Acid-base equilibria types

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

A frequently encountered pH-rate profile exhibits a bell-like shape or hump, with two inflection points. This graphical feature is essentially two sigmoid curves back-to-back. By analogy with the earlier analysis of the sigmoid pH-rate curve, where the shape was ascribed to an acid-base equilibrium of the substrate, we find that the bell-shaped curve can usually be accounted for in terms of two acid-base dissociations of the substrate. The substrate can be regarded, for this analysis, as a dibasic acid H2S, where the charge type is irrelevant we take the neutral molecule as an example. The acid dissociation constants are... 

C18-0040. List all the types of calculations described in Chapter 18 in which acid-base equilibrium expressions play a role. 

Our goal in this chapter is to help you continue learning about acid-base equilibrium systems and, in particular, buffers and titrations. If you are a little unsure about equilibria and especially weak acid-base equilibria, review Chapters 14 and 15. You will also learn to apply the basic concepts of equilibria to solubility and complex ions. Two things to remember (1) The basic concepts of equilibria apply to all the various types of equilibria, and (2) Practice, Practice, Practice. 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

Table 4-1. Solvent influence on the acid/base equilibrium HA + -l-SH charge types f...

In media of high dielectric constant, the assumption that /a and /b depend only on charge type is plausible since A and B differ only by a proton and similar specific effects would not change the value of the ratio /a//b. In practice, since the proton would be solvated, we deal with the transfer of a proton from one acid to the base of another acid and have a double acid-base equilibrium... 

The rapid initial reduction of the type 1 copper is very similar to that reported for tree laccase (49). The amplitude of this reduction increases with substrate concentration to a maximum value of approximately 50% of total absorbance change at 10°C. In laccase this effect is explained by the existence of two forms of the enzyme in an acid-base equilibrium. The active form allows rapid type 1 to type 3 electron transfer, whereas in the inactive form this process is inhibited. At higher substrate concentrations, the reduction of the type 1 copper is faster than the interconversion of inactive enzyme into its active form, leading to an increase in initial phase amplitude. Turnover-induced activation of ascorbate oxidase (67) could also be explained in terms of displacement of this inactive-active equilibrium. 

The second important solvent effect on Lewis acid-Lewis base equiUbria concerns the interactions with the Lewis base. Since water is also a good electron-pair acccplor, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intennolecular association of a solvent affects the Lewis acid-base equilibrium-" -. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

We enthusiastically endorse models as an aid to this type of research. Our model of acid-base equilibrium was essential for the design and operation of... 

In the nucleophilic addition reactions of amines to substituted aromatic aldehydes where acid catalysis is required, the use of EAN seems to be convenient. The EAN can take part in an acid-base equilibrium with the aromatic aldehydes substituted by electron-withdrawing groups. The imine products from the selected aldehydes could be obtained, confirming the dual behaviour of EAN as Brbnsted acid and potential nucleophile in these type of processes (Fig. 13.7). 

Metal ions of +3 oxidation state have a high surface charge density and consequently solvent molecules are tightly held. Resulting from this is the strong tendency for hydrolysis to take place according to an acid—base equilibrium of the type... 

Add-base indicators are generally weak protolytes that change color in solution according to the pH. The acid-base equilibrium of a weak acid type of indicator (HI) in water can be represented as... 

Acid-base indicators for titrations in nonaqueous solvents are normally weak protolytes. For dissociation model I as given in Table 1, the acid-base equilibrium of a weak acid type of indicator in the pure solvent can be represented in water by the following reaction ... 

An interesting extension of this type of reaction is the transalkylation reaction between 2-alkoxy-l-methylbenzimidazole (35) and benzenethiol. The kinetics of this reaction indicate a rapid acid-base equilibrium, followed by an attack at the ether saturated carbon by the PhS ... 

Several types of reactions are commonly used in analytical procedures, either in preparing samples for analysis or during the analysis itself. The most important of these are precipitation reactions, acid-base reactions, complexation reactions, and oxidation-reduction reactions. In this section we review these reactions and their equilibrium constant expressions. 

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. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

We see in Table 11-IV that the equilibrium view of acid strengths suggests that we regard water itself as a weak acid. It can release hydrogen ions and the extent to which it does so is indicated in its equilibrium constant, just as for the other acids. We shall see that this type of comparison, stimulated by our equilibrium considerations, leads us to a valuable generalization of the acid-base concept. 

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