Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.3 MOLARREACTIONQUANTITIES OF THECELLREACTION 457

the electric potentials of their phases. Consequently, in order for the sum to be zero, the
metal and solution must in general have different electric potentials.
For example, consider the zinc–copper cell of Fig.14.2. The electrode reaction of the
copper electrode at the right is

Cu^2 C.aq/C2 e.Cu/! Cu

where the metal phase of the electrons is indicated in parentheses. In order for this elec-
trode reaction to be at equilibrium, the interfacial potential difference between the copper
conductor and the solution containing Cu^2 Cions must be such that the following condition
is satisfied:
.Cu/.Cu^2 C/2e.Cu/D 0 (14.2.4)
The interfacial potential difference can arise from a combination of charge separation
across the interface, orientation of polar molecules on the solution side of the interface, and
specific adsorption of ions. The thickness of the zones in which properties differ from those
in the bulk phases is probably no greater than 10 ^11 m on the metal side and 10 ^7 m on the
solution side.

Liquid junctions

Some galvanic cells contain two electrolyte solutions with different compositions. These
solutions must be separated by a porous barrier or some other kind of junction in order to
prevent rapid mixing. At this liquid junction in the zero-current cell, there is in general
aliquid junction potentialcaused by diffusion of ions between the two bulk electrolyte
phases.
To understand this phenomenon, imagine the situation that would exist at the junction if
both solution phases had the same electric potential. An ion species with different chemical
potentials in the two solutions would spontaneously diffuse across the junction in the direc-
tion of lower chemical potential. Different ions would diffuse at different rates, resulting in
a net charge transfer across the junction and an electric potential difference. It is this elec-
tric potential difference in the equilibrium state of the cell that prevents further net charge
transfer under zero-current conditions.
The liquid junction may consist of a bridging solution in asalt bridge. A commonly
used kind of salt bridge is a glass tube filled with gel made from agar and concentrated
aqueous KCl or KNO 3 ; this type of liquid junction is believed to reduce the liquid junction
potential to several millivolts or less.

14.3 Molar Reaction Quantities of the Cell Reaction

This book will denote the molar reaction Gibbs energy of a cell reaction byÅrGcell. This
notation distinguishes it from the molar reaction Gibbs energyÅrGof the direct reaction,
which may have a different value because in the cell the chemical potential of an ionic
species is affected by the electric potential of its phase.ÅrGcellis defined by

ÅrGcelldefD

X

i

ii (14.3.1)