Dissociation, acids

The purpose of the modeling was to examine the influence of the solution environment on the extent of dissociation of an organic acid. A series of studies was performed initially to establish the validity of the model in preparation for later work. An initial test of the model was to vary the PD value and monitor the concentration of products. As expected, an increase in the PD rule produced an increase in the calculated acid dissociation constant K. A second study examined the influence of acid concentration on the observed properties. As expected, the KA was approximately constant over a modest concentration range. A third study considered the effect of water temperature on the acid dissociation. As the modeled water temperature was increased by increasing the Pb(W) value, the value of the KA decreased, in agreement with a common, but not universal, observation of the effect of temperature on acid dissociation. These three preliminary studies thus revealed emerging attributes consistent with experimental observations. 

The next study on solution environment influences included the presence of another molecule in the solution. This cosolute was endowed with an attribute of nondissociation. Its lipophilicity was varied in a series of studies revealing that the dissociation of the acid decreased when the lipophilicity of the cosolute decreased. These results may present a new insight into this effect. 

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Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

An alternative approach is to assume, in the light of the experimental evidence just mentioned, that the reactions of cations and neutral molecules have similar values of (or, equivalently, of log ( /l mol and to try to calculate the difference which would arise from the fact that the observed entropy of activation for a minority free base includes a contribution from the acidic dissociation of the conjugate acid in the medium in question (see (5) above). Consider the two following reaction schemes one (primed symbols) represents nitration via the free base, the other the normal nitration of a non-basic majority species (unprimed symbols) ... 

To evaluate the dissociation constants it would be necessary to measure the equDibrium constant for the reaction in Scheme 4. The dissociation of thiazolecarboxylic acids has been studied principally by Erlenmeyer et al. (47, 48). It seems that no systematic and reliable determination of the acidity dissociation constants have been realized until now. 

The strength of a weak acid is measured by its acid dissociation constant, which IS the equilibrium constant for its ionization m aqueous solution... 

The acid dissociation constant has the same form m Brpnsted-Lowry as m the Arrhenius approach but is expressed m the concentration of H30" rather than The concentration terms [H30" ] and [H" ] are considered equivalent quantities m equilibrium constant expressions... 

The C—H bonds of hydrocarbons show little tendency to ionize and alkanes alkenes and alkynes are all very weak acids The acid dissociation constant for methane for exam pie IS too small to be measured directly but is estimated to be about 10 ° (pK 60)... 

Section 19 5 Although carboxylic acids dissociate to only a small extent in water they are deprotonated almost completely m basic solution... 

Acid dissociation constant (Section 1 12) Equilibrium constant for dissociation of an acid... 

The values listed in Tables 8.7 and 8.8 are the negative (decadic) logarithms of the acidic dissociation constant, i.e., — logj, For the general proton-transfer reaction... 

Strong and Weak Acids The reaction of an acid with its solvent (typically water) is called an acid dissociation reaction. Acids are divided into two categories based on the ease with which they can donate protons to the solvent. Strong acids, such as Fid, almost completely transfer their protons to the solvent molecules. 

The equilibrium constant for this reaction is called an acid dissociation constant, K-, and is written as... 

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. 

The decrease in the acid dissociation constant from K i to tells us that each successive proton is harder to remove. Consequently, H3PO4 is a stronger acid than H2P04, and H2P04 is a stronger acid than HP 04 . 

A species that can serve as both a proton donor and a proton acceptor is called amphiprotic. Whether an amphiprotic species behaves as an acid or as a base depends on the equilibrium constants for the two competing reactions. For bicarbonate, the acid dissociation constant for reaction 6.8... 

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. 

To illustrate the systematic approach, let us calculate the pH of 1.0 M HF. Two equilbria affect the pH of this system. The first, and most obvious, is the acid dissociation reaction for HF... 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

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. 

Since the position of an acid-base equilibrium depends on the pH, the distribution ratio must also be pH-dependent. To derive an equation for D showing this dependency, we begin with the acid dissociation constant for HA. 

A sample contains a weak acid analyte, HA, and a weak acid interferent, HB. The acid dissociation constants and partition coefficients for the weak acids are as follows Ra.HA = 1.0 X 10 Ra HB = 1.0 X f0 , RpjHA D,HB 500. (a) Calculate the extraction efficiency for HA and HB when 50.0 mF of sampk buffered to a pH of 7.0, is extracted with 50.0 mF of the organic solvent, (b) Which phase is enriched in the analyte (c) What are the recoveries for the analyte and interferent in this phase (d) What is the separation factor (e) A quantitative analysis is conducted on the contents of the phase enriched in analyte. What is the expected relative erroi if the selectivity coefficient, Rha.hb> is 0.500 and the initial ratio ofHB/HA was lO.O ... 

The principal limitation to using a titration curve to locate the equivalence point is that an inflection point must be present. Sometimes, however, an inflection point may be missing or difficult to detect, figure 9.9, for example, demonstrates the influence of the acid dissociation constant, iQ, on the titration curve for a weak acid with a strong base titrant. The inflection point is visible, even if barely so, for acid dissociation constants larger than 10 , but is missing when is 10 k... 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

Since citric acid is a triprotic weak acid, we must first decide to which equivalence point the titration has been carried. The three acid dissociation constants are... 

In the second limiting situation the analyte is a weaker acid or base than the interferent. In this case the volume of titrant needed to reach the analyte s equivalence point is determined by the concentration of both the analyte and the interferent. To account for the contribution from the interferent, an equivalence point for the interferent must be present. Again, if the acid dissociation constants for the analyte and interferent are significantly different, the analyte s determination is possible. If, however, the acid dissociation constants are similar, only a single equivalence point is found, and the analyte s and interferent s contributions to the equivalence point volume cannot be separated. 

Ethylenediaminetetraacetic acid, H4Y, is a tetraprotic weak acid with successive acid dissociation constants of 0.010,... 

Using its titration curve, determine the acid dissociation constant for the weak acid in problem 6. 

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