Callister - An introduction - 8th edition

17.2 Electrochemical Considerations • 677
0.780 V
Figure 17.2 An electrochemical cell consisting
Fe Fe 2+ Cu Cu 2+
– +
of iron and copper electrodes, each of which is
V
immersed in a 1 M solution of its ion. Iron
e –
e –
corrodes while copper electrodeposits.
Voltmeter
Fe 2+ solution,
1.0 M
Cu 2+ solution,
1.0 M
Membrane
and, in the second stage, to Fe 3 [as Fe(OH) 3 ] according to
2Fe1OH2 2 1 2O 2 H 2 O ¡ 2Fe1OH2 3
(17.12)
The compound Fe(OH) 3 is the all-too-familiar rust.
As a consequence of oxidation, the metal ions may either go into the corroding
solution as ions (reaction 17.8), or they may form an insoluble compound with
nonmetallic elements as in reaction 17.12.
Concept Check 17.1
Would you expect iron to corrode in water of high purity? Why or why not?
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
Electrode Potentials
Not all metallic materials oxidize to form ions with the same degree of ease. Consider
the electrochemical cell shown in Figure 17.2. On the left-hand side is a piece
of pure iron immersed in a solution containing Fe 2 ions of 1 M concentration. 1 The
other side of the cell consists of a pure copper electrode in a 1 M solution of Cu 2
ions. The cell halves are separated by a membrane, which limits the mixing of the
two solutions. If the iron and copper electrodes are connected electrically, reduction
will occur for copper at the expense of the oxidation of iron, as follows:
Cu 2 Fe ¡ Cu Fe 2
(17.13)
or Cu 2 ions will deposit (electrodeposit) as metallic copper on the copper electrode,
while iron dissolves (corrodes) on the other side of the cell and goes into solution
as Fe 2 ions. Thus, the two half-cell reactions are represented by the relations
Fe ¡ Fe 2 2e
Cu 2 2e ¡ Cu
(17.14a)
(17.14b)
molarity
1 Concentration of liquid solutions is often expressed in terms of molarity, M, the number
of moles of solute per liter (1000 cm 3 ) of solution.