Acid-base Equilibria in Aqueous Solution

Acid-base equilibria in aqueous solutions are a source of enormous possibilities in terms of altered (increased or decreased) reactivity for practical purposes or as a tool in the mechanistic toolbox. The same is true for coordination and ion-pairing equilibria involving typically cationic metal complexes and anions, although oppositely charged reactants can also be involved. 

In Chapter 8, we learned the Arrhenius definition of acids and bases—that an acid is a snbstance that can increase the concentration of ions in water and a base is a snbstance that can increase the concentration of OH ions in water. In Chapter 18, we learned about equilibrium systems. This chapter extends both of these concepts in discussing acid-base equilibria in aqueous solutions, which are extremely important to biological as well as chemical processes. 

A great many reactions are carried out in a convenient solvent for reactants and products. Dissolved reactants can be rapidly mixed, and the reaction process is easily handled. Water is a specially favored solvent because its polar structure allows a broad range of polar and ionic species to be dissolved. Water itself is partially ionized in solution, liberating and OH ions that can participate in reactions with the dissolved species. This leads to the important subject of acid-base equilibria in aqueous solutions (see Chapter 15), which is based on the equilibrium principles developed in this chapter. We limit the discussion in this subsection to cases in which the solvent does not participate in the reaction. 

The lower the value of this constant, the larger the deferences in acidity indices (pH) between the standard solutions of strong acids and bases, that results in a wider acid-base range for the solvent. This refers not only to the acid-base equilibria in aqueous solutions but also applies to any donor-acceptor interaction in molecular solvents which are prone to heterolytic dissociation with the formation of acidic and basic particles, as provided by an appropriate definition of acids and bases. It follows from equations (1.1.3) and (1.1.4) that the Arrhenius definition can only be used for the description of acid-base interactions in aqueous solutions, since the reaction between the acid of solvent and the base of solvent can result in the formation only of the solvent molecules. In the case considered, this solvent is water. 

In dealing with acid-base equilibria in aqueous solution, three equilibrium constants are of special significance ... 

C.E. Housecroft and E.C. Constable (2002) Chemistry, 2nd edn, Prentice Hall, Harlow - Chapter 15 includes acid-base equilibria in aqueous solutions, and reviews calculations involving pH, and pAi],. 

G. P. Ford and B. Wang, THEOCHEM, 102,49 (1993). Energetics of Acid-Base Equilibria in Aqueous Solution. MNDO, AMI and PM3 Results for Compounds with Functionalities Analogous to the Nucleic Acids. 

What are the Important acid-base equilibria in an aqueous solution of pyridinium chloride (C5 H5 NHCl) What are the values of their equilibrium constants ... 

In conclusion, the peculiarities of hydrogenation of olefins in aqueous solutions show that by shifting acid-base equilibria the aqueous environment may have important effects on catalysis through changing the molecular state ofthe substrate or the catalyst or both. 

Among amphiprotic solvents of high permittivities, there are water-like neutral solvents (e.g. methanol and ethanol), more acidic protogenic solvents (e.g. formic acid), and more basic protophilic solvents (e.g. 2-aminoethanol). There are also amphiprotic mixed solvents, such as mixtures of water and alcohols and water and 1,4-dioxane. The acid-base equilibria in amphiprotic solvents of high permittivity can be treated by methods similar to those in aqueous solutions. If the solvent is expressed by SH, the acid HA or BH+ will dissociate as follows ... 

All the reactions discussed in the previous section could be described as acid/base phenomena, defining acids and bases quite liberally. The importance of ionic equilibria in aqueous solution was recognised in the 1880s by Arrhenius, who proposed that acids were sources of H+(aq) while bases were sources of OH-(aq), and it was soon realised that this definition was closely related to the self-dissociation of water ... 

E. Chufan, F. Suvire, R. Enriz, and J. C. Pedregosa, A potentiometric and spec-trophotometric study on acid-base equilibria in ethanol-aqueous solution of acetazolamide and related compounds, Talanta 49 (1999), 859-868. 

Histamine, a biologically important imidazole derivative, is in multiple acid-base and tautomeric equilibria in aqueous solution. Tautomeric and basic center preferences for isolated neutral and monoprotonated histamine have been studied by ab initio calculations (HF, MP2, and DFT), and the polarizable continuum model (PCM) has been... 

The solubility of carbon dioxide in aqueous or nonaqueous media depends on three primary factors temperature, partial pressure of carbon dioxide, and acid-base reactions in the solution. Accurate data for solubility and equilibria are well-known for aqueous solutions (1-3), but not for nonaqueous solutions. Neither the standard compilations of equilibrium constants (1,2) nor recent reviews on nonaqueous electrolytes ( ) cover what appears to be a large and poorly indexed literature. 

SECTION 17.1 In this chapter we have considered several types of important equilibria that occur in aqueous solution. Our primary emphasis has been on acid base equilibria in solutions containing two or more solutes and on solubility equilibria. The dissociation of a weak acid or weak base is repressed by the presence of a strong electrolyte that provides an ion common to the equilibrium. This phenomenon is called the common-ion effect. 

There are several major kinds of equilibria in aqueous solution. The one under consideration here is acid-base equilibrium, as exemplified by the ionization of acetic acid, HAc,... 

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