Entropy redox couple

Nickel(III) peptide complexes have a tetragonally-distorted octahedral geometry as shown by electron spin resonance studies (19) and by reaction entropies for the Ni(III,II) redox couple (17). Axial substitutions for Ni(III)-peptide complexes are very fast with formation rate constants for imidazole greater... 

Reaction entropy of redox couple, determined at same ionic strength as E. Data from ref. 9 unless otherwise indicated. 

Calculation of the entropy variation of the [CuIU(H 3G4)] / [Cu1I(H3G4)]2 redox couple from the negative slope of the linear trend gives ... 

In this case, the decrease in entropy of the redox couple i.e. a decrease of the molecular disorder) is proposed to be due to the passage of the complex from the CuHIN4 square planar geometry to the usual octahedral geometry of Cu(II) complexes which requires the axial coordination of two solvent molecules (water). This reduces the disorder of the solvent molecules around the complex. 

Negative values for redox couple entropy have also been obtained for the Cu(II)/Cu(I) reduction, in aqueous medium, of the blue copper proteins stellacyanin, plastocyanin and azurin.14 The decrease in molecular disorder has been attributed in this case to the fact that the charge neutralization of the redox site (from + 1 to 0) favours the formation of hydrogen bonds between the solvent (water) and the copper centre.17... 

Since the difference in entropy between the two redox couples is difficult to ascribe to different complex-solvent interactions, as the charges of the complexes are identical in the two cases, other causes of the entropy difference must be considered. 

Table 3 provides entropies for those species that are needed to determine the temperature dependence of standard potentials for alkali metal redox couples. AU of these entropies were obtained from values published by NIST [11]. The resulting temperature dependences agree well with values tabulated by Bratsch [17]. 

In Table 1 are summarized representative examples of self-exchange rate constant data for a variety of different types of redox couples based on metal complexes, organometallic compounds, organics and clusters. Where available the results of temperature dependence studies are also cited. For convenience, data obtained from temperature dependence studies are presented as enthalpies and entropies of activation as calculated from the reaction rate theory expression in equation (14). 

A consideration of these relationships reveals8 that because E° is a thermodynamic parameter and represents an energy difference between two oxidation states and in many cases the spectroscopic or other parameter refers to only one half of the couple, then some special conditions must exist in order for these relationships to work. The special conditions under which these relationships work are that (a) steric effects are either unimportant or approximately the same in both halves of the redox couple and (b) changes in E° and the spectroscopic or other parameters arise mainly through electronic effects. The existence of many examples of these relationships for series of closely related complexes is perhaps not too unexpected as it is likely that, for such a series, the solvational contribution to the enthalpy change, and the total entropy change, for the redox reaction will remain constant, thus giving rise to the above necessary conditions. 

Here the reversible formal potential, E° of the redox couple of interest has been measured at a series of temperatures versus a reference electrode maintained at constant temperature. Values of AShr have been interpreted in terms of differences in water-ligand hydrogen bonding [1] and have been compared to the entropy change for spin crossover in iron complexes [2]. 

To summarise E° values for redox couples of the type M(s)/M"+(aq) can largely be rationalised in terms of the atomisation enthalpies, ionisation energies and hydration enthalpies. The entropy terms can be neglected in most cases. 

As with electrode potentials themselves, confusion can occur regarding alternative entropy scales for inorganic ions and redox couples. In addition to values of AS obtained from nonisothermal cells, it is common to encounter reaction entropies for redox couples that actually refer to the entropy of reaction, AS, of a complete cell containing a hydrogen electrode. These latter quantities will, therefore, differ from AS by an amount equal to the reaction entropy of the hydrogen electrode half-cell it follows that AS° SJ, + 85 n J deg mol. A scale of ionic and redox reaction entropies is also established by arbitrarily assigning the entropy of the hydrogen ion, SJ,., a value... 

ZeJ = 10 cm/s). The reaction entropy AS% can be calculated from the temperature dependence of the formal potential of the redox couple ... 

This reaction entropy difference between reduced and oxidized forms of the redox couple can also be evaluated using the Bom electrostatic model ... 

Hupp IT, Weaver Ml (1984) Solvent, ligand, and ionic charge effects on reaction entropies for simple transition-metal redox couples. Inorg Chem 23 3639-3644... 

Yamato Y, Katayama Y, Miura T (2013) Effects of the interaction between ionic liquids and redox couples on their reaction entropies. 1 Electrochem Soc 160 H309-H314... 

Naegeli, R. Redepenning, J. Anson, F. C. Influence of supporting electrolyte concentration and composition on formal potentials and entropies of redox couples incorporated in Nation coatings on electrodes. J. Phys. Chem. 1986, 90, 6227-6232. 

Similar to the [Fe(CN)6] / " redox couple, EPH of an electrode reaction can directly be measured by the thermoelectrochemical experiments. However, it is hard to measure EPH of an electrode reaction at the standard state directly, because the standard state chosen usually in thermodynamics is even physically unrealizable in most cases. According to Eq.(8), EPH of a standard electrode reaction can be determined provided that AS of the reaction is known. For example, for the [Fe(CN)6]"V couple at each standard state of the components, its entropy change is calculated as AS [Fe(CN)6]" /" ) = -240.6 J.K i.moF, therefore, AS ( [Fe(CN)6]"V" ) = -153.0 J.K-i.mopi, and U [Fe(CN)6] V ) = -45.6 kj.mol-i. The EPHs of some standard electrode reactions at 298.15K are given in table 4. Similarly, the electrochemical Peltier coefficient, a characteristic quantity of electrode reaction, can be also determined by Eq.(20). 

As stated above, even the availability of reversible redox couples with high entropy values is not sufficient to do continuous potential recording. The reason is that in many cases only a single partner of the redox system is available, whereas the second one is not stable in solution, or cannot be synthesised. Generating the missing partner by coulometry sometimes was helpful, but often even such procedures did not solve the problem. 

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