Thermodynamic standard potentials

The (thermodynamic) standard potential, (Ox/Red), of a couple Ox/Red is measured by constructing an electrochemical cell in which the couple of interest forms the right-hand electrode and the standard hydrogen electrode is on the left. For example, the standard potential of the Ag /Ag couple is the standard potential of the cell 

We saw in Section 4.2 that in biochemical work it is common to adopt the biological standard state (pH = 7, corresponding to neutral solution), rather than the thermodynamic standard state (pH = 0). To convert standard potentials to biological standard potentials, , we must first consider the variation of potential with pH. The two potentials differ when hydrogen ions are involved in the half-reaction, as in the fumaric acid/succinic acid couple fum/suc with fum = HOOCCH=CHCOOH and sue = HOOCCH2CH2COOH, which plays a role in the citric acid cycle (Case study 4.3)  

When hydrogen ions occur as reactants, as in this example, an increase in pH, corresponding to a decrease in hydrogen ion activity, favors the formation of reactants, so the fumaric acid has a lower thermodynamic tendency to become reduced. We expect, therefore, the potential of the fumaric/succinic acid couple to decrease as the pH is increased. 

To establish the quantitative variation of reduction potential with pH for a reaction as a first step in determining the effect of changing from pH = 0 to pH = 7 we use the Nernst equation. Thus, for fixed fumaric acid and succinic acid concentrations, the potential of the fumaric/succinic redox couple is 

Note that this result is valid only for a half-reaction in which Vg = 2 and the stoichiometric coefhcient of H+(aq) is 2 and appears as a reactant. In general  

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The thermodynamic standard potential of the methanol electrode has a value of + 0.02 V (RHE) that is, it is quite close to the hydrogen electrode potential. The steady-state potential of a platinum electrode in aqueous methanol solutions is about + 0.3 V (RHE). 

C = concentration of the active species in the bulk of the solution E° = formal electrode potential of the couple Ox/Red. It differs from the thermodynamic standard potential E° by a factor related to the activity coefficients of the two partners Ox and Red ... 

Cyclic voltammetric methods, or other related techniques such as differential pulse polarography and AC voltammetry,3 provided a convenient method for the estimation of equilibrium constants for disproportionation or its converse, comproportionation. In this respect, the experimentally measured quantity of interest in a cyclic voltammetric experiment is E>A, the potential mid-way between the cathodic and anodic peak potentials. For a one-electron process, E,A is related to the thermodynamic standard potential Ea by equation (4).13 In practice, ,/2 = E° is usually a good approximation. 

Hence the basic (reversible) thermodynamic standard potential ( therm) of decomposition (as also the overpotentials) decreases as the temperature increases. 

The electric work W required for an electrochemical process in a practical electrolytic cell will be larger than AG °. For example, the indicated value of AG° in Equation (3) corresponds to a thermodynamic standard potential difference of ° = 1.23 V, while the voltage typically applied to the cell of Figure 3.1.1 is approximately U- 1.8 V. 

Note, however, that the half-wave potential Ey is usually similar but not exactly equivalent to the thermodynamic standard potential First, the product of reduction may be stabilized by amalgam formation in metal ion reductions second, there will always be a small liquid junction potential in electrochemical cells of this type that should be corrected for and hnally, it can be shown that the potential Ey is the sum of two terms ... 

Copper (I) complexes exhibit catalytic activity for the four-electron (4-e) reduction of O2 to water. Natural occurring enzymes like Cu-containing fungal laccase reduce O2 directly to water very efficiently at very positive potentials, not far from the thermodynamic standard potential of the O2/H2O couple. These enzymes involve a trinuclear Cu active site [149-153]. For this reason some authors have investigated the catalytic activity of Cu(I) complexes for ORR, in particular Cu phenanthrolines confined on graphite or glassy carbon surfaces [154-169], with the aim of achieving the total reduction of O2 via the transfer of four-electrons. 

The formal standard potential of a redox system. Closely related to the thermodynamic standard potential. 

