The electrical force produced by an electrochemical cell is measured by the cell voltage. Cell voltage depends on the redox reactions occuring in the cell and the concentration of the reactants, but not on the number of electrons passing through the cell: it is an intensive property. The standard voltage E⁰ of a cell is measured when all reactants are at 25^oC and at 1M concentration or 1 atm pressure.

For a copper-zinc cell like

Zn|Zn⁺²||Cu⁺²|Cu

Since we can split a redox reaction into two parts, we can also define standard voltages for both the oxidation and reduction parts of the reaction, E_ox⁰ and E_red⁰. We arbitrarily pick the hydrogen reduction half reaction

2H⁺(aq) + 2e^- -> H₂(g)

There are large tables published with the values of standard reduction voltages for half reactions. A small example is on pages 504-505 in your book. The oxidation half reactions are just the reaction run in reverse, and the half cell oxidation voltage is just the negative of the reduction voltage. For example:

• Reduction: Zn⁺²(aq) + 2e^- -> Zn(s)    E_red⁰ = -0.762 V
• Oxidation: Zn(s) -> Zn⁺²(aq) + 2e^-    E_ox⁰ = 0.762 V

The standard voltage of a cell, E⁰, is just the sum of the standard voltages of the oxidation and reduction half reactions. This is independent of the number of electrons in the half reaction: if you need to multiply a half reaction by 3 to balance the redox equation, you do not multiply the cell voltage by 3. For example, the zinc-copper cell above is made up of the reactions

• Oxidation: Zn(s) -> Zn⁺²(aq) + 2e^-    E_ox⁰ = 0.762 V
• Reduction: Cu⁺²(aq) + 2e^- -> Cu(s)    E_red⁰ = 0.339 V

Example: What is the standard voltage for the cell

Mn|Mn⁺²||Fe⁺³|Fe⁺²

Solution: First, seperate the cell into two half reactions

• Oxidation: Mn(s) -> Mn⁺²(aq) + 2e^-
• Reduction: Fe⁺³(aq) + e^- -> Fe⁺²(aq)

To compute the cell voltage, simply add the two voltages. It doesn't matter that the manganese reaction has two electrons and the iron only one

E⁰ = E_red⁰ + E_ox⁰ = 0.769 + 1.182 = 1.951 V

Information provided by: http://learn.chem.vt.edu