Dissociation constant of an acid

The solvent isotope effect on the dissociation constant of an acidic proton in aqueous solution has been used to deduce a value for the fractionation... 

Just as pH = -logjJH+j and pOH = -logjJOH ], then the dissociation constant of an acid can be represented by pK = -logju f. 

Hasselbalch equation, which is important for understanding buffer action and acid-base balance in the blood and tissues of vertebrates. This equation is simply a useful way of restating the expression for the dissociation constant of an acid. For the dissociation of a weak acid HA into H+ and A-, the Henderson-Hasselbalch equation can be derived as follows ... 

In very dilute solutions the molar fraction -> activity of the solvent ( hs,mr) approaches unity, and the -> activity coefficients of the acid-base species approach 1, the conventional stoichiometric concentration dissociation constant of an acid H A in a given solvent HS is ... 

It was seen in Chap. IX that the dissociation constant of an acid undergoes relatively little change with temperature between 0 and 100 on the other hand the ionic product of water increases nearly five hundredfold. It is evident, therefore, from equation (3) that the hydrolysis constant will increase markedly with increasing temperature the degree of hydrolysis and the pH at any given concentration of salt will thus increase at the same time. 

In solvents of moderately high dielectric constant, the dissociation constant of an acid is determined primarily by the basicity of the solvent. Thus in two solvents S H and S"H with similar dielectric constants, for a particular acid HA we may write the equilibria... 

The dissociation constant of an acid is not the proper criterion for judging its strength in various solvents since the value of the constant in a given medium depends partially on the basic strength of the solvent. Bronsted therefore has proposed another method of expressing the strength of an acid. 

It is easy to see from this that the dissociation constant of an acid in alcohol, even though Kam (Water) and AAcid (Aicohoi) may be equal, will be smaller than in water due to the fact that the conventional basicity constant of alcohol is less than that of water. 

Small amounts of water. We have already seen in Chapter Four ( 5) that the dissociation constant of an acid in alcohol is increased strikingly when water is added. It is of equal interest to know how a trace of water will change the color of an indicator which is exhibiting an intermediate color in a given acid-base solution in pure alcohol. The influence of water is determined by the nature of the particular acid-base system contained in the alcoholic solution. 

Since the activity coefficients of salts in alcoholic solutions are unknown, it is simplest to refer the value of the dissociation constant of an acid to the particular salt concentration concerned by assuming,that the activity of the salt is equal to its concentration (cf. page 67). In the following table will be found values of pK (= — log K) for solutions which are 0.01 molar with respect to both acid and sodium salt. 

The effect of tautomerism on the observed dissociation constant of an acid is to ihtroduce a complicating reaction whose equilibrium is established in addition to that of the dissociation. The measured equilibrium constant involves the sum of concentrations of tautomers in place of concentrations of definite compounds. Structural changes... 

Determination of Acid-Base Equilibria. Since the absorption spectra of organic molecules with acidic or basic functional groups depend upon the pH of the medium, the absorption maxima and intensities vary with the hydrogen-ion concentration. The dissociation constant of an acid or a base may be determined spectrophotometrically as a result of such changes. For a weak acid in water... 

The term linear free energy relationship (LFER) appHes to a variety of relationships between kinetic and thermodynamic quantities that are important in both organic and inorganic reactions. About 80 years ago, J. N. Bronsted found a relationship between the dissociation constant of an acid, Ka, and its abihty to function as a catalyst in reactions that have rates that are accelerated by an acid. The Bronsted relationship can be written in the form... 

If the dissociation constant of an acid is small, is the acid strong or weak ... 

Equilibrium constant in terms of activities ionization (dissociation) constant of an acid. 

SH (in SH), i.e., for the transfer of a proton from one solvent to another. (A similar scheme would apply for the transfer of any ion if solvation were taken explicitly into account.) Further, since the solvent activities are conventionally put equal to unity in defining dissociation constants, we can write for the ratio of dissociation constants of an acid A in any solvent and in water. 

In sufficiently dilute solutions in which the concentration of water is effectively constant and the activities of the ionic species may be replaced by their concentrations, the dissociation constant of an acid HA is given by... 

The dissociation constant of an acid or base may therefore be given by ATw divided by the dissociation constant of the corresponding conjugate base or acid. 

If the pH value determined by using Equation (6.54) is to identify with that used in Equation (6.38), to give the thermodynamic dissociation constant of an acid, then the following equality is necessary... 

In a half-neutralised solution, the concentrations of free acid and salt are equal, and becomes equal to K. Hence the dissociation constant of an acid (or a base) can be found by determining the Cj, or pH of a half-neutralised solution. The pH of such a solution is... 

Experiment.—Determine the Molecular Conductivity and the Dissociation Constant of an Acid. 

Use Pauling s second rule to predict (approximately) the first dissociation constant of an acid, from a consideration of the value of t in the general formula AOXOH) . (Questions 11.4-11.7 and 12.2)... 

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