Cathodic and anodic reactions

As shown in Fig. 7-1, the electrode reaction in which a particle of negative charge (electron or anion) transfers finm an electrode to an electrol3de (aqueous solution) is called the cathodic reaction-, and the electrode reaction in which a particle of positive charge (hole or cation) transfers from an electrode to an electrolyte is called the anodic reaction. Further, the electrode at which the cathodic reaction takes place is called the cathode and the electrode at which the anodic reaction takes place is called the anode. 

For redox electron transfer reactions, the transfer of electrons from an electrode to an oxidfmt particle to form a reductant particle is the cathodic reaction (the electron-accepting reduction of oxidants) and the transfer of electrons from a reductant particle to an electrode to form an oxidant particle is the anodic reaction 

(a) Cathodic reaction and (b) anodic reaction M = metal electrode S = aqueous solution (electrolyte Mm = metal ion in metallic bonding state M., = metal ion in hydrated state Om = electron in metals. 

Similarly, for metal ion transfer reactions, the cathodic and anodic reactions are expressed by Eqn. 7-3 and Eqn. 7-4, respectively  

From the flow of electric charge it follows that the cathodic transfer of metal ions requires the electrode to accept electrons from an external cell circuit, and that the anodic transfer of metal ions requires the electrode to donate electrons to an external cell circuit. No electron transfer, however, takes place across the electrode interface this is the reason why no electrons are involves in the metal ion transfer reactions in Eqns. 7-3 and 7-4. 

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The major cathodic and anodic reactions in near-neutral pH water are shown in Reactions 5.1 to 5.3 ... 

The effect of paint films on the cathodic and anodic reactions will now be considered and the factors which influence the electrolytic resistance of paint films will be discussed. 

Evans Diagram diagram in which the E vs. I relationships for the cathodic and anodic reactions of a corrosion reaction are drawn as straight lines intersecting at the corrosion potential, thus indicating the corrosion current associated with the reaction. 

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

It had been shown in the preceding sections that the initial step in a number of cathodic and anodic reactions yields organic radicals, which then undergo further oxidation, reduction, or dimerization. In some cases reactions of another type are possible reaction of the radical with the electrode metal, yielding organometallic compounds which are then taken up by the solution. Such reactions can be used in the synthesis of these compounds. 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

The electrolysis of some typical solutions are now considered for the purpose of elaborating the nature of the products of the cathodic and the anodic reactions. A classification of cathodic and anodic reactions in a tabular form is shown in Table 6.15. 

They have a signal from the carbonyl ion of carbamide, a signal that corresponds to ammonium ion, and a signal that characterizes the C-N bond. On the basis of the results obtained by us, only a rough scheme of cathode and anode reactions can be proposed because a number of bands in the IR spectra have not been identified ... 

A simple example of the redox behaviour of surface-bound species can be seen in Figure 2.17, which shows the behaviour of a bare platinum electrode in N2-saturated aqueous sulphuric acid when a saw tooth potential is applied. There are two clearly resolved redox processes between 0.0 V and 0.4 V, and these are known to correspond to the formation and removal of weakly and strongly bound hydride, respectively (see section on the platinum CV in chapter 3). The peak currents of the cathodic and anodic reactions for these processes occur at the same potential indicating that the processes are not kinetically limited and are behaving in essentially an ideal Nernstian fashion. The weakly bound hydride is thought to be simply H atoms adsorbed on top of the surface Pt atoms, such that they are still exposed to the... 

The concentration of the given species may appear with different exponents in the rate equations for the cathodic and anodic reactions. 

Verify Levich s equation for both the cathodic and anodic reactions and calculate the diffusion coefficients of species ox and red. 

Research on the direct conversion of chemical energy to electricity via fuel cells has received considerable attention in the past decades. Fuel cells are indeed attractive alternatives to combustion engines for electrical power generation in transportation applications and also as promising future power sources, especially for mobile and portable applications. Thus, the search for excellent electrocatalysts for the electro-catalytic oxygen reduction and methanol oxidation reactions, which are the two important cathodic and anodic reactions in fuel cells, is intensively pursued by scientists... 

Polarization of the galvanic cell. The different phenomena of polarization of the anodic and cathodic reactions (activation, diffusion, convection, etc.), should be well known as a function of the evolution and change of the properties of the interface as a function of time. The polarization behavior of the cathodic and anodic reactions on the two electrodes should be examined (see Figure 6.5). In natural atmospheres, the cathodic reaction controls frequently the attack rate. The diffusion of oxygen is an important parameter to avoid control and polarization of the corrosion by the rate of the cathodic reaction (Figure 6.12).7... 

If the battery is to be rechargeable, the reaction must be reversible, and if the load is replaced by a power supply, the reaction in equation (15) is reversed, or the cathode and anode reaction go in the reverse direction. The system depicted in Figure 28(a) was not viable, because of the uneven (dendritic) growth of Li metal upon subsequent discharge-recharge cycles, which lead to explosion hazards. 

We conclude this section by noting that underpotential deposition is a rather general phenomenon, occurring in both cathodic and anodic reactions. The surface is modified by the UPD layer and its catalytic activity is altered, usually for the better. The UPD layer is "transparent" to electrons (even when it consists of a layer of halogen atoms), and probably should be considered to be an extension of the metal rather than a superficial layer of a foreign substance. A... 

Electrolytic cleavage of a carbon-carbon bond may occur in the course of both cathodic and anodic reactions. During an uncontrolled anodic oxidation, many organic compounds end up as carbon dioxide and water, which involves splitting of C-C bonds, but only oxidations in which the reaction does not have the character of an extensive degradation are discussed here. The Kolbe reaction that involves a cleavage of a carbon-carbon bond has been discussed in Chapter 14. 

The use of chiral modified electrodes (see Chapter 27) may be the most promising method for electrochemical asymmetric synthesis because of (1) use of extremely small amounts of inducers, (2) low sensitivity to reaction conditions, (3) variety of kind, and (4) possible application of both cathodic and anodic reactions, although there are some problems to be solved. 

The strategy is to consider the possible competing cathode and anode reactions. At the cathode, choose the reduction reaction with the most positive (least negative) standard reduction potential ( ° value). At the anode, choose the oxidation reaction with the least positive (most negative) standard reduction potential E° value, as given in the table). Then calculate = °(cathode) - °(anode). The negative of this value is the minimum potential that must be supplied. 

From the temperature dependence of Eqs (19.12) and (19.13), the activation energy of the cathodic and anodic reactions at different values of Tj can be obtained. 

The cathodic Tafel slope corresponding to reaction (19.33) in the reverse direction is close to —0.120 V per decade at 25°C. For both cathodic and anodic reactions, the interfacial capacity results in 30—35 xF cm. This figure is consistent with a low surface coverage by chlorine atoms. 

The kinetics of the hydrogen electrode reaction on dense porous graphite electrodes in molten KHSO4 from 245° C to 280°C [88-90] showed that the cathodic and anodic reactions are not strictly conjugated processes. The cathodic reaction was discussed in terms of conventional mechanisms, but the anodic reaction involves the simultaneous oxidation of hydrogen and graphite surface. The reaction exhibits a one-half power dependence on hydrogen pressure. 

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