Redox potentials for common half reactions

The analysis of the electrical potential, or voltage, developed by pairing various half reactions in electrochemical cells has led to the determination of redox potentials for a substantial number of common half reactions. While a detailed description of redox potentials requires the methods of thermodynamics (the branch of physics concerned with the role played by heat in the transformation of matter or energy), a great deal of useful information can be obtained from redox potentials with minimal recourse to formal theory. Basically, a table of half-cell potentials is a summary of the relative tendencies of different oxidations and reductions to occur.

The table of standard reduction potentials lists selected half reactions and their corresponding reduction potentials (which are symbolized by E°). The physical significance of the values is directly linked to several agreements about their use. First, the greater the value of E° (the reduction potential), the greater the tendency of a half reaction to proceed from left to right (as written). The half reactions in the table are listed from top to bottom in order of decreasing E°: the higher a reaction’s position on the list, the greater the tendency of the reactants to accept electrons. In other words, reagents high on the list, such as fluorine gas (F₂) and permanganate ion (MnO₄⁻), are strong oxidizing agents. Second, the reduction of hydrogen ions (H⁺) to hydrogen gas (H₂) is arbitrarily assigned the value 0 volts. Half cells with positive reduction potentials involve reactants that are more readily reduced than H⁺; conversely, those with negative potentials involve reactants that are more difficult to reduce than hydrogen ions.

With the aid of reduction potentials, it is possible to predict whether a particular oxidation-reduction reaction can occur. The predictions require breaking down the overall reaction into two half reactions of known reduction potentials. For example, if a strip of zinc metal is dipped into a solution containing copper(II) ion, the possibility exists for a redox process, which can be regarded as the sum of the half reactions aqueous zinc ion (Zn²⁺[aq]) to zinc metal (Zn[s]) and aqueous copper ion (Cu²⁺[aq]) to copper metal (Cu[s]), as follows:

Combining these two half reactions requires writing the zinc ion to zinc metal half reaction the reverse of the way it appears in the table of standard reduction potentials. When the direction of a half reaction is reversed, so that it can be added to another half reaction, the sign of its redox potential is also reversed (in this case, from negative to positive), and the two reduction potential values are then added.

The resulting E° value for the net reaction, +1.10 volts, measures the tendency of the net reaction to occur. If E^o for a particular net reaction is positive, the process may be expected to occur spontaneously when the reactants are mixed at specified concentrations (one mole per litre; see below Oxidation-reduction equilibria). Therefore, it is predicted that copper metal should be deposited on a strip of zinc metal when the latter is immersed in a solution of a copper(II) salt. This reaction is, in fact, readily observed in the laboratory. A more specific physical interpretation of the +1.10 volt value is that it represents the voltage that would be produced by an ideal electrochemical cell based on the copper(II) ion to copper metal and zinc(II) ion to zinc metal half reactions with all the reagents at specified concentrations.

When the same two half cells are combined, with both their directions (and therefore the signs of their redox potentials) reversed, it is predicted that the reverse reaction, the depositing of zinc metal from a zinc(II) ion solution onto a copper strip, will not occur spontaneously. As in the case of E° values for half reactions, those for net redox reactions also change sign when the direction of the reaction is reversed.

The results of the copper-zinc system can be applied more generally to the half reactions in the table of standard reduction potentials. For example, copper(II) ion in water (Cu²⁺[aq]) is an oxidant strong enough to force a half reaction lower on the table to proceed spontaneously in the opposite direction of that written. Therefore, not only is copper(II) ion expected to oxidize zinc metal (Zn[s]) to zinc(II) ion (Zn²⁺[aq]); it is also predicted to oxidize hydrogen gas (H₂[g]) to hydrogen ion (H⁺) and sodium metal (Na[s]) to sodium ion (Na⁺).

Similarly, fluorine gas (F₂[g]), the strongest oxidant listed in the table of standard reduction potentials, is predicted to oxidize spontaneously the products of all the other half reactions in the table. In contrast, the strongest reducing agent is solid sodium metal (Na[s]), and it is expected spontaneously to reduce the reactants of all the other half cells.

Selected values of standard reduction potentials are given in the table.