Redox reactions terminology

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

For a metal, the negative of the work function gives the position of the Fermi level with respect to the vacuum outside the metal. Similarly, the negative of the work function of an electrochemical reaction is referred to as the Fermi level Ep (redox) of this reaction, measured with respect to the vacuum in this context Fermi level is used as a synonym for electrochemical potential. If the same reference point is used for the metal s,nd the redox couple, the equilibrium condition for the redox reaction is simply Ep (metal)= Ep(redox). So the notion of a Fermi level for a redox couple is a convenient concept however, this terminology does not imply that there are free electrons in the solution which obey Fermi-Dirac statistics, a misconception sometimes found in the literature. 

We recommend this scheme of oxidation states only as an aid to identify and balance redox reactions. Also, the terminology redox should not be confused with the mechanism of a reaction, as there is no connection between them. A moment s reflection also will show that virtually all reactions theoretically can be regarded as redox reactions, because in almost every reaction the reacting atoms experience some change in their electronic environments. Traditionally, however, reactions are described as redox reactions of carbon only when there is a net change in the oxidation state of the carbon atoms involved. An indication of just how arbitrary this is can be seen by the example... 

We now look at some examples of redox reactions involving simple cations in aqueous solution. Electrochemical terminology will often be encountered, since e.m.f. measurements on electrochemical cells are important sources of thermodynamic data in this area. For example, the reduction potential ° for the half-reaction ... 

Voltammetric methods are based on the potential-dependent redox reaction of the species to be analysed at the surface of a suitable electrode. According to the terminology recommended by IUPAC (International Union of Pure and Applied... 

Redox reactions are the third and, perhaps, most important type of chemical process. They include the formation of a compound from its elements (and vice versa), all combustion reactions, the reactions that generate electricity in batteries, the reactions that produce cellular energy, and many others. In this section, we examine the process and introduce some essential terminology. 

Figure 4.11 A summary of terminology for oxidation-reduction (redox) reactions.

Whereas the addition of a donor or acceptor molecule to the polymer is called doping, the reaction that takes place is actually a redox reaction and is unhke the doping of Si or Ge in semiconductor technology, where there is substitution of an atom in the lattice. The terminology in common use will be retained here, but it should be remembered that the doping of conductive polymers involves the formation of a polymer salt, and this can be effective either by immersing the polymer in a solution of the reagent or by electrochemical methods. 

Oxidation-Reduction (Redox) Reactions The Key Event Movement of Electrons Redox Terminology... 

Using Saveanfs terminology, such a process is called redox catalysis in its proper meaning, while Shono formed the expression homomediatory system . This type of mechanism was already schematically presented in the case of an oxidation in Eqs. (2) to (4). To this category of redox catalysts belong, for example, the radical anions and cations of aromatic and heteroaromatic compounds and some reactions of triaryl amine radical cations. 

In a chemically reversible redox system the rate of the following reaction(s) is insufficient to perturb the concentration of within the electrochemical reaction layer near the electrode surface. When the following reaction is rapid so that Y is depleted during the experiment, the couple is chemically irreversible. The term quasireversible should not be used in conjunction with chemical reversibility. That terminology is restricted to the electron-transfer step itself. This error is made frequently. Redox systems that are less than totally chemically reversible should be described as having limited chemical reversibility. 

Summary. It is pointed out that, in order to avoid misconceptions, the introduction of ions is very important ions have been dealt with as basic particles of matter according to Dalton s atomic model (see Chap. 5). In order to understand the charges of ions and the change of ions and atoms by electron transfer, the differentiated atomic model with nucleus and electron shells should be introduced. With the assistance of a clear terminology, it is easy to formulate half-reaction for the oxidation and reduction steps, the number of electrons to be transferred can be clearly recognized. Finally, if mental models -for instance, from involved atoms or ions in Galvanic cells or in batteries - are relayed and drawn by the students themselves, then they could more easily see through the redox processes or even perhaps be able to repeat them independently. In all explanations, one should pay attention that the observations should be done at the substance level, but that the interpretations and discussions of reaction equations should consequently take place at the level of the smallest particles as atoms, ions and molecules. 

Figure 21.1 A summary of redox terminology. In the reaction between zinc and hydrogen ion, Zn is oxidized and H is reduced.

Be sure to remember that transferred electrons are never free because the reducing agent loses electrons and the oxidizing agent gains them simultaneously. In other words, a complete reaction cannot be an oxidation or a reduction it must be an oxidation-reduction. Figure 4.12 summarizes redox terminology. 

Figure 21.1 A summaiy of redox terminology, as applied to the reaction of zinc with hydrogen ion.

In humid air, the metal surface will be covered by several monolayers of water as long as no hygroscopic impurities are adsorbed. Under these circumstances, standard electrochemistry will not be valid anymore, as no electrolytic double layer is formed and the reaction products will not be transported away from the surface. Therefore, the rate of any electrochemical reaction will be rather small and the electrode potential , which is measured by the Kel-vinprobe cannot be defined in the usual terminology of electrochemistry but will reflect, e.g., the redox properties of the oxide scale. In particular, the electrode potential ... 

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