means of the RDS mainly reacts back to adsorbed HO
2
-
. After inserting the
potential terms, rate equation (4.35) looks as follows:
kk
k
cc
12
12
F
R
12
12
F
R
kk
¢¢
k
¢¢
¢¢
¢¢
(
)
(
)
-
-
EE
-
EE
-
OH
12
3
HO
ref
-
3
ref
4
5
-
4
-
5
v
=K K
2
e
T
-
c
e
T
3
1
2
k
O
12
12
¢¢
k
¢¢
12
12
k
¢¢
k
¢¢
12
2
c
-
5
5
-
4
4
O
2
[4.36
]
Although the anodic term is dependent both on the hydrogen peroxide con-
centration and on the hydroxide ions concentration, the theoretically pre-
dicted value of the anodic transfer coefficient, i.e. 1, is twice that at the
experimentally observed value. Moreover, Equation 4.36 predicts that the
anodic current depends on the concentration of dissolved oxygen, which is
not observed experimentally. When k
-2
>> k
3
is assumed, this mechanism
also should consequently be rejected.
4.6.4
Stage 4 as RDS
By means of the equations below, the rate equation is calculated for mech-
anism 1 with stage 4 as RDS:
(
)
-
2
()
()
(
)
v
=
k
E
q
2
1
-
q
k
E
q
1
-
q
[4.18]
4
4
-
-
4
-
-
-
O
O
O
O
2
2
(
)
=
[4.32]
k
c
1
-
q
k
q
1
-
-
-
1
-
HO
HO
HO
2
2
2
(
)
(
)
(
)
=
k
q
1
-
q
1
-
q
k
q
q
1
-
q
[4.21]
2
-
-
•
-
2
-
•
-
O
OH
O
OH
HO
HO
2
2
