Physical Chemistry Third Edition

366 8 The Thermodynamics of Electrochemical Systems

c.NeglectingELJ,

E◦ 0 .3402 V−(− 0 .7628 V) 1 .1030 V

SinceE>0, the reaction would proceed spontaneously as written in the standard-state cell

if the terminals were connected to a passive circuit. Electrons would flow spontaneously

from left to right in the external circuit.

Exercise 8.7

Find the potential difference of a Daniell cell if the activity of zinc ions is equal to 0.500 and the

activity of copper ions is equal to 0.350. Neglect the liquid junction potential.

Since a liquid junction is not an equilibrium or metastable system, it cannot be

treated thermodynamically. Nonthermodynamic analyses have been carried out, but

we do not discuss these theories.^3 A common device for minimizing the liquid junction

potential is asalt bridge, shown schematically in Figure 8.7. The salt bridge contains

a concentrated electrolyte such as KCl that forms two liquid junctions, one with each

cell solution. Since the KCl is more concentrated than the cell solutions, the K+and

Cl−ions dominate the liquid junction potential. The K+ions and the Cl−are nearly

the same size and have nearly equal tendencies to diffuse into the cell solutions. Their

contributions to the liquid junction potential are of opposite signs and nearly cancel, so a

fairly small liquid junction potential results. Furthermore, the liquid junction potentials

at the two ends of the salt bridge oppose each other. The net liquid junction potential

with a salt bridge is generally no larger in magnitude than a few millivolts and can be

negligible in some cells. Liquid junction potentials without a salt bridge can range up

to 40 millivolts. In a cell symbol a salt bridge is represented by a double vertical line.

Electrolyte

solution

Electrolyte

solution

Porous

barriers

Saturated KCI solution

Figure 8.7 A Salt Bridge.

(^3) J. G. Kirkwood and I. Oppenheim,Chemical Thermodynamics, McGraw-Hill, New York, 1961,
p. 211ff.