Electrode reaction stoichiometry

Reaction mechanisms are also defined for electroorganic reactions, induced by or including an electron transfer at an electrode. Knowledge of such electrode reaction mechanisms includes, preferably but not exclusively, the potential at which the reaction proceeds, the proof of intermediates, the electron stoichiometry, the kinetics of the various reaction steps, and the transport properties of the species involved. Recently, the terms... 

Strategy. Before we start, we need to know (i) what is the electrode reaction. Next, we need to determine (ii) the number of moles (or fractions thereof) of charge which flows through the cell. This is Faraday s first law in action. Knowing the number of moles, we then invoke Faraday s second law and decide from the reaction stoichiometry (iii) how many moles of metal are formed. Finally (iv), now knowing the number of moles of metal, we can calculate the mass of metal from the known atomic mass. The following procedure is therefore adopted ... 

Strategy. We will need to decide first (i) the identity and stoichiometry of the second electrode reaction. Then, we will work out (ii) how much of the charge was required to form the hydrogen by this route. Therefore, knowing the overall charge and the amount consumed in the side reaction, (iii) we can work out the faradaic fraction utilized to form copper metal. 

So, by measuring the current and the time for which it flows, we can determine the moles of electrons supplied. By combining the number of moles of electrons supplied with the mole ratio from the stoichiometry of the electrode reaction, we can deduce the amount of product obtained (see 1). 

Note that the carbon electrode takes part in the reaction. From the reaction stoichiometry, we can calculate that a current of 1 A must flow for 80 h to produce 1 mol A1 (27 g of aluminum, about enough for two soft-drink cans). The very high energy consumption can be greatly reduced by recycling, which requires less than 5% of the electricity needed to extract aluminum from bauxite (Box 14.1). Note also that the production of 1 tonne of aluminum is accompanied by the release of more than 1 tonne of carbon dioxide into the atmosphere. 

In general, the amount of product formed in an electrode reaction follows directly from the stoichiometry of the reaction and the molar mass of the product. 

If we switch from a situation with uniform chemical potential (A/i0 = 0) to a situation in which on one side a different but constant PQ is established, a transient occurs during which the homogeneous stoichiometry profile changes to an approximately linear profile (see chemical polarization, see Appendix 3). As long as the electrode reactions are fast, the emf measured at such a sample is always determined by the invariant boundary values of the oxygen potential (ju0,ju0 + Aju0) but, owing to the internal virtually neutral short-circuit, lower than the Nernst-value. The result is, instead of Eq. (20),56 57 now... 

Voltammetric methods also provide a convenient approach for establishing the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier... 

The techniques of voltage sweep and cyclic voltammetry provide the analytical and physicochemical capabilities of classical voltammetry and in addition provide the means for performing these measurements much more rapidly for a broader range of conditions. Cyclic voltammetry is particularly useful for the rapid assessment of thermodynamic reversibility, and for the evaluation of the stoichiometry for the electrode reaction. 

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. 

In controlled-potential coulometry the total number of coulombs consumed in an electrolysis is used to determine the amount of substance electrolyzed. To enable a coulometric method, the electrode reaction must satisfy the following requirements (a) it must be of known stoichiometry (b) it must be a single reaction or at least have no side reactions of different stoichiometry (c) it must occur with close to 100% current efficiency. 

If the experimental data do not fit Eqs. (n.2.10) and (II.2.11), this means that the electrode reaction in the first-generation step is not of stoichiometry 1 1, or simply the product of the first step is transferred in a chemical process to an electroinactive compound. The deviations may also be caused by the adsorption of either Ox or Red. Similar to NPV, the adsorption of Red at the electrode surface will make the RP voltammogram peak-shaped, and the adsorption of Ox will produce a pre-wave. 

Poulsen, F. W. (2000). Defect chemistry modelling of oxygen stoichiometry, vacancy concentrations, and conductivity of (Lai xSrx)j,Mn03+j. Solid State Ionics 129 143-162. Mizusaki,., Saito, T., and Tagawa, H. (1996). A chemical diffusion-controlled electrode reaction at the compact Lai- Sr MnOa-stabilized zirconia interface in oxygen atmospheres. J. Electrochem. Soc. 143 3063-3073. 

While the precise stoichiometry of the electrode reaction is unimportant, it is very important that the reaction be consistent for the duration of any one experiment. 

Dankhazi, T., Fekete, E., Paal, K. and Farsang, G. (1993) Electrochemical oxidation of lysergic acid-type ergot alkaloids in acetonitrile. Part 1. Stoichiometry of the anodic oxidation electrode reaction. Anal. Ghint. Acta, 282, 289-296. 

Voltammetric methods also provide a convenient approach to establish the thermodynamic reversibility of an electrode reaction and for the evaluation of the electron stoichiometry for the electrode reaction. As outlined in earlier sections, the standard electrode potential, the dissociation constants of weak acids and bases, solubility products, and the formation constants of complex ions can be evaluated from polarographic half-wave potentials, if the electrode process is reversible. Furthermore, studies of half-wave potentials as a function of ligand concentration provide the means to determine the formula of a metal complex. 

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