Chemistry - A Molecular Science

However, if a SHE is connected to a different redox couple, one process will be favored over the other.

Process a•

is favored when the SHE is paired with a redox couple with a more negative

standard reduction potential. Thus, when the SHE is attached to Fe

2+/Fe at -0.44 V,

electrons move to H

1+ from Fe because H

1+ is at the more positive

potential. In this case, H

1+^

is reduced and Fe is oxidized.

Process b•

is favored when the SHE is paired with a redox couple with a more positive

standard reduction potential. Thus, when the SHE is attached to Cu

2+/Cu at +0.34 V,

electrons flow from H

to Cu because Cu 2

2+ is at the more positive

potential. In this case, H

(^2)
is oxidized and Cu
2+ is reduced.
Electrons transfer from more negative to more positive electrical potential, so the
reducing power of a reductant increases as its electrical potential decreases, and the oxidizing power of an oxidant increases as its electrical potential increases. That is,
Good oxidizing agents are those
oxidants (OX) in redox couples with high (positive)
standard reduction potentials, while good redu
cing agents are those reductants (RED) in
redox couples with low (negative) standard reduction potentials.
The standard reduction potential of a Fe
2+/Fe couple (-0.44 V) is lower than that of a
Cu
2+/Cu couple (+0.34 V), so Fe is a better reductant than Cu, and Cu
2+ is a better oxidant
than Fe
2+.
As shown in Figure 11.3, the standard cell potential is the difference between the
standard reduction potentials of the two redox
couples. The electrons start on an Fe atom
(the better reducing agent) at -0.44 V and transfer to Cu
2+ (the better oxidant) at +0.34 V.
The change in electrical potential experienced
by the electrons is equal to the difference
between the two standard reduction potentials:
oE
cell
=E^
ocathode

• E^

oanode

= +0.34 - (-0.44) = +0.78 V.

The electrons increase their electrical potential

by 0.78 V by transferring from Fe to Cu.

The

standard reduction potential of a couple is proportional to the negative of the

standard free energy of the electron in that couple

. Electron transfer to a couple at higher

standard reduction potential is extensive because

the free energy of the electron is lower in

the couple at the higher potential.

Table 11.1 (following page) lists the standa

rd reduction potentials for several common

half-reactions. They are arranged so that th

e standard reduction potential increases (free

energy decreases) going down the table. Thus,

electrons flow spontaneously when the

reductant is situated above the oxidant in the table

.

-0.44 V ---------Fe + 2e

Fe

2+

1-

+0.34 V ---------Cu + 2e

Cu

2+

higher energydonor electrons1-

lower energyacceptor orbital

betterreductant

betteroxidant

ElectricalPotential

FreeEnergy

oE

cell

= 0.78 V

2e

1-

Figure 11.3 Standard reduction potentials of the Fe

2+/Fe and

Cu

2+/Cu couples referenced to a SHE

The reduced form of the couple at

more negative potential has the

higher energy electrons and is the better reductant. The oxidized form of the couple at more positive potential has the lower energy orbital and is the better oxidant. The green arrow shows the direction of spontaneous electron

transfer as the higher energy

electrons on the donor flow into the empty orbital at lower energy on the acceptor.

Chapter 11 Electron Transfer and Electrochemistry