Properties from half-cell potentials

Measuring Thermodynamic Properties of Ions from Half-Cell Potentials... 

To calculate standard cell potentials from the half-cell potentials in Table 9-1, there are four principles that we must know (1) When we reverse the direction of the chemical reaction, we change the sign of the potential. (2) If we multiply the coefficients in the equation by some number, we do NOT change the potential. Potential is an intensive property, and does not depend on the quantity of reagents. (3) When we add chemical equations for half-cells, we add the corresponding potentials. (4) A positive potential for a complete cell reaction means that the reaction proceeds spontaneously in the direction of the equation, and a negative potential means that the reaction goes spontaneously in the opposite direction. 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

When two interval scales are used to measure the amount of change in the same property, the proportionality of differences is preserved from one scale to the other. For example. Table 1.4 shows reduction potentials of three electrochemical half-cell reactions measured in volts with reference to the standard hydrogen electrode (SHE, E°) and in millivolts with reference to the standard silver-silver chloride electrode (Ag/AgCl, ). For the SHE potentials the proportion of differences between the intervals +0.54 to +0.80 and +0.34 to +0.80 is... 

The free energies in (18) are illustrated in Fig. 10. It can be seen that GA is that part of AG ° available for driving the actual reaction. The importance of this relation is that it allows AGXX Y to be calculated from the properties of the X and Y systems. In thermodynamics, from a list of n standard electrode potentials for half cells, one can calculate j (m — 1) different equilibrium constants. Equation (18) allows one to do the same for the %n(n— 1) rate constants for the cross reactions, providing that the thermodynamics and the free energies of activation for the symmetrical reactions are known. Using the... 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

Intermingled with the neurons in the brain are a variety of other cell types. The most common of these satellite cells are glial cells. These make up virtually about one half of the total volume of the brain and exist in several forms such as astrocytes, oligodendrocytes, and Schwann cells. Membrane properties of glial cells exhibit fundamental differences from neurons, the chief difference being their passive nature. Unlike neurons, most glial cells are not excitable and do not fire action potentials. Membrane potential measurements of... 

There is, for example, an extensive databank of thermodynamic data, including half-cell enthalpies and entropies of reduction, that has been built up from investigations that use small mediators to carry electrons between protein and electrode. In these potentiometric studies, one measures the equilibrium concentrations of components in oxidized and reduced states at various values of the electrode potential. There are a number of variations on this theme. For example, a determination may also be carried out without using an electrode, by equilibrating the couple of interest with a titrant whose reduction potential is known accurately. Most importantly, it is necessary that the component of interest (or the titrant) exhibits some difference, in a readily measurable property, between oxidized and reduced forms. Light adsorption is the most convenient parameter since it may be monitored conveniently in situ. An excellent method, which has now gained wide... 

Although thiolate-surface complexes form at electrodes [58] and electrode potential measurements under reversible conditions are not usually possible, a great deal of useful information has been obtained by various electrochemical methods [35,36]. However, thermochemical properties of sulfur-centered radicals in aqueous solution have generally been obtained from studies of redox equilibria, such as those by BonifSeic and Asmus [52] and Surdhar and Armstrong [59], by methods described in Wardman [60]. In this section it will be convenient to consider sulhdes, disulfides, and sufhydryl compounds in that order. Afso alkyl and aryl compounds will be treated separately. Reduction potentials will be identified by placing the reactants and products of the half cell reaction in parenthesis [60], for example, (RS, H /RSH) is the reduction potential for ... 

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. 

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