Thermodynamics standard reduction potentials

The oxidizing power of the halate ions in aqueous solution, as measured by their standard reduction potentials (p. 854), decreases in the sequence bromate > chlorate > iodate but the rates of reaction follow the sequence iodate > bromate > chlorate. In addition, both the thermodynamic oxidizing power and the rate of reaction depend markedly on the hydrogen-ion concentration of the solution, being substantially greater in acid than in alkaline conditions (p, 855). 

Synthetic electron acceptors have been shown to react very rapidly with free flavins The combination of a flavin with an apoenzyme often inhibits the reaction with certain electron acceptors. The reaction with an electron acceptor will be thermodynamically favored if its standard reduction potential is larger than that of the flavin. A list of electron acceptors along with their reduction potentials can be foimd in Table 3. 

Reduction potential is a measure of how thermodynamically favourable it is for a compound to gain electrons. A high positive value for a reduction potential indicates that a compound is readily reduced and consequently is a strong oxidising agent, i.e. it removes electrons from substances with lower reduction potentials. The oxidised and reduced form of a substance are known as a redox pair. Table 3.2 lists the standard reduction potentials for some typical redox pairs. 

Practically in every general chemistry textbook, one can find a table presenting the Standard (Reduction) Potentials in aqueous solution at 25 °C, sometimes in two parts, indicating the reaction condition acidic solution and basic solution. In most cases, there is another table titled Standard Chemical Thermodynamic Properties (or Selected Thermodynamic Values). The former table is referred to in a chapter devoted to Electrochemistry (or Oxidation - Reduction Reactions), while a reference to the latter one can be found in a chapter dealing with Chemical Thermodynamics (or Chemical Equilibria). It is seldom indicated that the two types of tables contain redundant information since the standard potential values of a cell reaction ( n) can be calculated from the standard molar free (Gibbs) energy change (AG" for the same reaction with a simple relationship... 

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]. 

These values were compared with independent estimates of the °roor/ro ,ro values from thermochemical cycles, where data were available to evaluate them. In the case of di-cumyl peroxide, for example, the E° obtained experimentally differs from the result of a thermodynamic calculation by only 30 mV. It is of interest to note that the uncorrected a data would have led to °roor/rovro values only slightly negative to the corrected ones (0.06-0.07 V). The good agreement in these cases was used as the basis to support the use of the convolution approach to estimate °roor/rovro for systems where the necessary values for thermochemical estimates are not available. This has been particularly useful in the study of endoperoxides and was used to estimate the standard reduction potential of the antimalarial agent, artemisinin. ... 

Dilute aqueous solutions of strong acids (e.g. HCl or H2SO4) contain sufficient concentrations of hydrated protons to oxidize many metals, to produce their most stable states in solution. The only thermodynamic condition for metal oxidation is that the reduction potential of the metal ion produced should be negative. In general, for the metal ion M + undergoing reduction to the metal, if the standard reduction potential for the half-reaction ... 

Only the + 2 state of cobalt has thermodynamic stability in acid solution. The instability of Co3+ is referred to in Section 5.3. Only the + 3 states of Rh and Ir are stable in acid solution their +3/0 standard reduction potentials are quite positive, consistent with their nobility . In alkaline solutions the + 2 and + 3 states of the elements exist as insoluble hydroxides. 

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. 

In the past the electrostatic convention has often been called the European convention and the thermodynamic convention popularized by Luitmer (The Oxidation Potentials of the Elements and Their Values in Aqueous Solution Prenlicc-HBlI Englewood Cliffs. NJ, (952) the American convention. In an effort to reduce confusion, the IUPAC adopted the "Stockholm convention" in which electrode potentials refer to the electrostatic potential and end s refer to the thermodynamic quantity. Furthermore, the recommendation is that standard reduction potentials be listed as electrode potentials" to avoid the possibility of confusion over signs. 

It is necessary to know the thermodynamic reduction potentials of the active metals in chloroaluminate melts. Scordilis-Kelley et al. [451,467] have studied standard reduction potentials in ambient temperature chloroaluminate melts for lithium and sodium, and they have calculated those of K, Rb, Cs. The values are, respectively, -2.066 V, -2.097 V, -2.71 V, -2.77 V and -2.87 V [versus A1(III)/A1 in a 1.5/1.0 A1C13/MEIC reference melt]. 

The need for quantitative bond energy data for oxygen-containing molecules in the condensed phase has prompted the development of an evaluation procedure that is based on electron-transfer thermodynamics. The approach is illustrated for H-O bonds and for Fe-0 bonds, but is applicable to any X-O bond for which appropriate electron-transfer potentials are available. Table 6(a) summarizes the one-electron standard reduction potentials for H3O+, HO-, HOH, O2, HOO-, 02 % -O-, and in aqueous solutions (see Table 1). 

Figure 5 Redox predominance diagrams for iron (a) and manganese (b) boundaries represent the standard reduction potential for reduction of the (thermodynamically-stable) species above the boundary to the (thermodynamically-stable) species below the boundary. If the redox predominance regions of two species (e.g., the gray regions of Fe + and Mn04 ) do not overlap along the y-axis when the two diagrams are superimposed, reaction between the two species is thermodynamically favored...

