Standard potentials for oxidative

Here, we have substituted E, the standard potential for oxidation of H2, for —2FAG°, where AG° is the free energy change for reaction under standard conditions. Obviously, similar relationships can be written to calculate the equilibrium potentials for other fuels. For example, for alkanes, C, Azn z, the analogous relationship between partial pressures and the equilibrium cell potential is the following... 

At the temperatures of interest for SOFC applications, the standard potentials for oxidation, Go, are similar for hydrocarbon fuels and for H2 and CO, as shown in Table 1. Since Eo for H2 is more temperature dependent than the Eg for hydrocarbons, the thermodynamic advantage for hydrocarbon fuels is more apparent at higher temperatures. However, the... 

Standard Potentials for Oxidation of First-Series Transition Metals... 

The oxidation stops at the iron(II) stage because the standard potential for oxidation of the iron(II) ion is negative ... 

Table 7.7 Kinetic Data for Reaction (7.8.15) with Various Donor Molecules Together with the Standard Potentials for Oxidation of the Donor Molecule to its Cation Radical [24]...

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

Chlorine dioxide gas is a strong oxidizer. The standard reversible potential is determined by the specific reaction chemistry. The standard potential for gaseous CIO2 in aqueous solution reactions where a chloride ion is the product is —1.511 V, but the potential can vary as a function of pH and concentration (26) ... 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

We can use the electrochemical series to predict the thermodynamic tendency for a reaction to take place under standard conditions. A cell reaction that is spontaneous under standard conditions (that is, has K > 1) has AG° < 0 and therefore the corresponding cell has E° > 0. The standard emf is positive when ER° > Et that is, when the standard potential for the reduction half-reaction is more positive than that for the oxidation half-reaction. 

To reverse this half-reaction and bring about the oxidation of water, we need an applied potential difference of at least 0.82 V. Suppose the added salt is sodium chloride. When Cl ions are present at 1 mol-L 1 in water, is it possible that they, and not the water, will be oxidized From Table 12.1, the standard potential for the reduction of chlorine is Cl.36 V ... 

FIGURE3.7 The potential window for the redox chemistry of life. Redox chemistry in living cells is approximately limited by the standard potentials for reduction and oxidation of the solvent water at neutral pH. Approximate standard reduction potentials are also indicated for the commonly used oxidant ferricyanide and reductants NADH and dithionite. 

It is thus reasonable to anticipate that HOC1 could behave as an outer-sphere one-electron oxidant. Indeed, the standard potential for the HOC1/HOC1 - couple is estimated at 0.25 V (9). In prior reports where such a pathway might have been uncovered, alternative pathways generally have been found, such as inner-sphere mechanisms and reactions via Cl2. Reaction via Cl2 is often a viable pathway because of the presence of Cl- either as a contaminant or reaction product and its reaction with HOC1 as in Eq. (4). 

Density, AGf, solubility, dissociation const, and standard potentials of oxidation-reduction reactions for aqueous species... 

If you select any two half-reactions from the chart of standard electrode potentials, the half-reaction higher on the list will proceed as a reduction, and the one lower on the list will proceed in the reverse direction, as an oxidation. Beware Some references give standard electrode potentials for oxidation half-reactions, so you have to switch higher and lower in the rule stated in the preceding sentence, though this is not common. 

A critical and comprehensive review of fluorine redox chemistry has been given by Kuhn [17] and much fundamental information is also available from Bard [14] and from Pourbaix [13]. The most important redox states for fluorine are - I (fluoride, fluormonoxide) and 0 (fluorine) and the hypothetical reversible standard potentials for these systems are given below. The highly unstable oxide F2O is... 

Chlorine (from the Greek chloros for yellow-green ) is the most abundant halogen (0.19 w% of the earth s crust) and plays a key role in chemical processes. The chlor-alkali industry has been in operation since the 1890s and improvements in the technology are still important and noticeable, for example, the transition from the mercury-based technology to membrane cells [60]. Most chlorine produced today is used for the manufacture of polyvinyl chloride, chloroprene, chlorinated hydrocarbons, propylene oxide, in the pulp and paper industry, in water treatment, and in disinfection processes [61]. A summary of typical redox states of chlorine, standard potentials for acidic aqueous media, and applications is given in Scheme 2. 

