Electrolysis, electrode reactions

Studies aimed at characterizing the mechanisms of electrode reactions often make use of coulometry for determining the number of electrons involved in the reaction. To make such measurements a known amount of a pure compound is subject to a controlled-potential electrolysis. The coulombs of charge needed to complete the electrolysis are used to determine the value of n using Faraday s law (equation 11.23). 

Explosion Hazards. The electrolysis of aqueous solutions often lead to the formation of gaseous products at both the anode and cathode. Examples are hydrogen and chlorine from electrolysis of NaCl solutions and hydrogen and oxygen from electrolysis of water. The electrode reactions. 

Sodium metal is obtained by the electrolysis of molten sodium chloride (Figure 20.3, p. 537). The electrode reactions are quite simple ... 

Coulometric analysis is an application of Faraday s First Law of Electrolysis which may be expressed in the form that the extent of chemical reaction at an electrode is directly proportional to the quantity of electricity passing through the electrode. For each mole of chemical change at an electrode (96487 x n) coulombs are required i.e. the Faraday constant multiplied by the number of electrons involved in the electrode reaction. The weight of substance produced or consumed in an electrolysis involving Q coulombs is therefore given by the expression... 

Electrical units 503, 519 Electrification due to wiping 77 Electro-analysis see Electrolysis and Electrogravimetry Electrochemical series 63 Electro-deposition completeness of, 507 Electrode potentials 60 change of during titration, 360 Nernst equation of, 60 reversible, 63 standard 60, (T) 62 Electrode reactions 505 Electrodeless discharge lamps 790 Electrodes antimony, 555 auxiliary, 538, 545 bimetallic, 575... 

One possible reason for the reluctance of non-electrochemists to venture into this field is that in contrast to the electrochemists claim that controlled potential electrolysis offers a method for the selective introduction of energy into molecules, many electrode reactions carried out at a controlled potential have still been reported to give low yields and a diversity of products. The electrode potential is, however, only one of several variables and the lack of selectivity in the electrode process may be attributed to a failure to understand and to control all the parameters of the overall electrode reaction. 

Perhaps the most important single function of the solution environment is to control the mode of decomposition of reaction intermediates and hence the final products. This is particiflarly true in the case of electrode reactions producing carbonium ion intermediates since the major products normally arise from their reaction with the solvent. It is, however, possible to modify the product by carrying out the electrolysis in the presence of a species which is a stronger nucleophile than the solvent and, in certain non-nucleophilic solvents, products may be formed by loss of a proton or attack by the intermediate on further starting material if it is unsaturated. The major reactions of carbonium ions are summarized in Fig. 6. 

Certain electrode reactions occur by a mechanism which involves initial oxidation or reduction of one of the components of the electrolysis medium. This species is commonly reformed later in the reaction se-... 

The electrode reactions that take place in the electrolysis of CuS04 will be those that proceed most readily, and in the absence of the effects due to irreversibility, these are the reactions which possess the greatest driving force, or emf E. At the cathode the reactions possible are... 

In electroanalysis, coulometry is an important method in which the analyte is specifically and completely converted via a direct or indirect electrolysis, and the amount of electricity (in coulombs) consumed thereby is measured. According to this definition there are two alternatives (1) the analyte participates in the electrode reaction (primary or direct electrolysis), or (2) the analyte reacts with the reagent, generated (secondary or indirect electrolysis) either internally or externally. 

The electrolysis efficiency must be 100 per cent, i.e. the charge passed must be consumed only in the studied electrode reaction. There are four possible sources of error ... 

Consider the relatively simple case where the chemical reactions of formation and dissociation of the complexes are sufficiently fast so that equilibrium among the complexes, free ions and complexing agent is maintained everywhere in the electrolyte during the electrolysis. The rate of deposition of the metal is then determined by the electrode reaction of the complex MX/, for which the product kc,MXicMX is, at the given potential, the largest for all the complexes. 

The simplest case occurs when a number of nuclei is formed at the beginning of the process and does not change with time instantaneous nucleation). This is the case when the electrolysis is carried out by the double-pulse potentiostatic method (Fig. 5.18B), where the crystallization nuclei are formed in the first high, short pulse and the electrode reaction then occurs only at these nuclei during the second, lower pulse. A second situation in which instantaneous nucleation can occur is when the nuclei occupy all the active sites on the electrode at the beginning of the electrolysis. 

The reaction mechanisms of organic electrode reactions are thus composed of at least one ET step at the electrode as well as preceding and follow-up bond-breaking, bond-forming, or structural rearrangement steps. These chemical steps may be concerted with the electron transfer [15, 16]. The instrumental techniques described in this chapter allow the investigation of the course of the reaction accompanying the overall electrolysis. 

A typical counter electrode reaction is the electrolysis of water. Here the cathodic evolution of hydrogen is coupled with the formation of base, the anodic development of oxygen produces acid additionally. Frequently, acid and base formation at both electrodes will be balanced. Otherwise, a buffer solution or a (continuous) base/acid addition, for example, by a pH-controlling system, can enable the application of an undivided cell. 

These chemical electron-transfer reactions are in contrast with the electrode reactions. For instance, stilbene gives a mixtnre of 1,2-diphenylethylene with 1,2,3,4-tetraphenylbutane on electrolysis (Hg) in DMF at potentials about that of the first one-electron wave. This solvent has faint proton-donor properties. The stilbene anion-radical is stable under these conditions it has enough time to diffuse from the electrode into the solvent and dimerize therein (Wawzonek et al. 1965). 

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional hulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the Np02 /Np02 and Np /Np " " couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

The electrode reaction of triamterene 15 was elucidated by means of DCP, Tast polarography, cyclic voltammetry, microcoulometry, controlled potential electrolysis, and spectroscopy (ultraviolet/visible (UVA is), NMR). Two steps of reduction independent of pH were observed two-electron reduction of 15 resulted in the formation of 17. The first reduction wave of 15 was assumed to be due to irreversible two-electron reduction forming unstable 16, which tautomerized to 17, and the second reduction wave was ascribed to two-electron reduction of 17 to the tetrahydro product, 18 (Scheme 2). 

A positive standard cell potential tells you that the cathode is at a higher potential than the anode, and the reaction is therefore spontaneous. What do you do with a cell that has a negative " gii Electrochemical cells that rely on such nonspontaneous reactions cire called electrolytic cells. The redox reactions in electroljdic cells rely on a process called electrolysis. These reactions require that a current be passed through the solution, forcing it to split into components that then fuel the redox reaction. Such cells are created by applying a current source, such as a battery, to electrodes placed in a solution of molten salt, or salt heated until it melts. This splits the ions that make up the salt. 

The method of complete electrolysis is also important in elucidating the mechanism of an electrode reaction. Usually, the substance under study is completely electrolyzed at a controlled potential and the products are identified and determined by appropriate methods, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis. In the GC method, the products are often identified and determined by the standard addition method. If the standard addition method is not applicable, however, other identification/determination techniques such as GC-MS should be used. The HPLC method is convenient when the product is thermally unstable or difficult to vaporize. HPLC instruments equipped with a high-sensitivity UV detector are the most popular, but a more sophisticated system like LC-MS may also be employed. In some cases, the products are separated from the solvent-supporting electrolyte system by such processes as vaporization, extraction and precipitation. If the products need to be collected separately, a preparative chromatographic method is use-... 

Dialkylaziridinium salts are reducible both in aqueous and aprotic media. In aqueous solution the data are consistent with one two-electron reaction. The products obtained from a preparative electrolysis stem in part from the electrode reaction, but side reactions between primary products and starting material complicate the product mixture.181... 

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