Latimer-Luther’s rule

This mathematical relationship expresses Latimer-Luther s rule. We infer from it the fact that it is unnecessary to calculate the standard molar free enthalpy changes, but this result is not general. The rule is valid only for processes that involve only redox phenomena. This was not the case in the examples given before. 

Let s recall that the handling of the free enthalpies of redox reactions permits us to calculate unknown standard potentials (see Sect. 2.11 in Chap. 2). This point is also important for the calculation of cell potentials, namely, for the calculation of the electrical charges brought by the electrodes (see below) not all redox couples appear in Tables 13.1-13.3. The standard potentials of the other couples may be calculated by using the additivity of the free enthalpies of the physical or chemical equivalent processes or with the help of Latimer-Luther s rule (see Chap. 2). 

Obviously, the slope values of the segments OA, OB, OA, and OB are the standard potential values of the corresponding couples. The slopes of segments AB andA B are, equal to the standard potentials Cu +/Cu+ and Hg +/Hg2+, respectively. This is a consequence of the additivity of the free enthalpies and, more precisely in this case, of Latimer-Luther s rule (see Chap. 2). 

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