Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.1 CELLDIAGRAMS ANDCELLREACTIONS 451

The same cell can be described by a slightly different cell diagram that omits the copper
terminals seen in the figure and shows the electrolyte solute instead of the ion species:

Pt H 2 .g/ HCl.aq/ AgCl.s/ Ag

The reason it is not necessary to include the terminals is that the property whose value
we seek, the zero-current cell potential, is the same regardless of the metal used for the
terminals.

14.1.3 Electrode reactions and the cell reaction

A cell diagram, with its designation of the left and right electrodes, allows us to write
reaction equations for the cell. These equations are written according to the convention that
electrons enter at the right terminal and leave at the left terminal.
At each electrode there is anelectrode reaction, or half-reaction, one for reduction at
the right electrode and the other for oxidation at the left electrode. The reaction equations
for the electrode reactions include electrons as either a reactant (at the right terminal) or a
product (at the left terminal). Thecell reactiondescribes the overall chemical change; its
reaction equation is the sum of the equations for the two electrode reactants with cancella-
tion of the electrons.
For instance, we can write the electrode reactions of the cell of Fig.14.1as follows.

oxidation at left: H 2 .g/! 2 HC.aq/C2 e

reduction at right: 2 AgCl.s/C2 e !2 Ag.s/C2 Cl.aq/

As written here, the stoichiometric numbers of the electrons have the same absolute value
(2) in both reaction equations. This allows the electrons to cancel when we add the electrode
reactions to form the cell reaction:

H 2 .g/C2 AgCl.s/! 2 HC.aq/C2 Cl.aq/C2 Ag.s/

The cell of Fig.14.1has a single electrolyte phase with essentially the same composi-
tion at both electrodes, and is an example of acell without liquid junctionorcell without
transference. As an example of acell with transference, consider the cell diagram

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

This is the zinc–copper cell depicted in Fig.14.2on the next page, sometimes called a
Daniell cell. The two electrolyte phases are separated by a liquid junction represented in
the cell diagram by the dashed vertical bar. If the liquid junction potential can be assumed
to be negligible, the liquid junction is instead represented by a pair of dashed vertical bars:

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

14.1.4 Advancement and charge

Theelectron numberor charge number,z, of the cell reaction is defined as the amount
of electrons entering at the right terminal per unit advancement of the cell reaction.zis a