The Foundations of Chemistry

1.Choose the appropriate half-reactions from a table of standard reduction potentials.

2.Write the equation for the half-reaction with the more positive (or less negative)

E^0 value for reductionfirst, along with its potential.

3.Then write the equation for the other half-reaction as an oxidationand write its

oxidation potential; to do this, reverse the tabulated reduction half-reaction, and

change the sign of E^0. (Reversing a half-reaction or a complete reaction also changes

the sign of its potential.)

4.Balance the electron transfer. We do not multiply the potentials by the numbers used to

balance the electron transfer!The reason is that each potential represents a tendency

for a reaction process to occur relative to the SHE; this does not depend on how

many timesit occurs. An electric potential is an intensive property.

5.Add the reduction and oxidation half-reactions, and add the reduction and oxida-

tion potentials. E^0 cellwill be positivefor the resulting overall cell reaction. This

indicates that the reaction as written is product-favored (spontaneous).A negative E^0 cell

value would indicate that the reaction is reactant-favored (nonspontaneous).

For the cell described here, the Cu^2 /Cu couple has the more positive reduction poten-
tial, so we keep it as the reduction half-reaction and reverse the other half-reaction.
Following the steps outlined, we obtain the equation for the spontaneous reaction.

21-15 Uses of Standard Electrode Potentials 869

TABLE 21-2 Standard Aqueous Reduction Potentials in Aqueous Solution at 25°C

Standard Reduction
Element Reduction Half-Reaction Potential E^0 , volts

Li Lie 888nLi 3.045

KKe 888nK 2.925

Ca Ca^2  2 e 888nCa 2.87

Na Nae 888nNa 2.714

Mg Mg^2  2 e 888nMg 2.37

Al Al^3  3 e 888nAl 1.66

Zn Zn^2  2 e 888nZn 0.763

Cr Cr^3  3 e 888nCr 0.74

Fe Fe^2  2 e 888nFe 0.44

Cd Cd^2  2 e 888nCd 0.403

Ni Ni^2  2 e 888nNi 0.25

Sn Sn^2  2 e 888nSn 0.14

Pb Pb^2  2 e 888nPb 0.126

H 2 2H 2 e 888nH 2 0.000 (reference electrode)

Cu Cu^2  2 e 888nCu 0.337

I 2 I 2  2 e 888n2I 0.535

Hg Hg^2  2 e 888nHg 0.789

Ag Age 888nAg 0.799

Br 2 Br 2  2 e 888n2Br 1.08

Cl 2 Cl 2  2 e 888n2Cl 1.360

Au Au^3  3 e 888nAu 1.50

F 2 F 2  2 e 888n2F 2.87

Increasing strength as oxidizing agent;

increasing ease of reduction

Increasing strength as reducing agent;

increasing ease of oxidation