Gibbs energy redox couples

Because, as we have already seen, the standard potential of hydrogen is zero, the electromotive force of the galvanic cell (eq. 8.161) directly gives the value of the standard potential for the Zn,Zn redox couple. Table 8.14 lists the standard potentials for various aqueous ions. The listed values are arranged in decreasing order and are consistent with the standard partial molal Gibbs free energies of table 8.13. 

Table 8.14 Standard potentials for various redox couples (aqueous cation-metal element-aqueous anion) arranged in order of decreasing E°. Valnes, expressed in volts, are consistent with standard partial molal Gibbs free energy values listed in table 8.13. valne for snlfur is from Nylen and Wigren (1971).

The partial equilibria of equations 8.165 8.168 reveal the usefulness of standard potentials the Gibbs free energy change AG of the redox equilibrium is always given by applying Faraday s equation to the algebraic sum of the standard potentials of the redox couples in question. For equation 8.163, the bulk potential is thus... 

The Gibbs energy of such a reaction varies with temperature, and thus the standard potential of the other redox couple than H+/H2. When not specified, the temperature is 25 °C (298.15 K) so that the standard potentials cited in the text or in (1, 2, 3) are in at 25 °C. 

Using isopentane 15 as a reference alkane, the authors calculated the Gibbs free energy between the redox couples of various alkanes and the 2MeBuH/2MeBu+ couple. This leads to the standard potential of the alkane redox couples in HF... 

For each of the redox couples, the potential can be calculated using the Nernst equation. This equation correlates Gibb s free energy, known as AG, and the electromotive force provided by an oxido-reduction reaction (such a reaction acts as a galvanic cell). Given the following equation due to a chemical reaction ... 

Redox reactions are more conveniently described in terms of relative electrical potentials instead of the equivalent changes in Gibbs free energy. The electrons in Equation 6.8 come from or go to some other redox couple, and whether or not the reaction proceeds in the forward direction depends on the relative electrical potentials of these two couples. Therefore, a specific electrical potential is assigned to a couple accepting or donating electrons, a value known as its oxidation-reduction or redox potential. This redox potential is compared with that of another couple to predict the direction for spontaneous electron flow when the two couples interact—electrons spontaneously move toward higher redox potentials. The redox potential of species /, ), is defined as... 

The water stability boundaries and the locus of measured Eh and pH measurements in natural waters, as reported by Baas-Becking et al. (I960), are shown in Fig. 11.3 (see also Fig. 11.4). It has been observed that frequently the Eh values measured with a Pt electrode differ significantly from values computed from Gibbs free energies or standard potentials and solution concentrations. When they exist, there are two important reasons for such differences. These include (1) misbehavior of the Pt or other indicator electrode (2) the irreversibility or slow kinetics of most redox couple reactions and resultant di.sequilibrium between and among different redox couples in the same water and (3) the common existence of mixed potentials in natural waters (see below). 

In this case, one electron is transferred from Fe(II) to Cr(III), and no bonds are broken or formed. The reaction is also classified as heteronuclear, since the reactants belong to different redox couples. This means that the overall reaction involves a net change in the standard Gibbs energy, A G°. There is considerable interest also in homonuclear reactions such as... 

In order to express the standard redox potentials of the two couples located in different solvents on a same potential scale, e.g., SHE in water, o2>/R2 can t>e expressed in terms of the corresponding redox potential in water ( R2) and the Gibbs energies of transfer of... 

In order to check whether or not the assumption of low halidophilicity of the Cn complexes is correct, the hahdophihcity of the Cn" complexes can be calculated directly from eq. 1, provided that Katrp, Krh. Ket, and Kea are known. For instance, the hahdophilicity of the complex Cn"(bpy)2 in MeCN can be estimated from a known valne of Katrp for the reaction of Cn (bpy)2Br with 1-PhEtBr in MeCN (Katrp = 8.5xl0 (32) and Kbh for the same alkyl bromide (2.93xl0" ).( 7<5 The valne of Kea for the bromine atom in MeCN can be calcnlated from the valne of the redox potential for the couple Br / Bf in water (1.95 V vs. SHE) (33) and the Gibbs energy of transfer of Bf from water... 

E° is the equilibrium potential for the degradation reaction, for example Eqs. (1) and (2), under standard conditions. Unfortunately, sufficient data (that is more than two complete data sets containing rate constant, reactant and product concentration, and Hj concentration) were available only for the redox couple ICE and DCE (Eq. (2)). The standard potential was converted from Gibbs free energy provided by Dolfing (2000) to be 0.72 V pH values were available only for one of the sites (pH 5.3), the unknown values were set to a value of seven under the assumption that such a value is a best guess for anoxic and reducing conditions. In case of chloride, concentrations were available for two of the studies (between 1 and 2 mmol L ), the unknown value was set to 1 mmol L . ... 

Here, flox and flred are the activities of the oxidized and reduced form of the species, respectively, and = —AG /zF is the standard electrode potential of the redox couple A/A"+defined by the Gibbs free energy A of the reaction A"+-l-ne A. 

The Gibbs free energy of electron transfer is determined by the formal electron transfer potentials associated with the excited state of the metalloporphyrin ( p, ) and the redox couple in the organic phase... 

---