Environmental Engineering Reference
In-Depth Information
For all of the reactants participating in standard state,
∆
G
°
nFE
°
(2.17)
It follows from the negative sign in Eq. 2.16 that a positive value for the cell
potential
E
will ensure a negative free energy change and thus a spontaneous
reaction.
The concept of electrode potential, cell potential, and means for their determi-
nation are discussed below.
2.2.2 Electrode Potential and Cell Potential
A metal electrode immersed in an electrolyte develops a charged interface, a
simplified picture of which is shown in Fig. 2.6. The interfacial structure of sepa-
rated charge is commonly referred to as the electrical double layer, and it behaves
much like a charged capacitor. Arising from this situation, a potential difference is
developed at the electrode-electrolyte interface that is termed
electrode potential
.
This corresponds to the establishment of an equilibrium of metal ionization
reaction and the reaction of its recombination with electrons as represented by
M i M
n
n
e
or
(2.18)
M
M
n
n
e
Since an electrochemical cell has two electrodes, each of the electrodes has its
own characteristic potential developed at the electrode-electrolyte interface,
which is referred to as single-electrode potential or half-cell potential and the
algebraic sum of the two single-electrode potentials constitutes the cell potential.
This can be expressed as
E
E
1
E
2
(2.19)
where
E
is cell potential and
E
1
and
E
2
are the single-electrode potentials of the
constituent electrodes.
Referring back to the electrochemical cell comprising a copper electrode in
equilibrium with cupric ions at unit activity and a zinc electrode in equilibrium
with zinc ions at unit activity (Fig. 2.2), the cell potential is 1.1 V, which is easily
measured with the help of a high-resistance voltmeter. This cell potential is an
algebraic sum of the single-electrode potentials of the copper and zinc electrodes
corresponding to the equilibria:
Zn i Zn
2
2e
(2.20)
Cu
2
2e
i Cu
(2.21)
