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Note that the standard electrode potential for the hydrogen electrode is zero because the SHE is the reference half-cell. Some of the entries in Table 10.1 involve only water and the gases oxygen and hydrogen. Another group consists of metals and their ions; a third group contains only ions of the same element which have multiple valence (or oxidation) states. The last group in the table contains half-cell reactions in which solids containing oxygen are involved.

Often the standard electrochemical potential of half-cell reactions that are not included in Table 10.1 (or similar tabulation) can be derived from reactions that appear there. In some cases, thermochemical data from sources other than the s° table need to be incorporated. In all cases, the method involves converting the standard electrochemical potentials to standard free energies of the overall cell reaction by Eq (10.30), combining the latter algebraically to determine A^" of the desired half-cell reaction, and finally converting back to so.

Example: The half-cell reaction Fe = Fe+3 + 3e

This half-cell reaction is the sum or reactions 7 and 9 in Table 10.1. The standard free energy changes for these reactions and for the desired one are:

A^o(Fe/Fe2+) = -2Fs7 A^(Fe2+ /Fe3+) = -2Fso A^o(Fe/Fe3+) = -2Fso Summing the first two free energy changes gives the third:

A^ o(Fe/Fe2+) + A^ o(Fe2+/Fe3+) = A^ o(Fe/Fe3+) Dividing by 3 gives the standard electrode potential:

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