In exactly the same way, we can define the electrochemical
potential of a cell using oxidationreduction 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 ZnCu cell), and if F
is the charge on one mole of electrons, then the electrochemical
potential, E, is given by
D G^{0} =nFE^{0}
The superscript 0 on the potential, E^{0},
has the same meaning as for free energy. It signifies the
value when reactants and products all are in standard states
of 1molar concentrations, or 1atm partial pressures for
gases. It usually also refers to a temperature of 25°
C.
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?
Solution
