Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.2 ELECTRICPOTENTIALS IN THECELL 454

galvanic

cell

 C

L R

 C

b

(a)

galvanic

cell

 C

L R

 C

b

G

(b)

Figure 14.3 Potentiometer to measure the zero-current cell potential of a galvanic

cell.

(a) Galvanic cell with zero current.

(b) Galvanic cell included in potentiometer circuit; G is a galvanometer.

reaction should show reduction at the right electrode and oxidation at the left, we must now
write it as
CuCZn^2 C.aq/! Cu^2 C.aq/CZn

even though the arrow is not in the direction of the reaction that actually occurs sponta-
neously. In other words, the cell reaction is written according to the cell diagram, not
according to the direction of the spontaneous change.

14.2.2 Measuring the equilibrium cell potential

Figure14.3shows how we can use a potentiometer to determine the equilibrium cell poten-
tial. Consider Fig.14.3(a). Outside the galvanic cell is an external circuit with a battery that
allows an electric current to pass through a slidewire resistor. The cell’s negative terminal is
connected to the negative terminal of the battery. Since the cell is not part of this circuit, no
current passes through the cell, andRLis the zero-current cell potentialEcell, eq. The
left end of the slidewire is at the same electric potential as the left terminal of the cell.
In the setup shown in Fig.14.3(a), the electric potential within the slidewire is a linear
function of the distance from the left end. At some position along the slidewire, the electric
potential is equal toR. We can determine this position by connecting the right terminal
of the cell to a slidewire contact as shown in Fig.14.3(b). When we place the contact
at this particular position along the slidewire, there is no electric potential gradient in the
connecting wire, and the galvanometer indicates a condition of zero current in the wire. It
is a straightforward procedure to evaluateRLfrom the zero-current position of the
contact; this value is still equal toEcell, eq. When we keep the slidewire contact in this
position, no current passes through the cell; but if we displace the contact from this position
in either direction along the slidewire, current will pass in one direction or the other through
the cell.
In practice, it is more convenient to measure the zero-current cell potential with a high-
impedance digital voltmeter (a voltmeter that draws negligible current) instead of with a