Electrode reactions in electrolysis

In the following three sections we shall discuss four applications of quantum mechanics to miscellaneous problems, selected from the very large number of applications which have been made. These are the van der Waals attraction between molecules (Sec. 47), the symmetry properties of molecular wave functions (Sec. 48), statistical quantum mechanics, including the theory of the dielectric constant of a diatomic dipole gas (Sec. 49), and the energy of activation of chemical reactions (Sec. 50). With reluctance we omit mention of many other important applications, such as to the theories of the radioactive decomposition of nuclei, the structure of metals, the diffraction of electrons by gas molecules and crystals, electrode reactions in electrolysis, and heterogeneous catalysis. 

A FIGURE 19-23 Predicting electrode reactions in electrolysis— Example 19-11 illustrated... 

An overpotential is the voltage in excess of the theoretical value required to produce a particular electrode reaction in electrolysis. 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

Hydrogen evolution reaction — [i—iii] is an - electrode reaction in which hydrogen gas is produced at the - cathode of an - electrolysis cell by the reduction of hydrogen ions or by the reduction of the water molecules of an aqueous solution. Abbreviated as her . 

This section treats the electrolysis of substituted heterocyclic compounds if the electrode reaction involves the substituent directly and if the electrode reaction in some way is different from what would be expected from the carbocyclic series. 

Measurements often are made at only one rotation velocity and the ratio i /C is used in comparison with a one-electron standard to obtain a value of n for the wave. However, measurements made at only one rotation rate may be deceptive, because there is no confirmation that convective diffusion is the only means of mass transport e.g., this approach would not be able to diagnose adsorption phenomena or electrode reactions in which the n value is a function of electrolysis time. 

Now if there were two highly reactive species (for the oxidation and reduction process), it would require only a low voltage to drive the two electrode reactions. In fact a battery is a device that contains species that are sufficiently reactive so that instead of needing any externally supplied voltage to move electrons, the electrons are driven externally under the energy of reaction. Alternatively, two unreactive species (or even just one, if sufficiently unreactive) will require a higher voltage for electrolysis. 

Moreover, a case-study approach is rarely seen and the need to select the reactor design to suit the process needs is not always evident. In particular, it is important to consider the desired (and other) electrode reactions in the light of the composition of the process liquor, the reactor geometry and the electrolysis conditions. Equally, the metal must be removed from solution in a suitable form and at the desired frequency. 

Electrolytic gas evolution is a significant and complicated phenomenon in most electrochemical processes and devices. In the Hall process for aluminum production, for example, bubbles evolved on the downward-facing carbon anodes stir the bath and resist the current, both of which directly affect the heat balance and the cell voltage. Bubbles appear as a result of primary electrode reactions in chlorine and water electrolysis, and as the result of side reactions in the charging of lead-acid batteries and some metal electrowinning. Stirring of the electrolyte by gas evolution is an important phenomenon in chlorate production. Electrolytically evolved bubbles have also been used in mineral flotation. Relatively few major electrochemical processes do not evolve gas. 

A. Kisza, J. Kazmierczak, J. Ihonstad, T. Eidet, J. Hives, The Kinetics and Mechanism of the Electrode Reactions in Aluminium Electrolysis, in H. Wendt (Ed.), Molten Salt Forum 5, Trans Tech Publications LTD, New York, 1998, p. 263. 

The element tin is generally recovered from deposits of the ore cassiterite (Sn02). The oxide is reduced with carbon, and the crude metal is purified by electrolysis. Write balanced chemical equations for the reduction process and for the electrode reactions in the electrolysis. (Assume that an acidic solution of 1SO4 is employed as an electrolyte in the electrolysis.)... 

Mainly various spectroscopic methods were applied to the study of intermediates and products of electrode reactions in situ, however, they are predominantly based on the electrolysis of the solution as a prerequisite for any other investigation. Even with special experimental set up the time of electrolysis cannot be shortened beyond a certain limit due to the detection limits of the spectroscopic techniques used. It is hardly possible to investigate intermediates with life times shorter than about few seconds. 

Bioelectrochemical Hydrogen Production, Fig. 2 Comparison of the electrode reactions in a microbial fuel cell and microbial electrolysis cell and... 

Hydrides. LiH melts at 689°C. Would you expect liquid LiH to conduct an electric current Write the equations for the reaction at each electrode. Write the equations for the electrode reactions in the electrolysis of the solution formed by mixing water with LiH. (Assume that the electrodes are Pt in all... 

The equality of electrons passing across each electrode surface and through the external circuit largely determines the way in which we seek to understand or to study electrode reactions and electrolysis cells. The current i is in fact the rate at which electrons move through the external circuit It is also a very... 

In addition to the above factors, part of any difference observed with molten salts must be attributed of course simply to the effects of temperature. First, the kinetic accessibility of new states is increased so that reactions such as metathesis, oxidation-reduction, and electron exchange are usually rapid. The same may be said concerning the attainment of equilibrium at electrodes and in electrolysis. 

EXAMPLE 19-12 Predicting Electrode Half-Reactions and Overall Reactions in Electrolysis... 

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). 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

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... 

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-... 

---