Bases conjugate acid, dissociation constants

Identify each of the following terms (a) hydronium ion, (b) Br0nsted-Lowry theory, (c) proton (Brpnsted sense), (d) acid (Brpnsted sense), (e) base (Brpnsted sense), (f) conjugate, (g) strong acid or base, Qi) acid dissociation constant, i) base dissociation constant, (/) autoionization, k) pH, and (Z) K. ... 

Arrhenius concept Bronsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant Section 14.2 strong acid weak acid diprotic acid oxyacids organic acids carboxyl group monoprotic acids amphoteric substance autoionization... 

Arrhenius concept Br0nsted-Lowry model hydronium ion conjugate base conjugate acid conjugate acid-base pair acid dissociation constant... 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

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... 

The p/<, of a base is actually that of its conjugate acid. As the numeric value of the dissociation constant increases (i.e., pKa decreases), the acid strength increases. Conversely, as the acid dissociation constant of a base (that of its conjugate acid) increases, the strength of the base decreases. For a more accurate definition of dissociation constants, each concentration term must be replaced by thermodynamic activity. In dilute solutions, concentration of each species is taken to be equal to activity. Activity-based dissociation constants are true equilibrium constants and depend only on temperature. Dissociation constants measured by spectroscopy are concentration dissociation constants." Most piCa values in the pharmaceutical literature are measured by ignoring activity effects and therefore are actually concentration dissociation constants or apparent dissociation constants. It is customary to report dissociation constant values at 25°C. 

Biguanides usually behave as mono- and di-acid bases, combining with H+ ions to form the conjugate acid, but may also act as acids. The acid dissociation constants of many biguanides have been measured, and are collected in Tables 1 and 2. 

X10 . The molarity in this problem tells you that the concentration of benzoic acid is 5.0 X10 . You also know that the concentration of benzoic acid s conjugate base is the Scime as the given H+ concentration. All that remains is to write an equation for the acid dissociation constant and plug in these concentrations. 

It is possible to compare the strengths of weak acids by the values of their acid dissociation constants Ka. Figure 3.1 shows the titration curves for acids (HA or BH+) of different Ka values. The ordinate shows poH, which is defined by paH = -loga(SI I)). paH corresponds to the pH in aqueous solutions (see Section 3.2). The poH of non-aqueous solutions can be measured with a glass pH electrode or some other pH sensors (see Sections 3.2.1 and 6.2). For the mixture of a weak acid A and its conjugate base B, poH can be expressed by the Henderson-Hassel-balch equation ... 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

If a solute is an acid or base, its charge changes as the pH is changed. Usually, a neutral species is more soluble in an organic solvent and a charged species is more soluble in aqueous solution. Consider a basic amine whose neutral form, B, has partition coefficient A" between aqueous phase 1 and organic phase 2. Suppose that the conjugate acid, BH+, is soluble only in aqueous phase I. Let s denote its acid dissociation constant as Kir The distribution coefficient, D, is defined as... 

In many reference works, it is customary to express the strengths of organic bases not as Kb values but as the acid-dissociation constants, Ka (or pica s) for the corresponding conjugate acids. These Ka values are then the acid constants of the corresponding ammonium ions in aqueous solution (Equation 23-4) ... 

For any conjugate acid-base pair, the product of the acid-dissociation constant for the acid and the base-dissociation constant for the base always equals the ion-product constant for water ... 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMe3 > NHMe2 > NH2Me > NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 3.4. Note it is important to specify temperature, solvent (usually water) and solution ionic strength, 1 Basicity constants are related to the acid dissociation constants (/Q of the base s conjugate acid via the dissociation constant of water, K = 10 14 at 25 °C. Thus Kbx K = Kw. 

The acid-dissociation constant, Ka, is the equilibrium constant for the ionization of a weak acid to a hydrogen ion and its conjugate base ... 

As a preliminary to a discussion of kinetics of reactions in aqueous mixtures, it is interesting to review briefly the behaviour of equilibrium quantities as a function of co-solvent mole fraction. Interpretation of the data is necessarily complex because, for example, in the case of acid dissociation constants, the quantity 5mAXie represents the result of the individual variations of the partial molar quantities for acid, conjugate base and hydrogen ion. Nevertheless patterns of behaviour are observed which demonstrate the impact of co-solvent on water structure and on solute properties along the lines discussed in the previous section. 

Acid dissociation constant (Ka) the equilibrium constant for a reaction in which a proton is removed from an acid by H20 to form the conjugate base and H30+. (7.1)... 

The equilibrium constants to be determined in this example are protonation constants, introduced in Chapter 20. The protonation constant of a base L is the reciprocal of the acid dissociation constant Kg for the corresponding conjugate acid HL. For a polyprotic acid, the general expression for the protonation constant is given by equation 22-5. 

The perturbation of the acid dissociation constant of an amino acid residue as a consequence of its environment within a protein represents another mechanism for enhancing its reactivity relative to a free amino acid in solution. Most amino acid residues react with their respective modification reagents in their unprotonated form instead of in their conjugate acid form. Eq. (4.10) describes the pH dependence of the simple bimolecular reaction (eq. 4.9) of the free base form of nucleophilic amino acid side chain with a non-ionizable modification reagent where is the acid dissociation constant and Aj is the total concentration of amino acid. 

Dissociation Constants for Conjugate Acid/Base Pairs Consider the base dissociation-constant expression for ammonia and the acid dissociation-constant expression for its conjugate acid, ammonium ion ... 

This relationship is general for all conjugate acid/base pairs. Many compilations of equilibrium-constant data list only acid dissociation constants, since it is so easy to calculate basic dissociation constants by using Equation 9-14. For example, in Appendix 3, we find no data on the basic dissociation of ammonia (nor for any other bases). Instead, we find the acid dissociation constant for the conjugate acid, ammonium ion. That is,... 

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