Rate of electron transfer

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

The corrosion potentials of the two metals in the environment under consideration will determine the direction of the transfer of electrons, but will provide no information on the rate of electron transfer, i.e. the magnitude of the galvanic current. Thus if E an.. is more positive than corr..B thc transfer of electrons will be from to with a consequent increase in the corrosion potential (more positive) of and a decrease in that of A/ the corrosion rate of will consequently increase and the corrosion rate of A/ will decrease compared with the rates when the metals... 

In this section we consider experiments in which the current is controlled by the rate of electron transfer (i.e., reactions with sufficiently fast mass transport). The current-potential relationship for such reactions is different from those discussed (above) for mass transport-controlled reactions. 

This considerable enhancement in redox properties may however remain chemically hidden. Several causes may converge to mask these properties. First of all electron transfer is an intermolecular act of reactivity even when thermodynamically feasible it may have to compete with very rapid intramolecular acts of deactivation (fluorescence, phosphorescence, internal conversion)99. The rate of electron transfer is given by the Rehm-Weller equation96,100... 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

However, metal ions in higher oxidation states are generally smaller than the same metal ion in lower oxidation states. In the above example, the Co(ii)-N bonds are longer than Co(iii)-N bonds. Consider what happens as the two reactants come together in their ground states and an outer-sphere electron transfer occurs. We expect the rate of electron transfer from one center to another to be very much faster than the rate of any nuclear motion. In other words, electron transfer is very much faster than any molecular vibrations, and the nuclei are essentially static during the electron transfer process (Fig. 9-6). 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

As indicated in Fig. 1, nitrogenase can reduce substrates other than Na. In the absence of other reducible substrates it will reduce protons to dihydrogen, but it can also reduce a number of other small triple-bonded substrates, as indicated in Section V,E,1. Large substrates are not reduced efficiently, indicating physical limitations on access to the enzyme s active site. CO is a potent inhibitor of all nitrogenase substrate reductions except that of the proton to Ha. In the presence of CO the rate of electron transfer is generally not inhibited, but all electrons go toward the production of Ha. 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

The electron transfer mechanism for antioxidant activity corresponding to eq. 16.5 makes the standard reduction potentials of interest for evaluation of antioxidative activity. The standard reduction potential of the phenoxyl radical of several flavonoids has been determined and forms the basis for correlation of rate of electron transfer for various oxidants from the flavonoid (Jovanovic etal., 1997 Jorgensen and Skibsted, 1998). The standard reduction potentials have also been used to establish antioxidant hierarchies. 

Yet the view that the rates of electron transfer in simple reactions are principally independent of the electrode metal (which for some time had been current in the electrochemical literature) cannot be maintained in this strict form. Many experimental data relating to the exchange current densities of reactions involving simple cations (such as Fe and Fe ) provide evidence that the electrode metal does exert a rather strong influence on the reaction rates. 

The usual Tafel evaluation yielded a transfer coefficient a = 0.52 and a rate constant k of 4x 10 cm s at the standard potential of the MV /MV couple. This k value corresponds to a moderately fast electrochemical reaction. In this electrode-kinetic treatment the changes in the rate of electron transfer with pH were attributed only to the changes in the overpotential. A more exact treatment should also take into account the electrostatic effect on the rate of reaction which also changes with pH. 

In the thermodynamically redox-stable resting state, CcOs all Cu ions are in the Cu state and all hemes are Fe . From this state, CcOs can be reduced by one to four electrons. One-electron reduced CcOs are aerobically stable with the electron delocalized over the Cua and heme a sites. The more reduced forms—mixed-valence (two-electron reduced), three-electron reduced, and fully (four-electron) reduced—bind O2 rapidly and reduce it to the redox level of oxide (—2 oxidation state) within <200 p-s [Wikstrom, 2004 Michel, 1999]. This rate is up to 100 times faster than the average rate of electron transfer through the mammalian respiratory chain under normal... 

An analogous apparatus to that of Ref. 9 was used to follow the effect of the lipid monolayer on the rate of electron transfer (ET). In this setup [47], an organic phase droplet (1,2-DCE) is continuously expanded into the aqueous phase, and the resulting current transient was monitored in the absence and presence of the adsorbed lipid mono-layer. The rate of ET was decreased as a function of the lipid concentration. 

The formation of the radical-cation, PQ+ was monitored using laser photolysis techniques at its absorption maxima at 603nm. A study of the rates of PQ+" formation at different PQ++ concentrations led to kg=l.Tx109 M-1s-1. Despite the fact that this reaction is extremely fast, the rate of electron transfer for the macrobiradical is significantly slower than those for the same group in small molecules (8,11). 

As the potential is increased, the ratio of pcll to [Cl ] should increase without limit according to (1.18). In fact, given that pCh cannot substantially exceed one atm under ambient conditions, this implies that [CP] must become vanishingly small at the electrode surface. The current will then be determined entirely by the rate of transport of CP to the electrode surface and indeed quite generally the current will be determined by two factors, (a) the rate of electron transfer at the electrode surface (b) the rate of transport of material to the electrode surface. 

Clearly, the intrinsic rate of electron transfer is dominated by /. and modern theories have shown that X can be expressed as the sum of two terms Xh the contribution from vibrational modes within the charged ionic unit, and XQy the contribution from the solvent dipole re-orientation effects. Expressions for both of these terms have been given by Marcus ... 

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