GTBL042-16 GTBL042-Callister-v2 September 13, 2007 13:10
Revised Pages16.2 Electrochemical Considerations • 6650.323 VVoltmeter+–
e–V
e–FeFe2+ solution,
1.0 MMembraneZn2+ solution,
1.0 MFe2+ Zn Zn2+Figure 16.3 An electrochemical cell consisting
of iron and zinc electrodes, each of which is
immersed in a 1Msolution of its ion. The iron
electrodeposits while the zinc corrodes.When a current passes through the external circuit, electrons generated from the oxi-
dation of iron flow to the copper cell in order that Cu^2 +be reduced. In addition, there
will be some net ion motion from each cell to the other across the membrane. This
electrolyte is called agalvanic couple—two metals electrically connected in a liquidelectrolyte
wherein one metal becomes an anode and corrodes, while the other acts as a cathode.
An electric potential or voltage will exist between the two cell halves, and its
magnitude can be determined if a voltmeter is connected in the external circuit. A
potential of 0.780 V results for a copper–iron galvanic cell when the temperature is
25 ◦C (77◦F).
Now consider another galvanic couple consisting of the same iron half-cell con-
nected to a metal zinc electrode that is immersed in a 1Msolution of Zn^2 +ions
(Figure 16.3). In this case the zinc is the anode and corrodes, whereas the Fe now
becomes the cathode. The electrochemical reaction is thusFe^2 ++Zn→Fe+Zn^2 + (16.15)The potential associated with this cell reaction is 0.323 V.
Thus, various electrode pairs have different voltages; the magnitude of such a
voltage may be thought of as representing the driving force for the electrochemical
oxidation–reduction reaction. Consequently, metallic materials may be rated as to
their tendency to experience oxidation when coupled to other metals in solutions of
their respective ions. A half-cell similar to those described above [i.e., a pure metal
electrode immersed in a 1Msolution of its ions and at 25◦C (77◦F)] is termed a
standard half-cell standard half-cell.The Standard emf Series
These measured cell voltages represent only differences in electrical potential, and
thus it is convenient to establish a reference point, or reference cell, to which other
cell halves may be compared. This reference cell, arbitrarily chosen, is the standard
hydrogen electrode (Figure 16.4). It consists of an inert platinum electrode in a
1 Msolution of H+ions, saturated with hydrogen gas that is bubbled through the
solution at a pressure of 1 atm and a temperature of 25◦C (77◦F). The platinum