Callister - An introduction - 8th edition

17.2 Electrochemical Considerations • 679
Figure 17.4
The standard hydrogen reference half-cell.
V
Voltmeter
Pt
H + solution,
1.0 M
Hydrogen gas, 1 atm pressure
Membrane
electromotive force
(emf) series
atoms may be oxidized or hydrogen ions may be reduced. The electromotive force
(emf) series (Table 17.1) is generated by coupling to the standard hydrogen electrode,
standard half-cells for various metals and ranking them according to measured voltage.Table
17.1 represents the corrosion tendencies for the several metals; those at the
top (i.e., gold and platinum) are noble, or chemically inert. Moving down the table,
the metals become increasingly more active, that is, more susceptible to oxidation.
Sodium and potassium have the highest reactivities.
The voltages in Table 17.1 are for the half-reactions as reduction reactions, with
the electrons on the left-hand side of the chemical equation; for oxidation, the
direction of the reaction is reversed and the sign of the voltage changed.
Table 17.1
The Standard emf Series
Standard Electrode
Electrode Reaction
Potential, V 0 (V)
c
Au 3 3e ¡ Au
1.420
O 2 4H 4e ¡ 2H 2 O
Pt 2 2e ¡ Pt
1.229
1.2
Ag e ¡ Ag
0.800
Increasingly inert Fe 3 e ¡ Fe 2
0.771
(cathodic) O 2 2H 2 O 4e ¡ 41OH 2
Cu 2 2e ¡ Cu
2H 2e ¡ H 2
Pb 2 2e ¡ Pb
Sn 2 2e ¡ Sn
Ni 2 2e ¡ Ni
Co 2 2e ¡ Co
Cd 2 2e ¡ Cd
Fe 2 2e ¡ Fe
0.401
0.340
0.000
0.126
0.136
0.250
0.277
0.403
0.440
Increasingly active Cr 3 3e ¡ Cr
0.744
(anodic) Zn 2 2e ¡ Zn
Al 3 3e ¡ Al
Mg 2 2e ¡ Mg
Na e ¡ Na
K e ¡ K
0.763
1.662
2.363
2.714
2.924
c