Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.2 ELECTRICPOTENTIALS IN THECELL 453

14.2.1 Cell potential

Thecell potentialof a galvanic cell is the electric potential difference between terminals of
the same metal, and is defined by Eq.3.8.6:

EcelldefD RL (14.2.1)

The subscripts R and L refer to the right and left terminals. Theequilibrium cell potential,
Ecell, eq, is the cell potential measured under conditions of zero current when the cell is
assumed to be in an equilibrium state.^2

Over a relatively long period of time, the state of an isolated galvanic cell is found to

change. Nevertheless, the assumption of an equilibrium state is valid if the changes

are very slow compared to the period during which we measureEcell.

The long-term changes can be of two types. If there is a liquid junction between

electrolyte solutions of different composition, slow diffusion of ions through the junc-

tion is inevitable.

In a cell without a liquid junction, the reactants of the cell reaction can react di-

rectly without the passage of an electric current. For instance, in the cell of Fig.14.1

the electrolyte solution is saturated with respect to gaseous H 2 and solid AgCl, and

therefore contains very small concentrations of dissolved H 2 molecules and AgCions.

The direct reaction H 2 C2 AgC!2 HCC2 Ag occurs irreversibly and continuously

in the solution, but is slow on account of the low concentrations.

It is entirely arbitrary whether we show a particular electrode at the left or the right
of the cell diagram, although often there is a preference to place the electrode attached to
the positive terminal at the right. If we exchange the positions of the two electrodes in the
diagram, then we must reverse the reaction equations for the electrode reactions and the cell
reaction.
For example, it is found that the zinc–copper cell of Fig.14.2, with typical electrolyte
molalities, has its positive terminal at the copper electrode. When we write the cell diagram
as
Zn Zn^2 C.aq/ Cu^2 C.aq/ Cu

thenEcellandEcell, eqare positive. If we connect the two terminals by an external resistor
as depicted in Fig.14.2(b), electrons will flow from the left terminal through the external
resistor and wires to the right terminal, and the cell reaction

ZnCCu^2 C.aq/! Zn^2 C.aq/CCu

will occur spontaneously in the forward direction.
If, however, we draw the cell diagram the other way around:

Cu Cu^2 C.aq/ Zn^2 C.aq/ Zn

then the positive terminal is at the left,EcellandEcell, eqare negative, and electrons will
flow through an external resistor from the right terminal to the left terminal. Since the cell

(^2) The equilibrium cell potential used to be called the electromotive force, or emf. These names are deprecated
by the IUPAC Green Book (Ref. [ 36 ], p. 71) because a potential difference is not a force.