23.2. Cell Potential http://www.ck12.org
Reduction Potential
The activity series that was presented in the previous lesson allows us to predict the relative reactivities of different
materials when used in oxidation-reduction processes. We also saw that appropriate combinations of half-reactions
can be used to generate an electric current. However, we have not yet looked at the energy changes associated
with any of these redox reactions.Electrical potentialis a measurement of the ability of a voltaic cell to produce
an electric current. It is typically measured in volts (V). Like energy, electrical potential is a relative term; it can
only be measured by comparison with something else. The voltage that is produced by a given voltaic cell is the
difference in electrical potential between the two half-cells, but it is not possible to measure the electrical potential
of an isolated half-cell. For example, if only a zinc half-cell were constructed, no complete redox reaction can occur,
so no electrical potential can be measured. It is only when another half-cell is combined with the zinc half-cell that
an electrical potential difference, or voltage, can be measured.
The electrical potential of a cell results from a competition for electrons between two different chemical species.
The "winner" of this battle is reduced (gains electrons), while the "loser" is oxidized (loses electrons). Astandard
reduction potentialmeasures the tendency of a given half-reaction to occur as a reduction in an electrochemical cell.
In a given voltaic cell, the half-cell that has the greater reduction potential is the one in which reduction will occur.
In the half-cell with the lower reduction potential, the reverse process (oxidation) will occur. Thecell potential
(Ecell) is the difference in standard reduction potential between the two half-cells in an electrochemical cell.
Standard Cell Potentials
Thestandard cell potential(E°cell) is the potential of an electrochemical cell when the temperature is 25°C, all
aqueous components are present at a concentration of 1 M, and all gases are at the standard pressure of 1 atm. The
standard cell potential can be calculated by finding the difference between the standard reduction potentials of the
two half-cells.
E◦cell=E◦red−E◦oxidSince the reduction potentials for half-cells cannot be measured independently, it is necessary to establish a standard
to serve as a reference. This reference is given a standard reduction potential of 0 volts by definition. Every other
half-cell can then be compared to this standard electrode in order to determine the reduction potential for any half-
cell. Thestandard hydrogen electrode(SHE) is a reference half-cell that can be used with another electrode
(half-cell) to determine its standard reduction potential. A standard hydrogen electrode is shown inFigure23.8.
The electrode itself is made of platinum, which serves as an inert surface upon which the oxidation or reduction
reaction takes place. The electrode is then placed in contact with both hydrogen gas (at a pressure of 1 atm) and an
acidic solution in which the concentration of H+is 1.0 M. Written as a reduction, the following half-reaction takes
place in a SHE:
2H+(aq)+2e−→H 2 (g) E◦= 0 .00 VDepending on the relative electrical potential of other half-cell that the SHE is combined with, the hydrogen ions
may be reduced or the hydrogen gas may be oxidized. In general, reversing a reaction will also reverse the sign of
the corresponding electrical potential. However, reversing the above reaction has no effect on the standard potential,
because the opposite of zero is still zero.
H 2 (g)→2H+(aq)+2e− E◦= 0 .00 V