Types of electrode reaction

Among electrode processes with at least one charge transfer step, several different types of reaction can be found. The simplest interfacial electrochemical reactions are the exchange of electrons across the electrochemical interface by flipping oxidation states of transition metal ions in the electrolyte adjacent to the electrode surface. The electrode in this case is merely the source or sink of electrons, uptaking electrons from the reduced species and releasing them to the oxidized redox species in solution. Examples of simple electron transfer reactions are 

As in the homogeneous case, heterogeneous redox electrode reactions involve the chemical reorganization of the coordination spheres of the participating ions, which determines the kinetics of the overall electron transfer process because the electronic transition is a very fast event. 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

It is important to notice that the rate of a given outer sphere electrode redox reaction should be independent of the nature of the metal electrode if allowance is made for electrostatic work terms or double layer effects which will, of course, be dependent on the nature of the electrode material. Inner sphere reactions, on the other hand, are expected to be catalytic with kinetics strongly dependent on the electrode surface due to specific adsorption interactions. 

Another type of charge transfer electrode reaction is the ion transfer reaction in which the electrical charge is transferred by an ion across the double layer. These reactions are typical of metal dissolution and deposition. For instance 

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Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

The type of electrode reaction that will occur depends on the electrode and electrolyte and also on external conditions the temperature, impurities that may be present, and so on. Possible reactants and products in these reactions are (1) the electrode material, (2) components of the electrolyte, and (3) other substances (gases, liquids, or solids) which are not themselves component parts of an electrode or the electrolyte but can reach or leave the electrode surface. Therefore, when discussing the properties or behavior of any electrode, we must indicate not merely the electrode material but the full electrode system comprising electrode and electrolyte as well as additional substances that may be involved in the reaction for example, ZnCl2, ag I (Clj), graphite [the right-hand electrode in (1.19)]. 

The flow of electric current through the electrolytic cell is connected with chemical, electrochemical and physical processes which, as a whole, are termed the electrode process. The main electrochemical step in the electrode process is the actual exchange of charged species between the electrode and the electrolyte, which will be termed the electrode reaction (charge transfer reaction). Substances participating directly in the charge transfer reaction are termed electroactive. These substances can be either soluble or insoluble in the electrolyte or electrode material. Common basic types of electrode reactions are as follows ... 

The type of electrode reaction employed, the cell geometry, and the manner in which the limiting-current measurement is carried out determine the shape of the current versus electrode-potential curve. Often the ideal horizontal inflection in such curves is absent, making the determination of true limiting current problematical if not impossible. Characteristics of satisfactory limiting current plateaus are as follows ... 

To learn that there are two basic types of electrode reaction, namely oxidation (loss of electrons) and reduction (gain of electrons). These reactions are the sum total of redox chemistry. 

For any type of electrode reaction in solution, the Arrhenius expression relates the activation enthalpy, A//, with the rate constant, k ... 

It is convenient to distinguish between two types of electrode reactions ... 

In conclusion, all systems analysed which belong to the ion-transfer (deposition) type of electrode reactions change their E° ( 1/2) potentials to more negative values versus a solvent-independent electrode when the DN of the solvent increases. This conclusion seems to be valid also for electron-transfer type reactions, if both the oxidized and the reduced form are cations. Sahami and Weaver [86] have stud-... 

Two types of electrode reactions were found in the reduction of thiocyanates. 2-Thiocyanatocacetophenones [119] are reduced in a two-electron reduction to the acetophenone and thiocyanate ion. In most other thiocyanates the reduction results in the loss of a cyanide ion the half-wave potential is nearly independent of pH. Macroscale reductions at a mercury cathode yielded [149] the mercaptan. 

Examples 19-1 and 19-2 illustrate an important fact. The magnitude of the potential difference between the two electrodes is 0.412 V independent of which electrode is considered the left or reference electrode. If the Ag electrode is the left electrode, as in Example 19-2, the cell potential has a negative sign, but if the Cu electrode is the reference, as in Example 19-2, the cell potential has a positive sign. No matter how the cell is arranged, however, the spontaneous cell reaction is oxidation of Cu and reduction of Ag, and the free energy change is 79,503 J. Examples 19-3 and 19-4 illustrate other types of electrode reactions. 

It is important to obtain experimental information on the thermodynamics of electrode processes to ascertain the tendency of a particular reaction to occur under a given set of experimental conditions namely temperature, pressure, system com H)sition and electrode potential. Such information is provided by the standard- or formal-electrode potentials for the redox couple under consideration. Appropriate combinations of these potentials enable the thermodynamics of homogeneous redox processes to be determined accurately. However, such quantities often are subject to confusion and misinterpretation. It is, therefore, worthwhile to outline their significance for simple electrochemical reactions. This discussion provides background to the sections on electrochemical kinetics which follow. The evaluation of formal potentials for various types of electrode-reaction mechanisms is dealt with in 12.3.2.2. 

For symmetrical processes, each of these ways of representing the significance of gives the latter quantity a value of 0.5. However, the unsymmetrical processes, the different representations do not lead to identical values of for a given reaction note that atom transfer processes are normally highly unsymmetrical, so how p is considered from the theoretical point of view is especially important for such types of electrode reaction. 

Equation (32) is an example of a type of electrode reaction which may be represented by... 

Fig. 4. List of kinetic parameters which determine the electrical response of model electrochemical reaction schemes. The single electrode reaction (E), two reactions in series (E.E), and two reactions in parallel (E + E) are common types of electrode reactions, discussed in the text, which every electrochemical investigation has to be capable of assessing as a matter of course.

In this chapter, the terms and concepts employed in describing electrode reactions are introduced. In addition, before embarking on a detailed consideration of methods for studying electrode processes and the rigorous solutions of the mathematical equations that govern them, we will consider approximate treatments of several different types of electrode reactions to illustrate their main features. The concepts and treatments described here will be considered in a more complete and rigorous way in later chapters. 

Detailed treatments like the one developed for the zinc-ammonia system are easily worked out for other types of electrode reactions, including... 

The three steps associated with electrochemical reactions, i.e., transport of reactant(s) to the interface, the electron transfer (surface) reaction and transport of product(s) from the interface, are sequential. Therefore, the overall rate of reaction is controlled by the slowest of the three steps. When the transport processes are capable of operating at high rates relative to the electron transfer reaction, the rate of the overall reaction can be described by equations of electrodekinetics. These types of electrode reactions are said to be "under activation control". 

Whatever the mechanism, a pharmaceutical incompatibility between sodium heparin and chlorpromazine hydrochloride solutions is confirmed. The clinical significance of an interaction between the two drugs after administration has not been demonstrated. The type of electrode reaction demonstrated in the model in vitro system may occur in vivo, because a biological membrane has been shown to be capable of acting as an electrode and it known that heparin is taken up by cell membranes. Binding of chlorpromazine to other mucopolysaccharides may be of significance in the pharmacokinetic disposition of the drug. 

Fig. 1.1- Schematic view of some types of electrode reactions met in applied and fundamental electrochemistry.

A review of the effect of the nature of the electrode on the kinetics of electrode reactions was given by Parsons [1] recently. The different types of electrode reactions may be grouped between two limiting cases ... 

As the name of this chapter implies, we shall deal here with the various rate expressions associated with different types of electrode reaction mechanisms. But first let us compare electrode kinetics with regular solution kinetics and evaluate the similarities and the differences between the two. 

For low-temperature fuel cells, electrocatalysts are required to achieve desirable reaction rates at the anode and cathode, whereas the high-temperature fuel cells do not require any electrocatalyst. The most common type of electrode reactions and their corresponding Nemst potential are as follows ... 

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