Dissociation constants, acetic acid

Problem 1. Wkat will be the pH value of a solution obtained by mixing 0.20 mole per litre sodium acetate with 0.15 mole per litre acetic acid Dissociation constant of acetic acid is 1.8 x 10. ... 

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

One could go on with examples such as the use of a shirt rather than sand reduce the silt content of drinking water or the use of a net to separate fish from their native waters. Rather than that perhaps we should rely on the definition of a chemical equilibrium and its presence or absence. Chemical equilibria are dynamic with only the illusion of static state. Acetic acid dissociates in water to acetate-ion and hydrated hydrogen ion. At any instant, however, there is an acid molecule formed by recombination of acid anion and a proton cation while another acid molecule dissociates. The equilibrium constant is based on a dynamic process. Ordinary filtration is not an equilibrium process nor is it the case of crystals plucked from under a microscope into a waiting vial. 

Co-catalysts other than water. Trichloro- and monochloro-acetic acids, when used as cocatalysts, induced instantaneous polymerisation at -140°. With the following co-catalysts the rate of polymerisation at -78° decreased in the order acetic acid > nitroethane > nitromethane > phenol > water [75a]. Since this is also the sequence of the acid dissociation constants of these substances in water, it appears that the catalytic activity , as shown by the rate of polymerisation, is correlated with the acidity of the cocatalyst in aqueous solution. Flowever, there are two reasons for questioning the validity of this correlation. 

You can determine the value of for a particular acid by measuring the pH of a solution. In the following investigation, you will add sodium hydroxide to acetic acid, which is a weak acid. (See Figure 8.8.) By graphing pH against the volume of sodium hydroxide that you added, you will be able to calculate the concentration of the acetic acid. Then you will be able to determine the acid dissociation constant, Ka, for this acid. 

Hydrogen bond formation between dissimilar molecules is an example of adduct formation, since the hydrogen atom that is bonded to an electronegative atom, such as oxygen or nitrogen, is a typical acceptor atom. The ability of molecules to donate a hydrogen bond is measured by their Taft-Kamlet solvatochromic parameter, a, (or a . for the monomer of self-associating solutes) (see Table 2.3). This is also a measure of their acidity (in the Lewis sense, see later, or the Brpnsted sense, if pro tic). Acetic acid, for instance, has a = 1.12, compared with 0.61 for phenol. However, this parameter is not necessarily correlated with the acid dissociation constant in aqueous solutions. 

The larger the acid dissociation constant, the stronger is the acid. Hydrochloric acid has an acid dissociation constant of 10, whereas acetic acid has an acid dissociation constant of only 1.74 x 10 . For convenience, the strength of an acid is generally indicated hy its pA a value rather than its A a value The of hydrochloric acid, strong acid, is —7, and the pA a of acetic acid, much weaker acid, is 4.76. 

In acetic acid it is possible to measure separately the equilibrium constant of proton transfer to form an ion pair and the constant for dissociation of ion pairs to form free ions. [I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc., 78, I (1956) S. Bruckenstein and I. M. Kolthoff, J. Amer. Chem. Soc., 78, 10 (1956)]. G. W. Geska and E. Grunwald, J. Amer. Chem. Soc., 89, 1371, 1377 (1967) applied this technique to a number of substituted anilines and concluded that the equilibrium constant of the ionization step, rather than the overall acid dissociation constant, is the quantity that should be considered in discussions of effects of structural changes on acidity. 

The characteristics of the addition of HBr to double bonds are similar to those of the addition of HC1. However, in acetic acid 1,2-dimethylcyclohexene gives more anti addition if HBr is the addend.25 Also, as Figure 7.1 shows, when HX is added to a double bond in acetic acid, the ratio of alkyl halide to alkyl acetate increases sharply as the concentration of HBr is increased but is almost independent of the concentration of HC1. Fahey suggests that the much larger acid dissociation constant of HBr (AXd = 103-104) is responsible for both of these facts. Hydrobromic acid acts as a better halide source, and Ad3 addition is favored.26... 