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 most stable oxidation states for protactinium are Pa(V) and Pa(IV). The chemical behavior of Pa(V) closely mimics that of Nb(V) and Ta(V), and experimental data are consistent with a 5f(l) rather than a 6d(l) electron configuration for the Pa(IV) species [37]. The electrochemical literature for Pa is mainly focused on the characteristics of the Pa(V)/Pa(IV) couple and the electrodeposition of Pa metal films from aqueous and nonaqueous electrolyte solutions. In aqueous solutions, only Pa(V) and Pa(IV) ions are known to exist, and the standard potential for the Pa(V)/Pa(IV) redox couple is in the range of —0.1 to -0.32 V [38]. 

Neptunium has been characterized from the +3 to +7 oxidation states in aqueous solution. The standard potentials for various Np ions have been determined from measured formal potentials of the various redox couples. These data have been thoroughly reviewed by Martinet [94] and Fahey [95]. Recently the standard potentials for the redox couples Np02 /Np02, Np +/Np +, and Np02 /Np" in acidic aqueous solution have been reevaluated with more detailed consideration of activity coefficients [49,50]. The standard potential accepted here for the Np02 /Np02 couple is 1.161 0.011 V as determined from... 

The vast majority of electrochemical data on americium ions has heen obtained in aqueous solutions. Americium can exist in aqueous solutions in the oxidation states III, IV, V, and VI. The divalent state is difficult to attain in aqueous solutions because of the proximity of the standard potential of the Am(III)/Am(II) couple to the solvent/supporting electrolyte breakdown potential. Previous reviews have presented the formal and standard potentials for the various americium couples and these reviews should be consulted by the interested reader for more detailed discussion [133, 134]. Table 3 contains a summary of selected formal potentials Ef from these reviews in 1 M HCIO4 for convenience. AU values are calculated from various measurement techniques except for the Am(VI)/Am(V) couple (Am02 /Am02" "), which was determined directly. 

Using equation (41), and the values of E roor/ro, ro determined as described above, values for BDFE can be determined if E°ro7ro is known. In fact, in favorable conditions, the standard potential for the RO /RO couple can be determined through analysis of the voltammetric oxidation of the RO anion. In this way, reasonable estimates of solution 0—0 BDFEs were obtained for some peroxides. The data are also reported in Table 4 and are the same within experimental error. Since the entropy term for this series of compounds is not expected to be very different, this implies that the BDE of these compounds is also the same consistent with what is known about the substituent effects on BDE for simple peroxides." " " Using a common entropy correction for the acyclic peroxides, the BDE of the peroxides is in the range 34-37 kcal moU. ... 

Table 8 Peak potentials at the platinum electrode and standard potential for the oxidation of X-Ph-S in DMF/0.1 M TBAP at 25°C."...

Standard voltages for oxidation halfreactions are obtained by changing the sign of the standard reduction potentials. When the reaction is reversed, the sign of the ° is changed to the opposite sign. 

Q Construct a fully balanced equation lor the oxidation of Cm by molecular oxygen, and calculate the standard potential for the... 

The overall standard potential for the equation is the difference between the potential for the reduction half-reaction (+1.45 V) and that for the oxidation half-reaction ( + 0.94 V) = 1.45 — 0.94 = +0.51 V, which, being positive, means that the reaction is feasible. 

I d/d+ standard redox potential for oxidation in absolute scale... 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

Calculate the standard potential for the reaction of hydrogen peroxide with Fe2+ in acidic solution. Will the oxidation of Fe2+ to Fe3+ by hydrogen peroxide be spontaneous See Appendix 2B. 

Write the half-reactions that show how hydrogen peroxide can function as an oxidizing and as a reducing agent in both acidic and basic solutions. Give the appropriate standard potentials for these halfreactions. 

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