Thermodynamic acidity constant

The expression of pKg values in terms of concentrations (equation 7.5) is not rigorously correct. Instead, the acidity of a solution at equilibrium should be defined by the activity a) of each species or by the product of the concentration of each species and its activity coefficient (y). Thus, the thermodynamic acidity constant, Kl, is defined as... 

In this experiment the equilibrium constant for the dissociation of bromocresol green is measured at several ionic strengths. Results are extrapolated to zero ionic strength to find the thermodynamic equilibrium constant. Equilibrium Constants for Calcium lodate Solubility and Iodic Acid Dissociation. In J. A. Bell, ed. Chemical Principles in Practice. Addison-Wesley Reading, MA, 1967. 

Partanen, J. I. Karki, M. H. Determination of the Thermodynamic Dissociation Constant of a Weak Acid by Potentiometric Acid-Base Titration, /. Chem. Educ. 1994,... 

Directions are provided in this experiment for determining the dissociation constant for a weak acid. Potentiometric titration data are analyzed by a modified Gran plot. The experiment is carried out at a variety of ionic strengths and the thermodynamic dissociation constant determined by extrapolating to zero ionic strength. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Using the thermodynamic data available in Appendix 2A, calculate the acidity constant of HF(aq). 

Fig. 3.14 An extrapolation graph for determination of the thermodynamic dissociation constant of acetic acid using Eq. (3.3.14)...

In writing the thermodynamic equilibrium constant, recall that neither pure solids (PbS(s) and S(s)) nor pure liquids (H20(1)) appear in the thermodynamic equilibrium constant expression. Note also that we have written H+(aq) here for brevity even though we understand that H30+(aq) is the acidic species in aqueous solution. 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

Thermodynamic Dissociation Constants for Alkylated Succinic Acids in 50% Aqueous Ethanol... 

If tautomerism occurs in dilute aqueous acid, then K and K l will be the thermodynamic acid ionization constants and (26) will hold thus... 

The negative sign indicates mobilization toward the anode. Hence, the thermodynamic dissociation constant for the weak acid is obtained from Eqs. (5) and (8) as follows ... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

ADCA 7-Amino-3-desacetoxycephalosporanic acid APA 6 Aminopenicillanic acid HPG D-p-Hydroxyphenylglycine HPGA D-p-Hydroxyphenylglycine amide Keq Concentration based equilibrium constant Kth Thermodynamic equilibrium constant... 

Before we go on to define the acidity constant of a given organic acid in water, we need to introduce a thermodynamic convention for scaling such constants. We do this relative to H30+ in that we define the dissociation of H30+ in water to have a standard free-energy change ArG° = 0, which means that the equilibrium constant of this reaction is equal to 1 ... 

Table 6-5 gives thermodynamic dissociation constants and values of AG 0 and AH 0 for a number of acids of interest in biochemistry. Some of these values were used in obtaining the values of AGf° for the ions of Table 6-4. The data of Table 6-5 can also be used in evaluation of Gibbs energy changes for reactions of ionic forms not given in Table 6-4.

The dependencies of kohs [Equation (14)], kE [Equation (11)], and kK [Equation (8)] on proton concentration are usually displayed in log-log plots called pH-rate profiles, which allow one to identify the reaction paths dominating at various pH values as well as the parameters of the rate law, namely the acidity constants K and the elementary rate constants of the rate-determining steps. Figure 3 shows pH- rate profiles of kE (dashed line), kK (thin full line, coincides with kohs below pH 17), and kobs = kE + /cK (thick gray line), which were plotted using Equations (8) and (11) with the six relevant kinetic and thermodynamic parameters that have been determined for acetophenone (see Table 1 in section Examples ).4,19 23... 

Table 1 Kinetic and thermodynamic parameters determined for various tautomeric equilibria in aqueous solution at 25°C. The symbols for the rate constants k and the equilibrium constants K are explained in the text (first paragraph of section Examples ). Acidity constants are concentration quotients of ionization at ionic strength 7=0.1 m... 

The protonation of amides (A) to yield the conjugated acids (AH+) in aqueous sulphuric acid takes place on the carbonyl oxygen158-161 and the ionization ratio (I = [AH+]/[A]) has been found to depend on the acidity of the solution as measured by the Ha acidity function25,162 163 where A ah+ is the thermodynamic dissociation constant of the conjugated acid (equations 33 and 34). 

Enolization of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl- 1-methylpyridinium ion (94) have been measured13811 and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate.13811 For (94), qh = 1.9 x 102 M-1 s 1, about 1.1 x 106 times that for a typical methyl ketone such as acetone. Thermodynamically, it is >108 times more acidic (pAa values of 11.1 vs 19.3). These increases in kinetic and thermodynamic acidity are derived from through-bond and through-space effects, and the implications for enzymatic catalytic sites with proximal, protonatable nitrogen are discussed. The results for (94) suggest a pAa value of 8.8 for (95), a value that cannot be measured directly due to competing hydrolysis. 

Once the generality of the logarithmic relationships for a variety of side-chain reactions was established, it was convenient to choose one reaction as a standard. In view of the many accurately determined thermodynamic equilibrium constants for the ionization of benzoic acids, this reaction was a most desirable choice. Hammett defined the substituent constant a in terms of the change in acidity of the acids (15). 

---