Chemistry - A Molecular Science

Chapter 11 Electron Transfer and Electrochemistry

could predict the extent of a redox reaction if we knew the relative values of the standard half-cell potentials of the redox couples.

11.4

STANDARD REDUCTION POTENTIALS Although we cannot measure a half-cell pot

ential, we can measure the potential

difference

between two half-cells.

Relative

half-cell potentials can be obtained by defining one half-

cell as a reference against which all other half-cells can be measured. Which half-reaction we choose for the reference half-cell and the value we assign its potential are arbitrary. The decision, made many decades ago, was to use the

standard hydrogen electrode

(SHE)

as the reference and assign it a value of exactly 0 V. The SHE half-reaction is 2H

1+(aq) + 2e

1-^ U

H

(g) 2

o E

≡ 0 V

By convention, the half-cell to be measured is

connected to the ‘Hi’ terminal, so it is

assumed

to be the cathode or the reduction half-reaction. The SHE is then connected to the

‘Lo’ terminal, so it is

assumed

to be the anode or oxidation

half-reaction. If both cells are

in their standard states, then the resulting cell potential is the

standard reduction

potential

of the half-cell. It is a ‘reduction potential’ because the half-cell is connected to

the ‘Hi’ terminal. The standard

reduction potentials for the Fe

2+/Fe and Cu

2+/Cu couples

determined in this manner are -0.44 V and +0.34 V, respectively.*

(^) *
The cell connected to the Hi terminal is assumed to be the cathode by convention, but it may well be acting as the anode in the spontaneous process. This will be the case whenever the cell potential is negative. A negative cell potential means that the reaction is spontaneous in the re
verse direction. Thus, the negative
potential observed for the Fe/Fe
2+ half-cell means that it is actually
the anode in the spontaneous redo
x process. If the SHE had been
connected to the ‘Hi’ side and the Fe/Fe
2+ couple to the ‘Lo’ side,
then +0.44 V would be measured. Cell potentials measured with the SHE connected to the ‘Hi’ side are referred to as oxidation potentials. We consider only reduction potentials in this text.
The standard reduction potential of a couple is
the potential at which the couple is at
equilibrium with the cathode. The reduction ha
lf-reaction is written in the following form:
OX + ne
-1U^
RED
oE
o (^) E
, the standard reduction potential of the couple,
is the electrical potential (relative to the
SHE) at which OX and RED are in equilibrium. The standard reduction potential of the Cu
2+/Cu couple is +0.34 V, so metallic copper is in
equilibrium
with its +2 ion at a
potential that is 0.34 V higher
than the SHE.
Cu
2+ + 2e
1-^ U
Cu
oE
= +0.34 V
The standard reduction potential of the Fe
2+/Fe couple is -0.44 V, so metallic iron is in
equilibrium with its +2 ions at a potential that is 0.44 V lower
than the SHE.
Fe
2+ + 2e
1-U^
Fe
oE
= -0.44 V
Equilibrium implies that both the forward a
nd reverse reactions occur at the same rate.
If a SHE half-cell is connected to another SHE,
oE
= 0 V (both cells are at the same
potential), and
both
of the following processes occur at the same rate:
a) 2H
1+(aq) + 2e
1-^ →
H
(g) and b) H 2
(g) 2
→ 2H
1+(aq) + 2e
1-^