Thermodynamics and Chemistry

CHAPTER 14 GALVANIC CELLS

14.1 CELLDIAGRAMS ANDCELLREACTIONS 450

HCl(aq)

Pt Ag

bb

Cu Cu

AgCl

H 2 (g)

Figure 14.1 A galvanic cell without liquid junction.

general arrangement of the physical elements of a galvanic cell is therefore

terminal – electron conductor – ionic conductor(s) – electron conductor – terminal

Both terminals must be the same metal (usually copper) in order for it to be possible to
measure the electric potential difference between them.
The combination of an electron conductor and the ionic conductor in contact with it is
called anelectrode,^1 or half-cell. To describe a galvanic cell, it is conventional to distin-
guish theleftandrightelectrodes. In this way, we establish a left–right association with the
reactants and products of the reactions at the electrodes.

14.1.2 Cell diagrams

Consider the galvanic cell depicted in Fig.14.1. This cell has a hydrogen electrode at the
left and a silver–silver chloride electrode at the right. The hydrogen electrode is a strip of
platinum in contact with hydrogen gas and with aqueous hydrochloric acid, which is the
ionic conductor. In the type of hydrogen electrode shown in the figure, hydrogen gas is
introduced through a side tube into a closed-end glass jacket that surrounds the platinum
strip and is immersed in the hydrochloric acid; the gas bubbles out through holes near the
bottom of the tube. The silver–silver chloride electrode is a silver strip or wire that dips into
the hydrochloric acid and is coated with solid silver chloride.
The cell in Fig.14.1is compactly described by the followingcell diagram:

Cu Pt H 2 .g/ HC.aq/;Cl.aq/ AgCl.s/ Ag Cu

A cell diagram indicates which electrode is at the left and which is at the right, and shows
the reactants and products of the two electrode reactions. A single vertical bar represents a
phase boundary. Commas are used to separate different species in the same phase.

(^1) The term “electrode” is sometimes used to refer to just the electron conductor.