Agriculture Reference
In-Depth Information
Which redox couple in a redox reaction has the oxidizing role and which the
reducing role depends on the relative abilities of the two couples to accept or
donate electrons. For example O
2
has a greater affinity for electrons than other
potential oxidants in natural systems, and is therefore reduced preferentially. The
means of quantifying the relative abilities of redox couples to accept or donate
electrons and the corresponding free energy changes is as follows.
Just as free protons do not exist in solution in acid - base reactions, there are no
free electrons in redox reactions. However it is possible to define the activity of
electrons relative to a specified standard state and thereby treat electrons as dis-
crete species in equilibrium calculations in the same way as ions and molecules.
The standard state of electron activity for this purpose is by convention defined
with respect to the redox couple made by hydrogen ions and hydrogen gas:
H
+
+
e
−
=
1
2
H
2
(
g
)
(
4
.
2
)
By convention the standard free energy change,
G
o
, for this reaction is set at
zero, i.e. since
G
o
=−
RT
ln
K
:
(
H
2
(
g
))
1
/
2
(
H
+
)(
e
−
)
=
1
K
=
(
4.2a
)
The thermodynamic relations of any particular redox couple can therefore be
calculated from the values for the reaction between the couple of interest and the
hydrogen ion — hydrogen gas couple. Thus for the reduction of oxidant Ox to its
conjugate reductant Red, we have:
Ox
+
n
e
−
=
Red
(
4.3a
)
and
1
2
n
H
2
(
g
)
=
n
H
+
+
n
e
−
(
4
.
3b
)
and the full reaction is
1
+
n
H
+
Ox
+
2
n
H
2
(
g
)
=
Red
(
4.3c
)
If
K
1
,
K
2
and
K
are the equilibrium constants for Reactions (4.3a), (4.3b) and
(4.3c), then
(
Red
)
(
Ox
)(
e
−
)
n
K
=
K
1
K
2
=
K
1
=
(
4
.
4
)
i.e.
1
K
1
/n
(
Red
)
(
Ox
)
(
e
−
)
=
(
4
.
5
)
or, on a logarithmic scale,
log
K
−
log
(
Red
)
(
Ox
)
1
n
pe
=−
log
(
e
−
)
=
(
4
.
6
)
