Redox thermodynamic standard data

Determination of Thermodynamic Standard Data of Redox Equilibria. . 1176... 

Temperature-dependent thermodynamic standard data of an-timony(III)/antimony(V) redox equilibrium (Equation (26-28)), in an oxidic glass-forming silicate melt see also Figure 26-18. 

In order to define a potential reference suitable for electrochemistry, a reference redox couple must firstly be chosen the H /H2 couple in its thermodynamic standard state. This reference system is called Standard Hydrogen Electrode (SHE). It is well-defined even though it is theoretical, and it is the reference used in all contemporary data tables in thermodynamics and electrochemistry... 

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. 

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 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 computerized aqueous chemical model of Truesdell and Jones (, 3), WATEQ, has been greatly revised and expanded to include consideration of ion association and solubility equilibria for several trace metals, Ag, As, Cd, Cu, Mn, Ni, Pb and Zn, solubility equilibria for various metastable and(or) sparingly soluble equilibrium solids, calculation of propagated standard deviation, calculation of redox potential from various couples, polysulfides, and a mass balance section for sulfide solutes. Revisions include expansion and revision of the redox, sulfate, iron, boron, and fluoride solute sections, changes in the possible operations with Fe (II, III, and II + HI), and updating the model s thermodynamic data base using critically evaluated values (81, 50, 58) and new compilations (51, 26 R. M. Siebert and... 

The (standard) reduction potentials at pH 7 of some important biogeochemi cal redox couples are given in Table 5 together with the reduction potentials ofl some half-reactions involving xenobiotic organic species. From the data in Tubtifl 5 we can, for example, conclude that, from a thermodynamic point of view, thtfl... 

AG° = 31.9 kJmol-1 for reaction 32. This value should be compared to the thermodynamic redox potentials for the process with 1 M 02 as the standard state. The relevant redox potentials are 0.158 and —0.16 V for the reduction of Cu(II) and 02 (Sawyer and Valentine, 1981), respectively. From these thermodynamic data we calculate AG° = — nFE° = 30.1 kJmol-1. The close agreement indicates that the redox kinetics of copper in natural waters is, indeed, governed by reaction 32 as the rate-limiting step. 

For some electrochemical cells, like those in Figure 1.1.1, it is possible to calculate the open-circuit potential from thermodynamic data, that is, from the standard potentials of the half-reactions involved at both electrodes via the Nemst equation (see Chapter 2). The key point is that a true equilibrium is established, because a pair of redox forms linked by a given half-reaction (i.e., a redox couple) is present at each electrode. In Figure... 

One of the key reference books in the field of electrochemical thermodynamics, covering a wide number of Inorganic systems in aqueous solution standard chemical potentials, redox potentials and E/pH diagrams. It also Includes data on the kinetic parameters involved for reducing protons in aqueous medium. Particularly useful for understanding corrosion in aqueous medium. 

The thermodynamics of a redox reaction may be regarded a basic parameter set which defines the theoretical or optimal properties of a system. The theoretical potential of an electrode can be calculated on the basis of the Nernst equation and the Gibbs free energy of the reaction. Such estimations are often done based on literature data of standard enthalpies and entropies of formation of reactants and products. However, there may be deviations in the electrochemical experiment, which may be due to the following reasons ... 

Thermodynamics. Owing to the occurring polymorphism for the trivalent R oxides and the redox instability for the mixed trivalent-tetravalent R oxides, even precisely obtained thermodynamic data may refer to uncertain compositions. In particular, this is the case for the cl to mC transformation, where the transformation temperatures are not well defined and the high-temperature heat capacities are therefore uncertain. On the other hand, the standard enthalpies and entropies themselves of these transformations are known, yet rather small (around +1.0 and +0.6 per R2O3, respectively). Owing to these problems, data involving these transitions as well as data for oxides subject to extensive redox interactions are not included in the essential thermodynamic data listed in tables 6 and 7. 

In the present system, CuO powder is dispersed in the reaction solution. When the pH of the reaction solution is 9 and the temperature is 298 K, the activity of Cu + aquo ion is calculated to be about 7.5 x lOii using the thermodynamic data shown in Table 1 (Criss Cobble, 1964 Kubaschewski Alcock, 1979 Latimer 1959 Stull Prophet, 1971). This value itself does not have a precise meaning the calculated value of the activity of Cu2+ aquo ion can be significantly affected by an experimental error of thermodynamic data, e.g. the standard entropy of CuO. Normally, this value just indicates that CuO does not dissolve into the solution in a practical sense. However using this value, one can calculate other important thermodynamic values such as the oxidation-reduction (redox) pxjtential of Cu +ZCu redox pair in this reaction system using the same thermodynamic data set. 

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