Energetics of Electrode Reactions

The previous analysis clearly shows that a substantial amount of energy has to be supplied to the metallic silver in order to make it dissolve as Ag+. On the other hand, Ag+ from the electrolyte will spontaneously react at the silver surface to form metallic Ag AG°(Ag+ — Ag) -95 kJ mol-1. Indeed, this is why silver is considered a noble metal. 

The establishment of a stable equilibrium potential between the metal electrode and the electrolyte can be straightforwardly explained as follows. As soon as the neutral silver electrode gets in contact with the electrolyte, the reaction Ag+ + e — Ag will proceed, while the rate of the back reaction is negligibly small. The excess positive charge injected into the silver electrode will render the potential of the electrode 

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Galizzioli, D., Tantardini, F. and Trasatti, S. (1975), Ruthenium dioxide A new electrode material. II. Non-stoichiometry and energetics of electrode reactions in acid solutions. J. Appl. Electrochem., 5(3) 203-214. 

Adsorption phenomena significantly influence the rate of electrode reactions. The heterogeneous nature of electrode reactions determines that energetics and local activities of reacting species in the vicinity of the electrode may be different from those in the bulk solution, even when mass transport limitations can be regarded as negligible. The structure and properties of the electrode—solution interface then play a key role in the adsorption of electroactive as well as electroinactive surface active substances (SAS) at electrodes. 

Though much research on the influence of the solvent on the rate of electrode reactions has been done in recent years the problem is still far from a profound understanding. The basic question is the role of the dynamic and energetic terms in the control of the kinetics of simple electron-transfer electrode reactions. To answer this question it is essential to have reliable kinetic data for analysis. Unfortunately some kinetic data are too low and should be redetermined, preferably using submicroelectrodes. 

Steady-state or dynamic potential and current relationships to provide information on the energetic dependence of electrode reaction Current or potential versus time to provide information concerning stability of... 

The presence of a derivatized surface layer can affect the energetics as well as the kinetics of electrode reactions. It has been found that the flatband potentials of -Si and p-Si electrodes coated with conducting polypyrrole films are shifted by 300 and 500 mV in CH3CN solution." The reaction kinetics on polymer derivatized surface can further be enhanced by impregnation of noble metals such as Pt particles into the... 

Specific adsorption can have several effects. If an electroactive species is adsorbed, the theoretical treatment of a given electrochemical method must be modified to account for the presence of the reactive species at the electrode surface in a relative amount higher than the bulk concentration at the start of the experiment. In addition, specific adsorption can change the energetics of the reaction, for example, adsorbed O may be more difficult to reduce than dissolved O. The effects of specific adsorption in different electrochemical methods are discussed in Section 14.3. 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

Figure 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

M. Tilset. Derivative Cyclic Voltammetry Applications in the Investigation of the Energetics of Organometallic Electrode Reactions. In Energetics of Organometallic Species, J. A. Martinho Simoes, Ed. NATO ASI Series C, Kluwer Dordrecht, 1991 chapter 8. 

The hydrogenase film on the electrode was very stable, and this allows the study of active/inactive interconversion under strict potential control. By comparing cyclic voltammetry and potential step chronoamperometry, we were able to integrate energetics, kinetics and H e stoichiometry of the reaction. The effects of pH on these processes could also be conveniently observed. 

Electrode reactions occur when they are energetically favourable and do not occur if the thermodynamics of an electrode reaction imply that it is non-spontaneous. The simplest cause ofl o -faradaic arises from competing electrode reactions, i.e. two electrode reactions can occur in tandem if their energies are similar. Probably the most common of these reactions is electrolytic side reactions such as solvent splitting. 

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

Electron transfer is a fast reaction ( 10-12s) and obeys the Franck-Condon Principle of energy conservation. To describe the transfer of electron between an electrolyte in solution and a semiconductor electrode, the energy levels of both the systems at electrode-electrolyte interface must be described in terms of a common energy scale. The absolute scale of redox potential is defined with reference to free electron in vacuum where E=0. The energy levels of an electron donor and an electron acceptor are directly related to the gas phase electronic work function of the donor and to the electron affinity of the acceptor respectively. In solution, the energetics of donor-acceptor property can be described as in Figure 9.6. 

The electrode is generally, though not necessarily, a good conductor, being the source or sink of electrons and the site of reaction, a reactant in the case of metal dissolution, a product in metal deposition, etc. It must be borne in mind that, in electrode reactions, one of the reactants is the electron, the concentration and energetics of which may be controlled through the electrode—electrolyte interfacial electrical potential difference. 

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