The biological standard potential of the couple pyruvic acid/ lactic acid is-0.19 V. What is the thermodynamic standard potential of the couple Pyruvic acid is CH3COCOOH and lactic acid is CH3CH(0H)C00H. 

The treatment of the activities problem may be achieved as it is with acids and bases (see Chap. 5). The iteration process starts with the mixing of activities and concentrations. This first operation, once achieved, begins to transform the (thermodynamic) standard potentials into potentials that are already somewhat formal ones, that is, into potentials expressed in concentration terms. The process is recommenced several times until a constant ionic strength is obtained. Therefore, the definitive potential value is a pure formal one, of course at the definitive and true constant ionic strength. 

The thermodynamics of electrochemical reactions can be understood by considering the standard electrode potential, the potential of a reaction under standard conditions of temperature and pressure where all reactants and products are at unit activity. Table 1 Hsts a variety of standard electrode potentials. The standard potential is expressed relative to the standard hydrogen reference electrode potential in units of volts. A given reaction tends to proceed in the anodic direction, ie, toward the oxidation reaction, if the potential of the reaction is positive with respect to the standard potential. Conversely, a movement of the potential in the negative direction away from the standard potential encourages a cathodic or reduction reaction. 

If there are no standard conditions or in the case where it is not be possible to measure the standard potential, the value can be determined by thermodynamic calculations (see Sec. 1.3.2). 

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. 

Chromium, molybdenum and tungsten thermodynamic properties, chemical equilibria and standard potentials. I. Dellien, F. M. Hall and L. G. Hepler, Chem. Rev., 1976, 76, 283-310 (400). 

The standard potential of an electrode is the standard emfofa cell in which the electrode on the left in the cell diagram is a hydrogen electrode. A metal with a negative standard potential has a thermodynamic tendency to reduce Irydrogen ions in solution the ions of a metal with a positive standard potential have a tendency to be reduced by hydrogen gas. 

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. 

The reduction is thermodynamically favored, because the standard potential of the couple Cu2+/Cu is positive (E° = +0.34 V). Metals with negative standard potentials, such as zinc (E° = —0.76 V) and nickel (E° = —0.23 V), cannot be extracted hydrometallurgically. 

Electrochemical cells can be constructed using an almost limitless combination of electrodes and solutions, and each combination generates a specific potential. Keeping track of the electrical potentials of all cells under all possible situations would be extremely tedious without a set of standard reference conditions. By definition, the standard electrical potential is the potential developed by a cell In which all chemical species are present under standard thermodynamic conditions. Recall that standard conditions for thermodynamic properties include concentrations of 1 M for solutes in solution and pressures of 1 bar for gases. Chemists use the same standard conditions for electrochemical properties. As in thermodynamics, standard conditions are designated with a superscript °. A standard electrical potential is designated E °. 

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

In addition to the thermodynamic quantity E°, the electrode reaction is characterized by two kinetic quantities the charge transfer coefficient a and the conditional rate constant k°. These quantities are often sufficient for a complete description of an electrode reaction, assuming that they are constant over the given potential range. Table 5.1 lists some examples of the constant k. If the constant k° is small, then the electrode reaction occurs only at potentials considerably removed from the standard potential. At these potential values practically only one of the pair of electrode reactions proceeds which is the case of an irreversible or one-way electrode reaction. 

Lithium electrodes, 3 408 standard potential, 3 413t Lithium fluoride, 15 138-139 Lithium fluoroborate, 4 153 manufacture, 4 155 physical properties of, 4 152t thermodynamic properties of, 4 154t uses of, 4 157... 

The thermodynamics of dissociative electron transfer reactions may be characterized by its standard potentials defined from the standard chemical... 

Coming now to the cyanobenzyl halides, the general tendency is that the cleavage is faster than with the nitroderivatives, so fast that the reaction eventually becomes concerted (Table 3.5). There is a thermodynamical reason for this acceleration that the standard potential Epx/Rx- is more negative in the first case than in the second (because CN is a weaker... 

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