Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.2 ELECTRICPOTENTIALS IN THECELL 455

(a)

b

b

LT

LE

I

RE

RT

(b)

b

b

LT

LE

I RE

RT

R

L

Ecell, eq

R

L

Ecell

Figure 14.4 Galvani potential profile across a galvanic cell (schematic). LT and RT

are the left and right terminals, LE and RE are the left and right electron conductors,

and I is an ionic conductor such as an electrolyte solution.

(a) Cell with zero current.

(b) The same cell with finite current.

potentiometer circuit.

14.2.3 Interfacial potential differences

What is the source of an open-circuit, zero-current cell potential? When no electric current
passes through the cell, the electric potential must be uniform within each bulk phase that
is an electrical conductor, because otherwise there would be a spontaneous movement of
charged particles (electrons or ions) through the phase. Electric potential differences in a
cell without current therefore exist only at phase boundaries. The equilibrium cell poten-
tial is the cumulative result of these potential differences at interfaces between different
conducting phases within the cell.
An interfacial potential difference appears as a vertical step in a profile of the Galvani
potential, as shown schematically in Fig.14.4(a). The zero-current cell potential,Ecell, eq,
is the algebraic sum of the interfacial potential differences within the cell.
When an external resistor is connected to the terminals to form a circuit, current passes
through the cell and the cell performs electrical work on the surroundings. Figure14.4(b)
shows what happens to the potential profile in this case: the interfacial potential differences
are still present within the cell, and the internal resistance of the electrical conductors causes
Ecellto be reduced in magnitude compared toEcell, eq.
We shall next look briefly at the origin and consequences of potential differences at
interfaces between (1) two different metals, (2) a metal and an electrolyte solution, and
(3) two different electrolyte solutions. Keep in mind that these potential differences are
theoretical concepts whose values cannot be measured experimentally.

Metal–metal contacts

An electric potential difference at an interface between two metals is called acontact poten-
tial. When two different metals are placed in contact, the local densities of the free (mobile)
electrons change so as to form an electrical double layer with an excess positive charge on