Standard potentials thermodynamic data

Bard, A. J., Parsons, R., and Jordan, J., Eds. (1985) Standard Potentials in Aqueous Solution, lUPAC, Dekker, New York. Contains, in addition to standard potentials, thermodynamic data. 

Table A5.6 is obtained from the extensive tabulation by W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Second Edition, Prentice-Hall, Inc., Engelwood Cliffs, N.J. (1952). His tabulation was published at the time when p = 1 atm was the standard state for gases. His results should be corrected to the p = 1 bar standard state for comparison with other modern standard state thermodynamic data. The correction is given by (RT/F) In (1.01325), which has a value of 0.0003 volts at T = 298.15 K. The correction is small and probably negligible for all except the most precise work. (E° values measured to better than 1(T3 volts are unusual.)...

Thermodynamic data (4) for selected manganese compounds is given ia Table 3 standard electrode potentials are given ia Table 4. A pH—potential diagram for aqueous manganese compounds at 25°C is shown ia Figure 1 (9). 

The values in Table 2.16 show how the potentials obtained under service conditions differ from the standard electrode potentials which are frequently calculated from thermodynamic data. Thus aluminium, which is normally coated with an oxide film, has a more noble value than the equilibrium potential 3 + / = — 1-66V vs. S.H.E. and similar considerations apply to passive stainless steel (see Chapter 21). 

Besides measuring the potential in the standard conditions, it is possible to calculate its value from thermodynamic data [9]. In addition one can determine the influence of changing pressure, temperature, concentration, etc. 

The relationships of the type (3.1.54) and (3.1.57) imply that the standard electrode potentials can be derived directly from the thermodynamic data (and vice versa). The values of the standard chemical potentials are identified with the values of the standard Gibbs energies of formation, tabulated, for example, by the US National Bureau of Standards. On the other hand, the experimental approach to the determination of standard electrode potentials is based on the cells of the type (3.1.41) whose EMFs are extrapolated to zero ionic strength. 

When one considers the potential high-energy release on rupture of a carborane unit, together with the thermodynamic stability of combustion products, it is hardly surprising that there is a body of literature that reports on the use of carbo-ranes within propellant compositions. Their use in energetic applications is to be expected when the enthalpy of formation (AH/) data for the products of combustion for boron are compared to those of carbon. Thermodynamic data for the enthalpy of formation of o-carborane and of typical boron and carbon combustion products is shown in Table 4. Measurements of the standard enthalpy of combustion32 for crystalline samples of ortho-carborane show that complete combustion is a highly exothermic reaction, AH = — 8994 KJmol. ... 

Note, in using Equations 50 and 53 above, that tabulations of thermodynamic data for electrolytes tend to employ a 1 molar ess concentration for all species in solution. For situations defined to have a standard-state pH value different from 0 (which corresponds to a 1 molar concentration of solvated protons), the standard-state chemical potentials for anions and cations are determined as... 

The important role which the inductive effects of the substituent groups plays in the thermodynamic aspects of redox processes (i.e. on the value of the standard potential) is shown by the data reported in Table 6. Ferrocene derivatives are listed which have either electron-donating (which render the oxidation easier) or electron-withdrawing (which render the oxidation more difficult) substituent groups. 

Nevertheless, since in thermodynamics the values of the standard potentials are normally referred to the NHE electrode, when one wishes to obtain thermodynamic data from electrochemical experiments it is usual to transform the redox potentials obtained employing other reference electrodes into values with respect to the NHE, according to the scales given in Chapter 3, Section 1.2. 

Let us consider a set of experimental determinations of the standard potential at a series of temperatures, such as is fisted in Table A.2. A graph of these data (Figure A.2) shows that the slope varies slowly but uniformly along the entire temperature range. For thermodynamic purposes, as in the calculation of the enthalpy of reaction in the transformation... 

Lack of thermodynamic data prevents our making full use of the relationship between standard redox potential and free enthalpy of solvation. For many simple redox systems the standard redox potential has been found to be related to the donicities of the respective ligands. This question has been extensively discussed in a previous paper (3). 

The standard potentials of the reactions involving bromine species with oxidation number of +1 or higher are calculated from thermodynamic data. 

The standard potential values presented above are mostly derived from thermodynamic data [3] however, where it was possible, the electrochemically measured values [2] are given. 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

The metals of Group 11 all form + l states that vary in their stability with respect to the metallic state. The standard reduction potentials for the couples Cu+/Cu and Ag + /Ag are +0.52 V and +0.8 V, respectively. That for Au + /Au has an estimated value of + 1.62 V. The thermodynamic data for the calculation of the reduction potentials are given in Table 7.18, which also contains the calculated potentials for Cu and Ag. 

The standard potential of the Na+/Na couple can be obtained from these emf values. However, it can also be obtained by calculation from thermodynamic data [2] and the result agrees well with the result by emf measurement. 

Although the entire discussion of electrochemistry thus far has been in terms of aqueous solutions, the same principles apply equaly well to nonaqueous solvents. As a result of differences in solvation energies, electrode potentials may vary considerably from those found in aqueous solution. In addition the oxidation and reduction potentials characteristic of the solvent vary with the chemical behavior of the solvent. as a result of these two effects, it is often possible to carry out reactions in a nonaqueous solvent that would be impossible in water. For example, both sodium and beryllium are too reactive to be electroplated from aqueous solution, but beryllium can be electroplated from liquid ammonia and sodium from solutions in pyridine. 0 Unfortunately, the thermodynamic data necessary to construct complete tables of standard potential values are lacking for most solvents other than water. Jolly 1 has compiled such a table for liquid ammonia. The hydrogen electrode is used as the reference point to establish the scale as in water ... 

The availability of appropriate thermodynamic data for organic redox couples often limits application of the simple formulation presented in section 3.1. This is primarily because few organic substances form reversible redox couples amenable to direct measurement of Nernstian standard potentials. 

The estimation of standard potentials from other thermodynamic data follows a simple additive procedure. Typically, these calculations are based on published, gas-phase free energies of formation, AG° (g), for reactants and products. These gas phase data are adjusted to aqueous phase tree energies, AG° (aq), using... 

Reference data on total energies of forms 19-23 optimized by means of different theoretical methods in the gas phase are given in Table 2. Various energetic characteristics of tetrazoles can be successfully estimated. The vertical adiabatic ionization potentials of both neutral tautomers 20 and 21 were calculated for a- and Tt-radical cations <2000CPL(330)212>. The standard molar thermodynamic functions (enthalpies, heat capacities, and entropies) of... 

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