Acid-base equilibria weak acids

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... 

C16-0105. Write the equilibrium reaction and equilibrium constant expression for each of the following processes (a) Trimethylamine, (CH3)3 N, a weak base, is added to water, (b) Hydrofluoric acid, HF, a weak acid, is added to water, (c) Solid calcium sulfate, CaSOq, a sparingly soluble salt, is added to water. 

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. 

Before examining the equilibrium behavior of aqueous solutions of weak bases, let s look at the behavior of water itself. In the initial discussion of acid—base equilibrium above, we showed water acting both as an acid (proton donor when put with a base) and a base (proton acceptor when put with an acid). Water is amphoteric, it will act as either an acid or a base, depending on whether the other species is a base or acid. But in pure water the same amphoteric nature is noted. In pure water a very small amount of proton transfer is taking place ... 

Dilute acids or bases have no noticeable effect on the composition of sugars in aqueous solution at high concentration, however, they alter it considerably. What is then observed is, in fact, not the equilibrium of the sugars themselves but that of their cations and anions, respectively, sugars being weak bases and weak acids. Thus, the a yj-pyranose ratio for D-glucose in 2,5 M DC1 was found90 to be 45 55 the same ratio was obtained when the solvent was formic acid. 

Many water-insoluble drugs are either weak bases or weak acids. There exists an equilibrium of ionized and unionized species for a weak base or a weak acid in an aqueous solution. The pH car affect the equilibrium between ionized and nonionized solute species, and consequently can have an effect on the capacity of micellar solubility. An example of this is the decreased uptake of 4-chlorobenzoic acid by polysorbate 80 micelles observed when the pH is changed from 3 to 4.4 (Collette and Koo, 1975). Another example regarding the effect of SDS micelles orKthefp atenolol, nadolol, midazolam, and nitrazepam is provided by Castro et al. (1998). Apparent acidity constants (p of the drugs were determined potentiometrically or spectrophotometrically in... 

As a rule, hydrogen ion is involved not only in the pH-dependency of the reaction term (Thiele modulus) but also as the actively participating species involved in the acid-base equilibrium of all the substrates, reaction intermediates, products, and even the gel matrix. Furthermore, enzymatic reactions are always carried out in the presence of the mobile buffer. By mobile we mean a weak acid or a weak base that can move in and out of the reaction layer, as opposed to the fixed buffer represented by the gel (and by the protein) itself. Thus, we have to include the normalized diffusion-reaction equations for hydrogen ion and for the buffer. 

An equation for the observed capacity factor (kobs) can be derived if it is assumed that the acid-base equilibrium kinetics are fast enough to be considered as instantaneous. This corresponds to the observation of a single sharp peak for the ionizable species in the chromatogram. In that case it may be assumed [340,342,343] that for the weak acid HA... 

Having a conceptual understanding of the effect is a good starting point, but we still need to be able to understand the quantitative relationships between the different components in the equilibrium mixture. In this section, we will see how to deal with the common-ion effect in acid-base equilibrium problems. You will find that these problems are very similar to the weak acid problems earlier in the chapter. 

If the conjugate-base-dissociation mechanism is the correct one, the lower substitution rates observed with nitrite, acetate, and azide may be considered to arise because these weakly basic anions are less effective than methoxide in generating conjugate base CB in the preliminary acid base equilibrium. 

This equation could be solved using the quadratic formula, as we did in Chapter 14. A quicker way is based on the fact that acetic acid is a weak acid and therefore only a small fraction is ionized at equilibrium. Therefore it is reasonable to assume that y is small relative to 1.000 (that is, the final equilibrium concentration of CH3COOH is close to its initial concentration). This gives... 

Outline the procedure for the exact treatment of acid-base equilibrium and use it to find the pH of a very dilute solution of a weak acid or base (Section 15.8, Problems 69-70). 

Apply acid-base equilibrium concepts to salts of strong bases and weak acids... 

We recognize that both NaCH3COO and NaCN are salts of strong bases and weak acids. The anions in such salts hydrolyze to give basic solutions. As we have done before, we first write the appropriate chemical equation and equihbrium constant expression. Then we complete the reaction summary, substimte the algebraic representations of equilibrium concentrations into the equilibrium constant expression, and solve for the unknown concentration(s). 

Soluble carboxylic acids behave as weak acids they dissociate only slightly in water to form an equilibrium mixture with the carboxylate ion. The equilibrium concentrations of the carboxylic acid and the carboxylate ion depend on pH. At low pH, the acid form predominates, and at pH 7.4 (the pH of cellular fluids) and above, the carboxylate ion predominates. Carboxylic acids react with bases to produce carboxylate salts and water. Objective 3 (Section 5.3), Exercise 5.26... 

Mahltig B, Jerome R, Stamm M (2003) The influence of an acid-base-equilibrium on the adsorption behaviour of a weak polyampholyte. J Polym Res 10 219-223... 

The final area that can be discussed involves the kinetics and mechanism of reactions of Grignard reagents with ketones, nitriles, Schiff bases, and weak acids. This area is intimately tied to the problem of the structure of the organomagnesium reagent itself and this in turn centers around the values of the equilibrium constants for the processes... 

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