Acetic acid ionization constant

Thus for substituted acetic acids in water at 25°, the maximum error to be expected is 0.015 a units. As S is probably about 0.1 and u — vch2x will generally be less than 0.30, the error in values obtained from acetic acid ionization constants should be negligible for most substituents. 

Acetic acid, HC2H302, which is represented as HA, has an acid ionization constant Ka of 1.74 x 10 5. 

Acetic acid, HC2H3O2, which is represented as HA, has an acid ionization constant of 1.74 x 10 (a) Calculate the hydrogen ion concentration, [H ], in a 0.50 molar solution of acetic acid. 

The action of a buffer system can be understood through the use of a practical example. Consider acetic acid, for which = 1-82 x 10 (pAT = 4.74). The following pH values can be calculated (for solutions having a total acetate content of 1.0 M) using its acid ionization constant expression ... 

An important relationship between the acid ionization constant and the ionization constant of its conjugate base can be derived as follows, using acetic acid as an example ... 

Although D2O chemically resembles H2O in most respects, it is a toxic substance. The reason is that deuterium is heavier than hydrogen thus, its compounds often react more slowly than those of the lighter isotope. Regular drinking of D2O instead of H2O could prove fatal because of the slower rate of transfer of compared with that of H+ in the acid-base reactions involved in enzyme catalysis. This kinetic isotope effect is also manifest in acid ionization constants. For example, the ionization constant of acetic acid... 

An essential part of Arrhenius s theory of ionization was the application of the law of mass-action to describe the state of ionic equilibrium. Thus, acetic acid (CH3COOH) is a weak acid which ionizes in water to give some hydrogen ions (H ) and some acetate anions (CHsCOO"). The product of the concentration of the ions (which is [H" "] [CHsGOO"]) always bears a fixed ratio to the concentration of the neutral molecules [CH3COOH]. This ratio is called the acidic ionization constant ( a) more simply the ionization constant. Thus ... 

Solution (a) The acid ionization constant for pyruvic acid should be somewhat greater than that of acetic acid because the carbonyl function on the o-carbon atom exerts an electron-withdrawing effect on the carboxylic acid group. In the C—O—H bond system the electrons are shifted from hydrogen, facilitating loss of the hydrogen as a proton. (Section 16.10)... 

For a strong acid, which ionizes completely in solution, the concentrations of ions are determined by the stoichiometry of the reaction from the initial concentration of acid. However, for a weak acid such as acetic acid, the concentrations of ions in solution are determined from the acid-ionization constant (also called the acid-dissociation constant), which is the equilibrium constant for the ionization of a weak acid. 

The acid ionization constant for a weak acid represents a small value. To determine the numerical value of the ionization constant for acetic acid at a specific temperature, the equilibrium concentrations of H3O+ ions, CH3C00 ions, and CH3COOH molecules must be known. The ionization of a molecule of CH3COOH in water yields one H3O+ ion and one CH3C00 ion. These concentrations can therefore be found experimentally by measuring the pH of the solution. 

The pH of the acetic acid solution is higher (it is less acidic) because acetic acid only partially ionizes. Calculating the [H30 ] formed by the ionization of a weak acid requires solving an equilibrium problem similar to those in Chapter 14. Consider, for example, a 0.10 M solution of the generic weak acid HA with an acid ionization constant K. Since we can ignore the contribution of the autoionization of water, we only have to determine the concentration of H30 formed by the following equilibrium ... 

In a series of organic acids of similar type, not much tendency exists for one acid to be more reactive than another. For example, in the replacement of stearic acid in methyl stearate by acetic acid, the equilibrium constant is 1.0. However, acidolysis in formic acid is usually much faster than in acetic acid, due to higher acidity and better ionizing properties of the former (115). Branched-chain acids, and some aromatic acids, especially stericaHy hindered acids such as ortho-substituted benzoic acids, would be expected to be less active in replacing other acids. Mixtures of esters are obtained when acidolysis is carried out without forcing the replacement to completion by removing one of the products. The acidolysis equilibrium and mechanism are discussed in detail in Reference 115. 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

The term is also called an ionization constant because it states the extent to which a substance forms ions in water. The relatively low value of for acetic acid reveals that the un-ionized form, CH3COOH, predominates over H and CH3COO in aqueous solutions of acetic acid. Viewed another way, CH3COO, the acetate ion, has a high affinity for H. ... 

Unionized mercuric acetate is also a mercurating species, for the second-order rate coefficient for mercuration of benzene by mercuric acetate in acetic acid at 25 °C is 0.41 x 10"7. If mercuration took place via ionized acetate ion pairs HgOAc+OAc" for which AT, the equilibrium constant can be estimated at 2 x 10"8, then since the rate of mercuration by this ion pair will be approximately the same as by the acetoxymercury perchlorate ion pair for which k2 the second-order rate coefficient has been determined (above) as 0.37x10"3 at 25 °C, the observed second-order rate should be 2 x 10"8 x0.37 x 10"3 = 0.74xl0-11. This is so different from the rate actually observed that mercuration by the ion pair can be eliminated which leaves ionized mercurcy acetate as the only possible mercurating species439. 

All equilibrium constants in the present discussion are based on the concentration (not activity) scale. This is a perfectly acceptable thermodynamic scale, provided the ionic strength of the solvent medium is kept fked at a reference level (therefore, sufficiently higher than the concentration of the species assayed). This is known as the constant ionic medium thermodynamic state. Most modern results are determined at 25 °C in a 0.15 M KCl solution. If the ionic strength is changed, the ionization constant may be affected. For example, at 25 °C and 0.0 M ionic strength, the pXj of acetic acid is 4.76, but at ionic strength 0.15 M, the value is 4.55 [24]. 

In this way Kolthoff and Bruckenstein59 determined spectrophotometrically at 25° C for acid-base equilibria in glacial acetic acid the following ionization and dissociation constants of the bases ... 

The authors studied, as they call it, "acid-base equilibria in glacial acetic acid however, as they worked at various ratios of indicator-base concentration to HX or B concentration, we are in fact concerned with titration data. In this connection one should realize also that in solvents with low e the apparent strength of a Bronsted acid varies with the reference base used, and vice versa. Nevertheless, in HOAc the ionization constant predominates to such an extent that overall the picture of ionization vs. dissociation remains similar irrespective of the choice of reference see the data for I and B (Py) already given, and also those for HX, which the authors obtained at 25° C with I = p-naphthol-benzein (PNB) and /f B < 0.0042, i.e., for hydrochloric acid K C1 = 1.3 102, jjrfflci 3 9. IQ-6 an jjHC1 2.8 10 9 and for p-toluenesulphonic acid Kfm° = 3 7.102( K ms 4 0.10-6) Kmt = 7 3.10-9... 

Shedlovsky, T Kay, R. L., The ionization constant of acetic acid in water-methanol mixtures at 25°C from conductance measurements, J. Am. Chem. Soc. 60,151-155 (1956). 

Log-log plot of ionization constants of benzoic and phenyl-acetic acids in water at 25°. (From Physical Organic Chemistry by J. S. Hine. Copyright 1962. Used with permission of McGraw-Hill Book Company.)... 

The value of its ionization constant (pKa =5.01) approximates that of acetic acid. 

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