The potential difference, which is measured in volts (v), depends upon the particular substances constituting the electrodes. For any electric cell, the total potential is the sum of those produced by the reactions at the two electrodes:
EMF cell = EMF oxidation + EMF reduction
The EMF denotes electromotive force, another name for electrical potential.
Chemists have measured the voltages of a great variety of electrodes by connecting each in a cell with a standard hydrogen electrode, which is hydrogen gas at 1 atmosphere bubbling over a platinum wire immersed in 1 M H +( aq). This standard electrode is arbitrarily assigned a potential of 0 volts, and measurement of the EMF of the complete cell allows the potential of the other electrode to be determined. Table 1 lists some standard potentials for electrodes at which reduction is occurring.
Near the middle of the list, you will see 0 volts arbitrarily assigned to the standard hydrogen electrode; all other potentials are relative to the hydrogen half‐reaction. The voltages are given signs appropriate for a reduction reaction. For oxidation, the sign is reversed; thus, the oxidation half‐reaction,
has an EMF of –1.20 volts, the opposite given in Table 1. Look this up in the chart to be sure that you understand.
Consider how these standard potentials are used to determine the voltage of an electric cell. In the zinc‐copper cell described earlier, the two half‐reactions must be added to determine the cell EMF. (See Table 2.)
The complete zinc‐copper cell has a total potential of 1.10 volts (the sum of 0.76v and 0.34v). Notice that the sign of the potential of the zinc anode is the reverse of the sign given in the chart of standard electrode potentials (see Table 1) because the reaction at the anode is oxidation.
In the chart of standard electrode potentials (see Table 1), reactions are arranged in order of their tendency to occur. Reactions with a positive EMF occur more readily than those with a negative EMF. The zinc‐copper cell has an overall EMF of +1.10 volts, so the dissolution of zinc and deposition of copper can proceed.
Calculate the total potential of a similar cell with zinc and aluminum electrodes. Table 3 shows the two pertinent half‐reactions.
Such a cell with zinc and aluminum electrodes would have an overall potential of +0.90 volt, with aluminum being dissolved and zinc metal being deposited out of solution.
If you select any two half‐reactions from the chart of standard electrode potentials, the half‐reaction higher on the list will proceed as a reduction, and the one lower on the list will proceed in the reverse direction, as an oxidation. Beware: Some references give standard electrode potentials for oxidation half‐reactions, so you have to switch “higher” and “lower” in the rule stated in the preceding sentence, though this is not common.
- Some silver mines dump shredded iron cans into ponds containing dissolved silver salts. Write the two redox half‐reactions and the overall balanced reaction that explains the deposition of silver from the solution.
- Considering only the elements in the chart of standard electrode potentials (see Table 1), which pair can make a battery with the greatest voltage? What would the voltage be?