Standard potential hydrated electron

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

We represent the hydrated proton as H + (aq) rather than H30 + (aq) because we re interested here in electron transfer, not proton transfer as in Chapter 15.] The standard potential for this cell, 0.34 V at 25°C, is a measure of the combined driving forces of the oxidation and reduction half-reactions ... 

This mechanism is reasonable as a) reduction of benzene occurs at a cathode potential of -2,5 V vs. S.C.E., roughly corresponding to the standard potential of the hydrated electron 293 while the potential for the direct electron transfer to benzene is more negative ( -3,0 V) and b) in situ electrolysis in the ESR cavity produces at -100 °C the characteristic singlet of the solvated electron 293a>, which changes to the septett of the benzene radical anion, when benzene is added to the solution. 

Like that of the hydrated electron and the hydrogen atom, the potential of the hydroxyl radical has long been the subject of estimates based on thermochemical cycles involving the free energy of hydration of OH the results of these calculations appear, for example, in Standard Potentials (pp. 59-64). Recently, however, there have been two direct determinations of E° for the OH/OH- couple. In the first, Schwarz and Dodson (279) used pulse radiolysis to measure the equilibrium constants for... 

The cathode potential necessary for the production of solvated electrons is rather negative the standard potential of the hydrated electron has been calculated to be —2.68 V versus NHE. Also, in other solvents compatible with its formation, very negative potentials must be used for example, in liquid ammonia the generation of ens is achieved at —2.47 V versus Ag/AgN03 (0.1 M) [306], but the dissolution standard potential measured in HMPA was found to be —3.44 V versus Ag/AgC104 (0.1 M) [307]. Similarly, in methy-lamine f — 50°C), a potential of —2.90 V versus Ag/AgN03 was reported [308]. 

Solvated electrons can also be obtained via atomic hydrogen The potential of the electrode which is in equilibrium with atomic hydrogen in an aqueous alkaline solution (pH 12) equals —2.8 V (NHE) . It is close to the standard equilibrium potential of an electron in water (see p. 179). Therefore, hydrated electrons... 

Since AG° = —F , this corresponds to a standard electrode potential of —2..11 V. The hydrated electron is thus an extremely potent reducing agent, much more powerful than the hydrogen atom, for which is —2.10 V. The free energy and heat of hydration are —37.4 and —38.1 Real mole", respectively. The entropy of hydration is —1.90 cal mole" °K" [31]. 

Following the discovery of the hydrated electron in radiation chemistry, the reexamination of some fields of aqueous chemistry gave rise to a new concept of primary reduction processes. This paper surveys aspects of these investigations in which it appears that e aq, as opposed to its conjugate acid (H atom), is invariably the precursor to H2 when water is reduced. Evidence is reviewed for the production of e aq (a) photochemically, (b) by chemical reduction of water, (c) electrolytically, (d) by photo-induced electron emission from metals, (e) from stable solvated electrons, and (f) from H atoms. The basis of standard electrode potentials and various aspects of hydrated electron chemistry are discussed briefly. 

Table 4.7 Standard reduction potentials (in V) of selected half-reactions in the aqueous H Oy system (Milazzo and Carol 1978, Bard et al. 1985, Stanbury 1989, Wardman 1989, Holze 2007), at 25 °C. e electron transferred from electrode, hydrated electron, (g) gase-ons, aq) dissolved.

The hydrated electron is strong reducing agent with a standard reduction potential (vs. NHE) of E° = —2.9 V. When the OH radicals disturb the observation of e reactions, the OH radicals are eliminated using saturated H-atom-containing compounds, like fcrf-butanol... 

In electrochemistry, the electron level of the normal hydrogen electrode is important, because it is used as the reference zero level of the electrode potential in aqueous solutions. The reaction of normal hydrogen electrode in the standard state (temperature 25°C, hydrogen pressure 1 atm, and unit activity of hydrated protons) is written in Eqn. 2-54 ... 

Fig. 2-43. Energy balance in the reaction of normal hydrogen electrode H2(sid.p>j = hydrogen molecule in the gaseous standard state (at 1 atm) H( gro. i) = hydrated proton of unit activity = real potential of the hydrated proton of unit activity a.ajHE) = real potential of the equilibrium electron of NHE (= Fermi level cpcnhe) of NHE).

The electrode potential in the equilibrium of redox electron transfer may also be defined by the free enthalpy change in the reaction of the hydrated redox particles with the standard gaseous electron eisro) as shown in Eqn. 4—20 ... 

It, thus, follows that the electrode potential in equilibrium of metal ion transfer is given by the free enthalpy for the formation of a solid metal from both hydrated metal ions and standard gaseous electrons as shown in Eqn. 4—25 ... 

Figure 5-64 shows the band edge potential for compound semiconductor electrodes in aqueous solutions, in which the standard redox potentials (the Fermi levels) of some hydrated redox particles are also shown on the right hand side. In studying reaction kinetics of redox electron transfer at semiconductor electrodes, it is important to find the relationship between the band edge level (the band edge potential) and the Fermi level of redox electrons (the redox potential) as is described in Chap. 8. 

TABLE 8-1. Preference for the conduction band mechanism (CB) and the valence band mechanism (VB) in outer sphere electron transfer reactions of hydrated redox particles at semiconductor electrodes (SC) Eo = standard redox potential referred to NHE c, = band gap of semiconductors. [From Memming, 1983.]... 

Fig. 8-90. Normalized cathodic cur> rent of redox reactions of hydrated redox particles as a function of standard redox potential at n-type electrodes of zinc oxide / (n, cqx) = normalized cathodic reaction current n, = concentration of interfacial electrons Cqx = concentration of oxidant particles au = arbitrary unit. [From Morrison, 1969,1980.]...

The first element, hydrogen, has an Allred Rochow electronegativity coefficient of 2.1, and an electronic configuration Is1. The atom may lose the single electron to become a proton, which exists in aqueous solutions as the hydroxonium ion, H30+(aq), in which the proton is covalently bonded to the oxygen atom of a water molecule. The ion is hydrated, as is discussed extensively in Chapter 2. The reduction of the hydrated proton by an electron forms the reference standard half-reaction for the scale of reduction potentials ... 

The lattice enthalpy U at 298.20 K is obtainable by use of the Born—Haber cycle or from theoretical calculations, and q is generally known from experiment. Data used for the derivation of the heat of hydration of pairs of alkali and halide ions using the Born—Haber procedure to obtain lattice enthalpies are shown in Table 3. The various thermochemical values at 298.2° K [standard heat of formation of the crystalline alkali halides AHf°, heat of atomization of halogens D, heat of atomization of alkali metals L, enthalpies of solution (infinite dilution) of the crystalline alkali halides q] were taken from the compilations of Rossini et al. (28) and of Pitzer and Brewer (29), with the exception of values of AHf° for LiF and NaF and q for LiF (31, 32, 33). The ionization potentials of the alkali metal atoms I were taken from Moore (34) and the electron affinities of the halogen atoms E are the results of Berry and Reimann (35)4. 

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