Thermochemical Data Half-Cell Potentials

Like the equilibrium constant, is obtained from thermochemical data. Since the overall reaction in the electrochemical cell is composed of two half-reactions, we can tabulate electrochemical data in terms of half-reactions. We then simply add together the appropriate reduction and oxidation half-reactions to calculate E for the entire electrochemical cell. 

An isolated half-cell reaction cannot occur by itself, so we need to choose a reference half-cell reaction to complete the reference cell. Thermochemical data for electrochemical half-reactions are commonly published as the standard half-cell potential for reduction reactions, E°. Representative values are reported in Table 9.1 however, these are but a few of the extensive sets of values that are available. This potential is measured with reference to a hydrogen—hydrogen ion oxidation reaction, whose potential is defined as zero  

The reactant and product species for both the reduction half-reaction and the hydrogen oxidation half-reaction are specified to be in their standard states. Recall that the standard state of a gas is an ideal gas at 1 bar, a liquid is a 1 m ideal solution in the Henry s law sense, and a solid is the pure solid with an activity of 1. In terms of our shorthand notation, we can measure the standard potential of any reduction half-reaction with a standard hydrogen electrode (S.H.E.)  

It is also possible to obtain the value of oxidation reactions from Table 9.1. The halfcell potential of an oxidation reaction is simply the negative of the reported reduction half-reaction. The half-reaction potential and the hydrogen reduction reaction reference are analogous to our use of Gibbs energy of formation and the elemental form of molecules in the other parts of this chapter. 

It is instructive to compare the half-cell reactions from Table 9.1. To construct an electrochemical cell, we use one reduction half-reaction and one oxidation half-reaction. The value of ° for the reduction reaction can be obtained directly from Table 9.1, while the oxidation reaction is the negative of the value listed. 

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