http://www.ck12.org Chapter 23. Electrochemistry
E^0 cell=E^0 red−E^0 oxid
0 .34 V=E^0 Cu− 0 .00 V
E^0 Cu= 0 .34 V− 0 .00 V= + 0 .34 VThe standard reduction potential for the Cu^2 +|Cu half-cell is thus equal to +0.34 V. In a similar way, the reduction
potential for any half-cell can be determined by connecting it to a SHE and measuring the voltage.
When a standard hydrogen half-cell is connected to a standard zinc half-cell (Figure23.6 (B)), the measured voltage
is 0.76 V. However, it is observed that the zinc electrode is oxidized to zinc ions, while the hydrogen ion is reduced
to hydrogen gas.
Oxidation: Zn(s)→Zn^2 +(aq)+2e−
Reduction: 2H+(aq)+2e−→H 2 (g)
Overall: Zn(s)+2H+(aq)→Zn^2 +(aq)+H 2 (g)The SHE is now the cathode, while the zinc electrode is the anode. Now it is the E^0 for the oxidation half-cell that
is the unknown in the standard cell potential equation.
E^0 cell=E^0 red−E^0 oxid
0 .76 V= 0 .00 V−E^0 Zn
E^0 Zn= 0 .00 V− 0 .76 V=− 0 .76 VThe standard reduction potential for the Zn^2 +/Zn half-cell is equal to−0.76 V. A negative standard reduction
potential for a particular species means that is easier to reduce H+than to reduce that species. A positive standard
reduction potential for a species means that it reduces more easily than H+. The table below (Table23.2) lists many
standard reduction potentials under standard conditions. From top to bottom, they are listed in decreasing order of
their tendency to occur in the forward direction, as a reduction. Fluorine gas is the most easily reduced, while lithium
ions are the most difficult to reduce. Note that this table is the exact opposite of the activity series. Lithium ions are
very difficult to reduce, which means that lithium metal is very easy to oxidize.
TABLE23.2: Standard Reduction Potentials at 25°C
Half Reaction Eo(V)
F 2 + 2e−→2F− +2.87
PbO 2 + 4H++ SO 42 −+ 2e−→PbSO 4 + 2H 2 O +1.70
MnO 4 −+ 8H++ 5e−→Mn^2 ++ 4H 2 O +1.51
Au^3 ++ 3e−→Au +1.50
Cl 2 + 2e−→2Cl− +1.36
Cr 2 O 72 −+ 14H++ 6e−→2Cr^3 ++ 7H 2 O +1.33
O 2 + 4H++ 4e−→2H 2 O +1.23
Br 2 + 2e−→2Br− +1.07
NO 3 −+ 4H++ 3e−→NO + 2H 2 O +0.96
2Hg^2 ++ 2e−→Hg 22 + +0.92
Hg^2 ++ 2e−→Hg +0.85
Ag++ e−→Ag +0.80