Acid-base and oxidation-reduction

Chapter 16 considers a broad but closely related set of chemical processes that control the acid-base and oxidation-reduction (or redox) balances of the planet. It will be seen that both sets of processes depend greatly on... 

The Acid-Base and Oxidation-Reduction Balances of the Earth... 

It is convenient to divide organic chemical reactions between acid-base and oxidation-reduction reactions as in inorganic chemistry. In acid-base reactions the oxidation states of carbon do not change, e.g. in hydrolysis, where reaction is, for example,... 

The many functions of hydrogen then allow it to be involved in many organic and inorganic acid/base and oxidation/reduction reactions. The danger is that its general role will allow its presence to be taken for granted, though the very presence of water (H20) is the basis of life. 

Clark, W. M. 1934. The acid-base and oxidation-reduction equilibria of milk. In Fundamentals of Dairy Science, 2nd ed. Assoc, of Rogers Reinhold Pulishing Corp, New York, pp. 137-154. 

One can draw a useful analogy between acid-base and oxidation-reduction reactions. Both involve the transfer of a species from a source, the donor, to a sink, the acceptor. The source and sink nomenclature implies that the tendency of the proton (in the case of acids) or of the electron (for reducing agents) to undergo transfer is proportional to the fall in free energy. From the relation AG° = RTIn Ka. you can see that the acid dissociation constant is a measure of the fall in free energy of the proton when it is transferred from a donor HA to the solvent H2O, which represents the reference (zero) free energy level of the proton in aqueous solution. 

Water is the most abundant (and important, besides oxygen) substance on Earth. It is found in large amounts in cells and blood. Water is an excellent solvent and has a high boiling point, high surface tension, high heat vaporization, and low vapor pressure. Three key types of reactions occur in water precipitation, acid-base, and oxidation reduction. 

Like other compounds, organic molecules undergo acid-base and oxidation-reduction reactions, as discussed in Chapters 2 and 4. Organic molecules also undergo substitution, elimination, and addition reactions. 

Many parallels can be drawn between acid-base and oxidation-reduction (redox) reactions. Instead of proton interchange and an acid-base conjugate pair, in redox reactions we have electron interchange and a redox conjugate pair... 

The endpoint may be detected by addition of colored indicators, provided the indicator itself is not electroactive. Potentiometric and spectrophotometric indication is used in acid-base and oxidation-reduction titrations. Amperometric procedures are applicable to oxidation-reduction and ion-combination reactions especially for dilute solutions. 

Relation between acid-base and oxidation-reduction behaviour... 

Chemical reactions are frequently carried out in solution, and their description requires modifications to the rules of stoichiometry described in Chapter 2. We illustrate these modified rules by the important analytical techniques of titration in acid-base and oxidation-reduction reactions. 

Recognize the various classes of chemical reactions precipitation, reactions with oxygen, acid-base, and oxidation-reduction. 

Now a number of additional chemical species are considered. The mass continuity of each chemical species is expressed by the reactive transport equation. In addition to the usual homogeneous reactions occurring in the liquid phase (aqueous complex formation, acid/base and oxidation/reduction), heterogeneous reactions that exchange matter between the liquid and the solid... 

Reactive transport equations In the formulation proposed by Guimaraes (2002), several dissolved chemical species (solutes) are transported in the water phase through the porous medium and, simultaneously, they react among themselves (homogeneous reactions) and, also, with species present in the solid phase (heterogeneous reactions). The homogeneous reactions include aqueous complex formation, acid/base and oxidation/reduction. The relevant heterogeneous reaction for the case presented in this paper refers to dissolution and precipitation of minerals. 

Since potentiometric titrations are an old and well-known technique, particularly in regard to acid-base and oxidation-reduction titrations, only a few selected examples will be presented here. For more detailed treatments, the student is urged to consult the bibliography at the end of the chapter. It will be assumed that the student is already familiar with titration curves and their calculation from ionic equilibria and other pertinent data. 

These competencies involve students in understanding chemical concepts such as acid/base and oxidation/reduction reactions, the production and handling of gases, separation and purification techniques, additives and preservatives, fermentation processes, and the use of temperature as control mechanisms for chemical reactions. 

It is now clear that complexes of transition metals in low oxidation states can act as bases. Proton NMR spectroscopy has been used to survey the protonation of a variety of transition metal complexes (54, 55). The entire idea of oxidative addition is based on the conceptual removal of electrons from such a basic metal center. The relationship between acid-base and oxidation-reduction reactions is particularly evident in cases where a proton becomes attached to a basic metal nucleus causing a two-unit increase in the formal oxidation state of the metal. Once coordinated, the proton is considered a hydride. Indeed the hydrogen often has hydridic character, reacting with another proton to yield hydrogen gas and the metal with an increase of two charge units. 

In Sections 4.2-4.4 we will study three types of reactions in the aqueous medium (precipitation, acid-base, and oxidation-reduction) that are of great importance to industrial, environmental, and biological processes. They also play a role in our daily experience. 

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