In exactly the same way, we can define the electrochemical
potential of a cell using oxidation-reduction reactions as
the free energy change per mole of electrons transferred.
If one mole of reaction occurs with the transfer of
n moles of electrons (n = 2
in the Zn-Cu cell), and if F
is the charge on one mole of electrons, then the electrochemical
potential, E, is given by
D G0 =-nFE0
The superscript 0 on the potential, E0,
has the same meaning as for free energy. It signifies the
value when reactants and products all are in standard states
of 1-molar concentrations, or 1-atm partial pressures for
gases. It usually also refers to a temperature of 25°
The charge on a mole of electrons,
F, is known as Faraday's constant.
It has various numerical values, depending on the units involved,
but if free energy is in kcal mol-1, n has units
of electrons, and potential is expressed in volts, then F=
23.056 kilocalories per mole per electron volt (eV), or 23.056
kcal mol-1 eV-1.
Example. What is the
standard electrochemical potential of the ZnCu cell?