Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.2 ELECTRICPOTENTIALS IN THECELL 452

Zn

ZnSO 4 (aq)

Cu

CuSO 4 (aq)

bb

L CR

(a)

Zn

ZnSO 4 (aq)

Cu

CuSO 4 (aq)

bb

 C

e

(b)

Figure 14.2 Zinc–copper galvanic cell with porous barrier (heavy dashed line) sepa-

rating two electrolyte solutions. The dashed rectangle indicates the system boundary.

(a) Open circuit with isolated system in equilibrium state.

(b) Closed circuit.

positive dimensionless quantity equal tojej, whereeis the stoichiometric number of the
electrons in either of the electrode reactions whose sum is the cell reaction.
Because both electrode reactions are written with the same value ofjej, the advance-
ments of these reactions and of the cell reaction are all described by the same advancement
variable. For an infinitesimal change d, an amount of electrons equal tozdenters the
system at the right terminal, an equal amount of electrons leaves at the left terminal, and
there is no buildup of charge in any of the internal phases.
TheFaraday constantF is a physical constant defined as the charge per amount of
protons, and is equal to the product of the elementary charge (the charge of a proton) and the
Avogadro constant:FDeNA. Its value to five significant figures isFD96; 485C mol^1.
The charge per amount of electrons isF. Thus, the charge entering the right terminal
during advancement dis
∂QsysDzFd (14.1.1)

14.2 Electric Potentials in the Cell

As explained at the beginning of Sec.3.8, theelectric potentialat a point in space is
defined as the change in the electrical potential energy of an infinitesimal test charge when
it is brought to this point from a position infinitely far from other charges, divided by the
charge.
We are concerned with the electric potential within a phase—the inner electric potential,
orGalvani potential. We can measure the difference between the values of this electric po-
tential in the two terminals of a galvanic cell, provided the terminals have the same chemical
composition. If the terminals were of different metals, at least one of them would have an
unknown metal–metal contact potential in its connection to the external measuring circuit.
Since we will be applying the concept of electric potential to macroscopic phases, the
value of the Galvani potential at a point in a phase should be interpreted as theaveragevalue
in a small volume element at this point that is large enough to contain many molecules.