Acid-dissociation constant relative strengths

The bipyridines are dibasic, and the two acid dissociation constants Ki and K2, for all the bipyridines have been determined. Typical values are recorded in Table I. There has been considerable interest in the first dissociation constants Ki of 2,2 -bipyridine and substituted 2,2 -bipyridines because of their use as metal complexing agents. In general, the order of relative basic strengths of derivatives of 2,2 -bipyridine is as expected. Electron-attracting substituents reduce the basicity, whereas electron-donating substituents increase the basicity of the molecule. " The dissociation constants of several substituted bipyridines correlate well with the Hammet equation. 2,2 -Bipyridines with an electron-donating substituent at position 4 are monoprotonated at N-1 and not at... 

Ethers, such as tetrahydrofuran and 1,2-dimethoxyethane, have low dielectric constants (7.4 and 7.2, respectively), and the choice of supporting electrolyte is very limited. The ethers are difficult to reduce and are inert toward many metalorganic reagents [243] that are soluble in the ethers. Ethers are thus suitable as medium for the anodic addition of Grignard reagents to olefins [420]. Dimethoxyethane and polyethylene glycol dimethyl ethers have the ability to solvate electrons [421]. Dissociation constants for some acids and the relative strength of some bases in THE has been determined [422]. 

The Arrhenius Acid-Base Definition The Acid-Dissociation Constant (KJ Relative Strengths of Acids and Bases... 

Using a table of acid-dissociation constants is the surest way to quantify relative strengths of weak acids, but you can often classify acids and bases qualitatively as strong or weak just from their formulas ... 

Acids and bases are essential substances in home, industry, and the environment. In aqueous solution, water combines with the proton released from an acid to form the hydrated species represented by HgO laq). In the Arrhenius definition, acids contain H and yield HaO in water, bases contain OH and yield OH in water, and an acid-base reaction (neutralization) is the reaction of and OH to form HgO. Acid strength depends on [HaO" ] relative to [HA] in aqueous solution. Strong acids dissociate completely and weak acids slightly. The extent of dissociation is expressed by the acid-dissociation constant, K. Weak acids have values ranging from about 10 to 10 . Many acids and bases can be classified qualitatively as strong or weak based on their formulas. 

Ka and Kb are particular types of equilibrium constants that give us an idea of the relative strengths of acids and bases, respectively. The acid-dissociation constant, Ka, is the equilibrium constant for the ionization of a weak acid to a hydrogen ion and its conjugate base. Likewise, the base-dissociation constant, Kb, is the equilibrium constant for the addition of a proton to a weak base by water to form its conjugate acid and an OH ion. 

In these reactions, there is a competition for protons between the two bases. Therefore, the relative amounts of the conjugate acid-base pairs that exist at equilibrium will be a measure of the strengths of the acids and bases. This is equivalent to saying that the dissociation constants measure strengths of acids and bases. 

The relative strength of a Bronsted-Lowry acid can be quantified by its acid-dissociation constant, /C, which is defined in Equation (14.7) ... 

The effect of solution ionic strength on the acid dissociation constants can be estimated for relatively dilute solutions by the use of Equation 2-3q and... 

The reaction of the solution will clearly depend upon the relative dissociation constants of the acid and the base. If they are equal in strength, the solution will be neutral if Ka> Kb, it will be acid, and if Kb> Ka> it will be alkaline. 

We express the relative strengths of weak acids and bases in terms of their dissociation constants. Shown... 

Because they vary by many orders of magnitude, base-dissociation constants are usually listed as their negative logarithms, or pATb values. For example, if a certain amine has Kb = 10-3, then pKb = 3. Just as we used pKa values to indicate acid strengths (stronger acids have smaller pKa values), we use pKb values to compare the relative strengths of amines as proton bases. 

THE STABILITY OF COMPLEXES In the previous section hints were made about the differences in stabilities of various complexes. In order to be able to make more quantitative statements and comparisons, a suitable way has to be found to express the stability of complexes. The problem in many ways is similar to that of expressing the relative strength of acids and bases. This was done on the basis of their dissociation constants (cf. Section 1.16), obtained by applying the law of mass action to these dissociation equilibria. A similar principle can be applied for complexes. 

Because the dissociation of an acid depends in a complex way on the chemical properties, molecular dipole and dielectric constant of the solvent, attempts to define absolute acid strength independently of the solvent have been unavailing. Nevertheless, relative strengths are independent, within a power of 10, of the nature of the solvent provided the acids belong to the same charge type, whether molecular, anionic or cationic. The independence of the nature of the solvent shown by acids of the same chemical character is even more marked. 

To determine Ki and K2 for H3PO4 from titration data, careful pH measurements ar e made after 0.5 and 1.5 mol of base are added for each mole of acid. It is then assumed that the hydrogen ion activities computed from these data are identical to the desired dissociation constants. Calculate the relative error incurred by the assumption if the ionic strength is 0.1 at the time of each measurement. 

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