The third largest class of enzymes is the oxidoreductases, which transfer electrons. Oxidoreductase reactions are different from other reactions in that they can be divided into two or more half reactions. Usually there are only two half reactions, but the methane monooxygenase reaction can be divided into three "half reactions." Each chemical half reaction makes an independent contribution to the equilibrium constant E for a chemical redox reaction. For chemical reactions the standard reduction potentials ° can be determined for half reactions by using electrochemical cells, and these measurements have provided most of the information on standard chemical thermodynamic properties of ions. This research has been restricted to rather simple reactions for which electrode reactions are reversible on platinized platinum or other metal electrodes. 

For solutions with unit activities of ions, the standard reduction potentials for these reactions are 0.337 Vsh and -1.630 Vsh. respectively The reduction potentials in our case are shifted in the active direction however because the activities of the metal ions in solution are less than unity. Because the reduction potential of titanium is more active (more negative) than that of copper, the reduction of copper ions by the titanium metal and the concurrent oxidation of titanium metal by the copper ions will be thermodynamically favored. 

Because of thermodynamic and electrochemical conventions, standard potentials are defined in the direction of reduction, independently of the respective chemical stabilities of the molecules involved. Thus for the oxidation of toluene to its cation radical, E° refers to the reduction of the highly unstable cation radical into the highly stable toluene. To overcome such a priori chemical nonsence, E is frequently designated as the standard oxidation potential of toluene for example. However, such a term should not be accepted according to canonical rules because it formally implies that the cell now operates in a driven mode, that is, is connected to an external power supply [19]. Thus in this chapter we prefer to use the denomination standard reduction potentials, rather than the usual temi standard potential, as a reminder of the E° definition, although such as expression is basically a pleonasm. 

In practice there are several limitations to such measurement. Obviously it implies that both members of the half-reaction are sufficiently stable for a cell to be realized. This is a serious difficulty in organic chemistry owing to usual great reactivities of the species formed upon electron transfers. For the most frequent cases it is then impossible to rely on reversible thermodynamic transformations to determine experimental values of standard reduction potentials. However, these important figures, or at least very precisely approximated values, can be obtained from current intensity potential curves or transient electrochemical methods as is discussed in a later section. 

Let us consider a simple half-reaction O -I- e R, with a standard reduction potential E°. It must be realized that E° corresponds to (1) stable solvation states for O and R and (2) stable nuclear configurations for O and R. On the other hand, ionization potentials Ip or affinities Aff correspond to nonsolvated O and R and radiative transitions (Sec. II.C), that is, to unstable nuclear configurations R or O, respectively, for R (affinities) or O (ionization potentials). The ensuing thermodynamic relationships between the three numbers E°, Ip, anc Aff are then derived from thermodynamic considerations based on the... 

Since the reaction of RX with Mg to generate RMgX is a formal oxidative addition, then a better way to look at the reactivity is to consider the reduction potential of the RX. For the reaction to be thermodynamically feasible, AE must be positive (i.e., a —AG). This means that the reduction potential for RX must be more positive than the reduction potential of Mg " to Mg, which has a standard reduction potential of —2.375 V [34]. Since the reduction potentials are greatly affected by solvent and reference electrode, comparisons and discussion of trends must be made under the same electrochemical system (Table 2). 

The standard reduction potentials of the most relevant half-reactions involved in the four- and two-electron reduction of dioxygen in acid and alkaline aqueous media are listed in Table 3.1. It follows from these values, that, under full thermodynamic control, the equilibrium concentration of peroxide at the reversible potential for the four-electron reduction of oxygen in acid media, that is, 1.23 V, is of the order of 10 18 M. Hence, a stepwise reduction of dioxygen to yield currents of about 1A cm 2 will require values for the standard heterogeneous rate constants for... 

Since the standard reduction potential for the SO /SO couple is +0,16 V and the standard reduction potential for the O2/H2O couple is +1,23 V then the potential difference for the equation above is = +1.23 V - 0.16 V = +1.07 V. Since this potential is large and positive, the Gibbs energy of this reaction is negative (reaction is hence spontaneous) and will be driven nearly to completion (K> 1). Thus, the expected thermodynamic fate of SO2 is its conversion to sulfate (or neutral sulfuric acid vapour). This aqueous solution of SO and H ions precipitates as acid rain, which can have a pH as low as 2 (the pH of rain water that is not contaminated with sulfuric or nitric acid is -5.6),... 

Recall that the Frost diagrams can tell us a lot about the thermodynamic stability of an oxidation state but under strict conditions. In this case we are constructing the diagram from the standard reduction potentials, thus the diagram below is strictly correct for pH = 0 and unit activity of ail species involved. Nevertheless, the conclusions hold rather generally. Note that a completely correct Frost diagram must clearly indicate chemical species associated with each data point. In some cases, such as for Cr +5 and +4, the chemical species are not well-defined. In these instances it is sufficient to indicate the element s oxidation state (i.e., Cr(V) and Cr(IV)). 

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... 

The thermodynamic and kinetic data are summarized in Table 4. The equilibrium constants as listed in Table 4 are calculated from log K = A °/0.059 and A °= — (°M) + (°0i) and involve the standard reduction potentials of the melal couple and of oxygen at [02] = 1 M. This standard state is more suitable for kinetic calculations than the more widely used convention p0l = 1 atm. The calculation of thermodynamic equilibrium constants for the surface complexes if more difficult (Sposito, 1983) and requires further work. Figure 8 displays the resulting LFER. A theoretical line of slope one (its the data over a broad range of 13 log k units. This analysis supports an outer-sphere mechanism for the... 

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