Standard electrode potential redox series

Referring to a list of standard electrode potentials, such as in Table 8.3, one speaks of an electrochemical series, and the metals lower down in the se-ries(with positive electrode potentials) are called noble metals. Any combination of half-reactions in an electrochemical cell, which gives a nonzero E value, can be used as a galvanic cell (i.e., a battery). If the reaction is driven by an applied external potential, we speak of an electrolytic cell. Reduction takes place at the cathode and oxidation at the anode. The reduction reactions in Table 8.3 are ordered with increasing potential or pe values. The oxidant in reactions with latter pe (or E°) can oxidize a reductant at a lower pe (or ) and vice versa for example, combining half-reactions we obtain an overall redox reaction ... 

Describe how standard electrode potentials (Ehair-ceii values) are combined to give Ecen and how the standard reference electrode is used to find an unknown haif-ceiii explain how the reactivity of a metal is related to its Ehaif-ceih write spontaneous redox reactions using an emf series like that in Appendix D ( 21.3) (SPs 21.3,21.4) (EPs 21.24-21.40)... 

The more negative the standard electrode potential, of a redox couple M (aq)/M(s), the more powerful a reducing agent is that metal. This means it is a more reactive metal because it loses electrons more easily. We can arrange the metals in order of reducing power, producing an activity series of metals (Table 7.2). The metals at the top of the series are more reactive than those below. 

The electrochemical series has been extended to give the redox series (Table 19.2), which includes the standard electrode potentials of redox systems in which transition metals are present in different oxidation states. 

Note that the standard electrode potential is not doubled when the stoichiometry is doubled. The standard electrode potential is an example of an intensive property, that is, it is independent of quantity or amount. Density and boiling point are other examples of intensive properties. This is in contrast to extensive properties, for example, mass and volume, which do depend on quantity or amount. The redox series can be extended to include the standard electrode potentials of non-metals and ions (Table 19.3). 

Much information can be gained by examining trends in redox potentials within series of compounds. Let us consider such a series of coordination compounds, M0, Mj, M2. .. M , which undergo reversible, one-electron oxidation reactions at potentials E°0, E°lf E°2,... E° respectively, with respect to the same reference electrode. If we define the oxidation of M0 as our standard reaction (equation 8), then we can examine the variation of the free energy difference, F(E°0— E° ), in terms of the structural difference between M0 and each other member M . Such an analysis is directly comparable to the classic approach of Hammett17 which relates a free energy difference term, log(AH/Ax), for equilibrium reactions such as (9) and (10), to the nature of the aryl substituent, X. 

Table 2.1 Standard aqueous half-cell potentials at 25 °C (also known as standard electrode, redox, or oxidation potentials, and as the standard emf series)(a)...

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

The redox potentials against the standard hydrogen electrode (SHE) for various reactions, usually called electrochemical series , are listed in the annually updated CRC Handbook of Chemistry and Physics, CRC Press Boca Raton, EL. On the other hand, unless otherwise indicated, the potential values used in this review are all referred to the Li+/Li reference electrode. 

While the redox titration method is potentiometric, the spectroelectrochemistry method is potentiostatic [99]. In this method, the protein solution is introduced into an optically transparent thin layer electrochemical cell. The potential of the transparent electrode is held constant until the ratio of the oxidized to reduced forms of the protein attains equilibrium, according to the Nemst equation. The oxidation-reduction state of the protein is determined by directly measuring the spectra through the tranparent electrode. In this method, as in the redox titration method, the spectral characterization of redox species is required. A series of potentials are sequentially potentiostated so that different oxidized/reduced ratios are obtained. The data is then adjusted to the Nemst equation in order to calculate the standard redox potential of the proteic species. Errors in redox potentials estimated with this method may be in the order of 3 mV. 

As for semiconductor/metal contacts, a change in the Fermi level of the liquid phase should result in a different amount of charge transferred across the semicondnctor/liqnid junction. For semiconductor/liquid junctions, the important energetic trends for a series of different liqnid contacts can thns be determined by measuring the solntion redox potential relative to a standard reference electrode system. Within this model, solutions with more positive redox potentials shonld indnce greater charge transfer in contact with n-type semicondnctors. 

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