Redox reactions types

Soluble sulfides (i.e., H S, HS" and S ", with sulfur at minus two oxidation state) are chemically very reactive. The two general types of soluble-sulfide reactions may be identified as precipitation reaction (type A) and redox reaction (type B). 

Equation (2.41) shows again that the Galvani potential difference of an electrode in contact with a redox electrolyte depends on the chemical potential of the electrons in the metal used as the electrode. This difference does not, however, appear in the measured cell voltage vs. a reference electrode, because it is compensated at the external connection to the metal of the reference electrode by the contact potential between the two metals as explained in Figure 2.5 and Equations (2.35) and (2.36). The consequence for the two cycles of ion transfer (type 2.23) and redox reactions (type 2.27) is that the chemical potentials of the electrons in the individual electrodes have to be replaced by the chemical potential of the electrons in the metal of the reference electrode. The relative position of an electrode reaction in the electromotive series is therefore exclusively controlled by the siun of all the other chemical potentials in these cycles. 

For more complicated redox reactions, a general fonn of the Nemst equation may be derived by analogy with A2.4.113. If we consider a stoichiometric reaction of the following type ... 

Type 2 tlie inliibiting species takes part in tlie redox reaction, i.e. it is able to react at eitlier catliodic or anodic surface sites to electroplate, precipitate or electropolymerize. Depending on its activation potential, tlie inliibitor affects tlie polarization curve by lowering tlie anodic or catliodic Tafel slope. 

The most important types of reactions are precipitation reactions, acid-base reactions, metal-ligand complexation reactions, and redox reactions. In a precipitation reaction two or more soluble species combine to produce an insoluble product called a precipitate. The equilibrium properties of a precipitation reaction are described by a solubility product. 

Thus far we have examined titrimetric methods based on acid-base, complexation, and redox reactions. A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration. We call this type of titration a precipitation titration. 

The potential of a metallic electrode is determined by the position of a redox reaction at the electrode-solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion. 

Balancing a redox reaction is often more challenging than balancing other types of reactions, because we must balance both electrons and elements. Perhaps the simplest way to balance a redox reaction is by the half-reaction method, which consists of the following steps. 

There are two types of impressed current anodes either they consist of anodically stable noble metals (e.g., platinum) or anodically passivatable materials that form conducting oxide films on their surfaces. In both cases, the anodic redox reaction occurs at much lower potentials than those of theoretically possible anodic corrosion. 

Type B (redox) reactions are more complex. Sulfide in this reaction is converted into some other oxidation state of sulfur. For example, sulfides can be converted to a zero oxidation state of elemental sulfur by oxygen ... 

Another common type of reaction in aqueous solution involves a transfer of electrons between two species. Such a reaction is called an oxidation-reduction or redox reaction. Many familiar reactions fit into this category, including the reaction of metals with acid. 

This expression enables us to calculate the exact concentration at the equivalence point in any redox reaction of the general type given above, and therefore the feasibility of a titration in quantitative analysis. 

When we mix two solutions the result is often simply a new solution that contains both solutes. However, in some cases the solutes can react with each other. For instance, when we mix a colorless aqueous solution of silver nitrate with a clear yellow aqueous solution of potassium chromate, a red solid forms, indicating that a chemical reaction has occurred (Fig. 1.1). This section and the next two introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. (The fourth type of reaction discussed in this text, Lewis acid-base reactions, is introduced in Section 10.2.) Because many chemical reactions take place in solution, particularly in water, in this section we begin by considering the nature of aqueous solutions. 

Redox reactions constitute the third of the three major classes of chemical reactions treated here. The variety of these reactions is remarkable. Many common reactions, such as combustion, corrosion, photosynthesis, the metabolism of food, and the extraction of metals from their ores, appear to be completely different. However, when we consider these changes at the molecular level with a chemist s eye, we can see that they are all examples of a single type of process. 

Balance each of the following skeletal equations and classify the reaction as Bronstcd-I. owry acid-base, Ix wis acid-base, or redox. For both types of acid-base reactions, identify the acid and the base. For redox reactions, identify the reducing agent and the oxidizing agent. 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

Polynuclear dendrimer complexes of this type can undergo redox reactions at the metal centre and have luminescent properties. They have been proposed as molecular photochemical devices, although no practical examples have yet been produced. 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

A detailed discussion of redox reactions must wait until Chapter 19, after we explore the nature of the atom, periodic properties of the elements, and thermodynamics. For now, we focus on only a few types of redox reactions that are common and relatively simple. 

Added stability in PEC can be attained through the use of non-aqueous solvents. Noufi et al. [68] systematically evaluated various non-aqueous ferro-ferricyanide electrolytes (DMF, acetonitrile, PC, alcohols) for use in stabilizing n-CdSe photoanodes. Selection of the solvent was discussed in terms of inherent stability provided, the rate of the redox reaction, the tendency toward specific adsorption of the redox species, and the formal potential of the redox couple with respect to the flat band potential (attainable open-circuit voltage). On the basis of these data, the methanol/Fe(CN)6 system (transparent below 2.6 eV) was chosen as providing complete stabilization of CdSe. Results were presented for cells of the type... 

The polarization equation describes polarization as a fnnction of current density. In the case of concentration polarization, the form of the polarization eqnation is nnre-lated to the natnre of reaction or electrode. In the case of activation polarization, the parameters of the polarization eqnations depend decisively on the natnre of the reaction. At identical values of current density and otherwise identical conditions, the values of polarization for different reactions will vary within wide limits, from less than 1 mV to more than 2 or 3 V. However, these equations still have common features. A relatively simple set of equations is obtained for simple redox reactions of the type... 

Reactions in batteries are chemical reactions between an oxidizer and a reducer. In reactions of this type, the reducer being oxidized releases electrons while the oxidizer being reduced accepts electrons. An example of such a redox reaction is the reaction between silver oxide (the oxidizer) and metallic zinc (the reducer) ... 

In their first paper on noise, Tyagai and Luk yanchikova (1967) considered a very simple redox reaction of the type of O + ze = R . According to these authors, when the reaction rate is limited by a slow-discharge step, the mean-square amplitude of the noise is given by... 

Corrosion, a type of redox reaction, can cause cars and buildings to rust. Rusting happens when iron is oxidized by oxygen in the air. This process happens when —... 

Redox reaction, ion activities and ion concentrations. In order to express the relationship between cell type 2.40 and the ion concentrations, we can write eqn. 2.38 as follows... 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

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