Factors influencing the magnitudes of standard reduction potentials

Factors influencing the magnitudes of standard reduction potentials 

In this section, we hrst consider factors that influence the magnitude of E° for the Na /Na and Ag /Ag couples, by correlating these values with those of other, independently determined thermodynamic quantities. This comparison allows us to investigate the reasons why, in aqueous media, Na is so much more reactive than Ag, and gives an example that can be extended to other pairs or families of species. 

Whereas the standard reduction potential for half-reaction 7.51 is readily measurable in aqueous solution (see Section 7.2), that for half-reaction 7.50 must be determined by a rather elaborate set of experiments involving Na amalgam electrodes (amalgams, see Box 22.3). 

Similar analyses for other metals can be carried out. For example, Cu and Zn are adjacent rf-block metals, and it is interesting to investigate factors that contribute to the difference between E° values for the Cu +/Cu and Zn +/Zn redox couples, and thus reveal how a balance of thermodynamic factors governs the spontaneous reaction that occurs in the Daniell cell (reaction 7.8). Table 7.3 lists relevant thermodynamic data it is apparent that the crucial factor in making E°q i+ significantly more positive than ii°zn2+/zn is the greater enthalpy of atomization of Cu Na (aq)-I-e Na(s) 

We can represent the general half-equation for reduction as taking place in steps as shown in Fig. 8.5. Since all standard reduction potentials are measured with respect to the standard hydrogen electrode (for which, by convention, AH°, AG° and AS° are all zero), we must also consider the second thermodynamic cycle (involving absolute values) in Fig. 8.5. Table 8.2 lists values of AH° for steps in the cycles 

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