B-2 Acid Dissociation Constants

Acid Formula Conjugate base Dissociation constant p K 

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Consider a neutral base B of such strength that it can be protonated in dilute aqueous solution in the acidic range, say pH 1-2. In the conventional manner the acid dissociation constant /ibh + is defined. 

Table 2. Acid dissociation pK values, 1 = 0.10 M(NaCl), relating to the active site protonation of different plastocyanins, PCu(I), as determined by (a) proton NMR (b) the variation of rate constants (25 °C) with pH for the [FelCN) ] oxidation of PCu(I), 1 = 0.10 M(NaCl), and (c) similar experiments with [Co(phen)3] " as oxidant. The latter is an apparent value only, and is believed to be composite due to reaction occurring at the remote site...

In the sulfite pulp process, the digestion fluid is typically made by dissolving sulfur dioxide in water in the presence of limestone until the solution contains 7.0% by weight S02, three-quarters of which is in the form of bisulfite ion. Why is this done In support of your answer to this question, calculate the pH of (a) the digestion solution prepared as above, (b) a hypothetical 7% solution of S02 alone, and (c) an actual saturated solution of S02 alone (2.9% S02), if the first acid dissociation constant of sulfurous acid is 1.7 x 10-2, at ambient temperature and pressure. 

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... 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

Example 2.5 (a) Derive the two equations to calculate /m and /ml as functions of only [L] for a system where only one product (ML) of M and L is formed, (b) Select a monoprotic acid as an example and find its corresponding acid dissociation constant (Analytical Chemistry textbooks are normally good initial sources), (c) Plot the results for pL = 0 to pL = 14. Note that in this example, pL = pH. 

Whether or not A and B represent identical ligands, the products KaKc and KbKa must be equal. The relationship between Ka and Kb or between Kc and Kd depends on whether or not A and B are identical. With complete generality, it can be shown that the thermodynamic acid dissociation constants (K and 2) are related to the microscopic constants by the following expressions ... 

You can assign descriptive names to cells, such as Ka 1 and Ka2 to refer to the two acid dissociation constants of a diprotic acid. It is usually much easier to write and read formulas that contain descriptive names rather than cell addresses such as B 2 and C 2. Names can be used only to refer to individual absolute addresses. Names must start with a letter, may contain only letters, numbers, periods and underscores, and cannot be R, C, or possible cell addresses. Consequently, Cl and Ca 1 are not valid names, but Ca, caa3 and (in all current versions of Excel) Ka 1 are, since the rightmost column label is IV in future versions of Excel you might have to use Kaaland Kaa2 instead. 

Fig. 19.7. Relationship between acid dissociation constant pK and frequency (v) of the antisymmetric stretching vibration of salts of carboxylic acids in DjO solution. 1, Trifluoroacetic 2, trichloroacetic 3, dichloroacetic 4, cyanoacetic 5, chloroacetic 6, 2-phenoxypropionic 7, methoxyacetic 8, formic 9, glycollic 10, lactic 11, 3-hydroxybutyric 12, acetic 13, propionic 14,cyclohexane carboxylic 15, polyacrylic 16, malonic 17, phthalic 18, benzoic 19, salicylic and 20. polyguluronic. polygalacturonic, galacturonic, alginic, hyaluronic, and chondroitin sulfates A, B, and C. ( C) From Goulden and Scott, 1968 (O) from Chapman et at. 1964. The pK values are from references given in Goulden and Scott (1968). (Goulden and Scott, 1968.)...

Succinic acid (H2C4H5O4), which we will denote H2SUC, is a biologically relevant diprotic acid with the structure shown below. It is closely related to tartaric acid and malic acid (Figure 16.1). At 25 °C, the acid-dissociation constants for succinic acid are = 6.9 X 10 and Ff 2 = 2.5 X 10 . (a) Determine the pH of a 0.32 M solution of HjSuc at 25 °C, assuming that only the first dissociation is relevant, (b) Determine the molar concentration of Suc in the solution in part (a), (c) Is the assumption you made in part (a) justified by the result from part (b) (d) Will a solution of the salt NaHSuc be acidic, neutral, or basic ... 

Acid dissociation constants appear in Appendix B. Each compound is shown in its fully protonated form. Methylamine, for example, is shown as CH3NH3, which is really the methylammonium ion. The value of (2.33 X 10 given for methyl-amine is actually for the methylammonium ion. To find for methylamine, we write = KJK = (1.0 X 10 V(2.33 X 10 ) = 4.29 X 10 ... 

Lactic acid, CH3CH(OH)COOH, received its name because it is present in sour milk as a product of bacterial action. It is also responsible for the soreness in muscles after vigorous exercise, (a) The pfQ, of lactic acid is 3.85. Compare this with the value for propionic acid (CH3CH2COOH, pfC = 4.89), and explain the difference. (b) Calculate the lactate ion concentration in a 0.050 M solution of lactic acid, (c) When a solution of sodium lactate, (CH3CH(OH)COO)Na, is mixed with an aqueous copper(II) solution, it is possible to obtain a solid salt of copper(II) lactate as a blue-green hydrate, (CH3CH(0H)C00)2 Cu-a H20- Elemental analysis of the solid tells us that the solid is 22.9% Cu and 26.0% C by mass. What is the value for x in the formula for the hydrate (d) The acid-dissociation constant for the Cu (aq) ion is 1.0 X 10. Based on this value and the acid-dissociation constant of lactic add, predict whether a solution of copper(Il) lactate will he addic, basic, or neutral. Explain your answer. 

When the deprotonation occurs in a reversible step, leaving group departure is rate-limiting, k.i lc2, and Eq. 10.71 reduces to Eq. 10.72. Recall from Chapter 5 that the ratio of a base to its conjugate acid (here [B ] / [HB]) sets the pH of a buffer, and is equal to R.,/ [H, where is the acid dissociation constant of the conjugate acid of the base. We find that the rate of the elimination does not depend upon the concentration of the base, if the pH is kept constant. When the rate of a reaction that involves a base added to the solution depends only upon the pH, it is specific-base-catalyzed (see Section 9.3.2). Hence, many standard ElcB reactions are specific-base-catalyzed eliminations. 

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