Potential, chemical redox

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

Certainly, the same arguments apply for chemical redox catalysis , but as discussed above, thinner films may be effective in this case. Hence, it will be reasonable to work with modified electrodes having a large effective area instead of thick films, i.e. three-dimensional, porous or fibrous electrodes. The notorious problem with current/potential distribution in such electrodes may be overcome by the potential bias given by selective redox catalysts. Some approaches in this direction are described in the next section. 

Various methods have been used to determine the redox potentials (Table XI). Very commonly, EPR-monitored chemical redox titration is performed, which can be used to measure the redox potential not only in isolated complexes but also in membrane preparations. In general, there is good agreement between redox potentials determined in membranes, isolated complexes, or isolated Rieske proteins or fragments the only exception is the water-soluble Rieske fragment from spinach bef complex where differences of more than 50 mV have been observed by the same group but using different methods (31). 

Potential chemical interactions across dissimilar solvent systems and the sensitivity of oxide semiconductors to redox conditions and surface adsorption combine to make integration of solution-processed dielectrics and semiconductors a nontrivial undertaking. By maintaining appropriate aqueous... 

Grenthe, I., W. Stumm, M. Laaksuharju, A. C. Nilsson, and P. Wikberg (1992), "Redox Potentials and Redox Reactions in Deep Groundwater Systems", submitted to Chemical Geology. 

The chemical potential h,(redoxs) of redox electrons (OX + (redoxs) = RED ) in the state of redox equilibrium is given by u,(redoxs) = 1 rbixs) - licws). We hence obtain from Eqn. 4-19 the expression in Eqn. 4-21 ... 

A rich variety of reagents and methods have been applied to generate radical ions. As illustrated above, the first methods were chemical redox reactions. Radical anions have long been generated via reduction by alkali metals. Because of the high reduction potentials of these metals, the method is widely applicable, and the reductions are essentially irreversible. 

The carbon cycle is illustrated in Fig. 5.2. All living systems require an external source of energy, either in the form of chemical bond energy, as chemical (redox) potential or as some form of electromagnetic radiation usually in or near the visible light region. 

Standard potentials for redox reactions involving oxygen at 25°C. (From Sawyer, D.T. and Nanni, E.J., Jr., Redox chemistry of dioxygen species and their chemical reactivity, in Oxygen and Oxy-Radicals in Chemistry and Biology, Rodgers, M.A.J. and Powers, E.L., Eds., Academic Press, New York, 1981,15 44. With permission.)... 

The standard reduction potentials (see Redox Potential) of the elements and their compounds have many important applied implications for chemists, not the least of which is being aware when a compound or mixture of compounds they are handling has the potential for exploding. This should be considered as a possibility when the appropriate potentials differ by more than about one volt and appropriate kinetics considerations apply. A simply predictable case is the sometimes-violent reaction of metals with acids, as illustrated in a recently produced discovery video. Redox activities of elements are most commonly (and most precisely) analyzed via thermo chemical cycles such as the familiar Bom-Haber cycle for the production of NaCl from Na and CI2. A similar analysis of the activities of different metals in their reactions with acids shows that the standard reduction potential for the metal (the quantitative measure of the activity of the metal) can be expressed in terms of the appropriate ionization energies of the metal, the atomization energies of the metal (see Atomization Enthalpy of Metals), and the hydration energies... 

During the PEVD process, a chemical redox reaction takes place and the whole PEVD system can he viewed as a chemical reactor where the reactants are distributed over both the source and sink sides. According to the previous discussion, the driving force for this PEVD process can be solely provided by a dc electric potential, so that isolation of the source and sink vapor phases is not necessary. Consequently, the PEVD process is equivalent to physically moving a solid phase Na COj through another solid phase (Na+-[3"-alumina) by electric energy. Furthermore, it should be pointed out that the overall cell reaction in this PEVD system is reversible. 

EPR and ENDOR spectroscopic studies [6-12] indicate that the oxidized BChl complex (P ) is a rr-radical cation in which the spin of the unpaired electron is delocalized over the tt electron systems of two BChl molecules. Chemical redox titrations of P-870 follow a one-electron Nernst curve. The midpoint potential at pH 7 (Em. ) ranges from -t-0.36 to +0.50 V depending on the species of bacteria, but is typically about +0.45 V [13-17]. 

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. 

Zaban and co-workers reported the use of chemical redox titrations to measure the potential of sensitizers bound to Ti02 [136], An unexpected result from these studies is that redox couples that are not pH sensitive in fluid solution become pH dependent when bound to the semiconductor surface. The magnitude of the pH-induced shift varied from 21 to 53 mV per pH unit depending on the physical location of the sensitizer. Sensitizers inside the semiconductor double layer track the 59 mV pH shift of the semiconductor. When sensitzers were outside the double layer, their potential was almost independent of the semiconductor. This finding has important implications for the determination of interfacial energetics for dye sensitization and interfacial electron transfer studies [136]. 

The basic components of a voltaic cell are a wire, two electrodes and two partially-separated solutions. When the electrodes are placed in their respective solutions and the wire is used to connect them, a spontaneous flow of electrons occurs in the wire from one electrode to the other. The impetus for current flow comes from the difference between the oxidation potentials of the electrodes and the solutions, or between the electrodes themselves or between the two solutions in which the electrodes are immersed. A chemical redox reaction occurs between these separated species such that the oxidation half of the reaction occurs in one solution and the reduction half occurs in the other. The partial separation of the solution can be accomplished by a membrane or a salt bridge, which allows an electrolytic connection but does not allow a general mixing of the two solutions. Within the cell, electrical current moves in the form of free electrons in the wire and as ions in the electrolyte. 

Another view is that electrochemistry is an alternative to chemical redox methods. Indeed, in certain cases the products are similar. This is to be expected if the chemical reagent reacts like an electrode via discrete electron transfer steps - not atom transfers. Even here, however, it is not unusual to observe significant differences between chemical and electrochemical processes. A peculiar advantage of electrochemistry is control of the electrode potential. In particular one can adjust the potential to selectively attack the most easily reduced or oxidized moiety in a complex molecule. This technique can also avoid the over-reductions and oxidations produced by chemicals. 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. What, we may ask, is the redox potential In the sense that they involve group transfer, redox reactions (more correctly oxidation—reduction reactions) are like other types of chemical reactions. Whereas, for example, in hydrolytic reactions a functional group is transferred to water, in oxidation-reduction reactions, electrons are transferred from electron donors (reductants) to electron acceptors (oxidants). Thus, in the reaction... 

Correlation between spectral and electrochemical features for CNTs has been studied by Kavan et al. (2004). Spectral features for SWNTs in the near infrared-visible region can be explained in terms of resonance enhancement and optical band-gap excitation in a one-dimensional conductor with Van Hove singularities in the electronic density of states (Kavan et al., 2000). Chemical redox processes can modify the population of such states so that the electronic structure of SWNTs can be tuned by chemical doping with molecules having different redox potentials or by controlling the interfacial potential of SWNTs in contact with an electrolyte solution, as studied in detail by Kavan et al. (2001). 

In addition to rate constant measurements and mechanistic determinations for reactions of radical species, pulse radiolysis techniques have been especially successful in measurements of one-electron reduction potentials of redox pairs where one of the partners is unstable so that traditional methods cannot be used. The reduction potentials of a large number of species, in various solvents, have been determined. The techniques and theoretical aspects have been presented by Pedi Neta in the Journal of Chemical Education (2). 

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