PROBLEM 18.14 What is the value of E° for a redox reaction involving the transfer of 2 mol of electrons if its equilibrium constant is 1.8 X 10-5 (the value of the acid-dissociation constant Ka for acetic acid) ... 

Fig. 3.2 Acid dissociation constant for acetic acid as a function of temperature (see Problem 3.4).

The plot in Fig. 3.2 of the acid dissociation constant for acetic acid was calculated using equation 3.2-21 and the values of standard thermodynamic properties tabulated by Edsall and Wyman (1958). When equation 3.2-21 is not satisfactory, empirical functions representing ArC[ as a function of temperature can be used. Clark and Glew (1966) used Taylor series expansions of the enthalpy and the heat capacity to show the form that extensions of equation 3.2-21 should take up to terms in d3ArCp/dT3. 

By combining equations (2.15), (2.16), and (2.18), a distribution diagram (Figure 2.10) for acetic acid can be prepared given that the acid dissociation constant is 1.8 x 10 5 with an assumed concentration of 0.01 M. The vertical line in Figure 2.10, positioned at x 4.74, is a reminder that when the pH of the solution is equal to the pKa of the analyte, the a value is 0.5, which signifies that the concentration of HA is equal to the concentration of A. The distribution diagram can be used to determine the fraction of ionized or nonionized acetic acid at any selected pH. 

The process of acid dissociation can be thought of as comprising two steps, namely ionisation and dissociation of the ion-pair into separate products. The first step may be studied in isolation if a solvent of low dielectric constant is employed, as the second process then occurs to a negligible extent. For example, the rates of proton exchange in acetic acid (dielectric constant, 6.27) are given [73] in Table 15. The measured rate,... 

These Oi values of Taft and Lewis were used as a basis set by Charton [Charton, 1963 Charton, 1964] with the aim of obtaining a large number of inductive constants. Acid dissociation constants of substituted acetic acids (XCH2COOH) in water were correlated with Oi constants of the basis set at temperatures from 5° to 50°C in terms of the equation ... 

Curve C is a plot of K (XIO ), the concentration quotient for the equilibrium involving the dissociation of acetic acid, as a function of electrolyte concentration. Here again, the ordinate function approaches a limiting value A, which is the thermodynamic acid dissociation constant for acetic acid. 

Illustrations. The simplest example is the case of ammonium acetate. The dissociation constants of both acetic acid and ammonia equal 10 - . From (42) it follows that in a solution of ammonium acetate... 

Figure 1.18 Transition states for the acetate-catalysed, acetic acid-catalysed and water reactions in the mutarotation of tetramethyl glucose. The additional waters for the acetate and acetic acid reactions are drawn to indicate solvation, rather than a change in bonding that would alter fractionation factors. Isotope effects are taken from ref 34 and fractionation factors calculated from their data using 1.0, rather than 1.23, for the fractionation factor of the anomeric hydroxyl. The latter was based on an implausible, equilibrium isotope effect of 4.1 on the acid dissociation constant of tetramethyl glucose. ...

To assist computation of the degree of dissociation of the Sephadex and IRC-50 at the lower pH values, the activity coefficient of hydrogen ion in the presence of the polystyrene sulfonate was determined by examining the potentiometric property of 0.001M reagent-grade acetic acid from Fisher in the presence of 0.03, 0.06, and 0.09M NaPSS (mol wt 500,000). At each polystyrene sulfonate concentration a unique activity coefficient (0.300, 0.290, and 0.290, respectively, in 0.03, 0.06, and 0.09M NaPSS), essential for the resolution of the acid dissociation constant, was resolved by this approach. 

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.)...

Let us return to the ether/aqueous immiscible distribution equilibrium model introduced earlier, refer to Figure 3.1. What if the aqueous solution, prior to adding any ether, was made alkaline by the addition of NaOH We know that the chloride ion concentration in the original aqueous solution would not change, but what about the HOAc We also know that acetic acid is a weak acid and undergoes dissociation to hydronium ions and acetate ions. The extent of this dissociation is governed by the acid dissociation constant, K. The double arrow notation is used in the following reaction to show that prior to the addition of hydroxide ion to an aqueous solution that contains dissolved acetic acid, the ionic equilibrium is already established. 

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