Autoprotolysis constant others

The water term in the denominator of Equation (6.3) is always large when compared with the other two concentrations on the top, so we say it remains constant. This assumption explains why it is rare to see the autoprotolysis constant written as Equation (6.3). Rather, we usually rewrite it as... 

As pointed out by Mayr,28 Ritchie,15 and Hine33,34 KR also measures the relative affinities of R+ and H30+ for the hydroxide ion. It can be regarded as providing a general affinity scale applicable to electrophiles other than carbocations.33,35 It can also be factored into independent affinities of R+ and H30+ as shown in Equations (2) and (3). Such equilibrium constants have been denoted If by Hine.33 AR corresponds to the ratio of constants for reactions (2) and (3) and, in so far as Kc for H30+ is the inverse of Kw the autoprotolysis constant for water, KR = KCKW... 

For each corresponding acid-base pair, e.g., HA and A", the acidity constant Ka and the basicity constant AT are related to each other by the equation Kaut0 = KaKb, where Kaut0 is the - autoprotolysis constant of the solvent. 

The equilibrium can be described with the autoprotolysis constant, i.e., the product of the activities of the species produced as a result of autoprotolysis. For solvents in which no other ionization processes are significant the term is synonymous with ionic product . 

The autoprotolysis constant The extent of ionization (4-19) of a pure amphiprotic solvent is measured by the autoprotolysis constant SH> defined as the product Since the autoprotolysis reaction results in the formation of both solvent cations and solvent anions, the autoprotolysis constant is a measure of the differentiating ability of a solvent. If a solvent has a large Ash value, the existence in it of a wide range of strengths of either adds or bases is not possible. In contrast, if the autoprotolysis constant is small, adds and bases of varying strengths show titration curves distinctly different from each other. 

Glacial acetic add represents the other extreme from ammonia, that of a solvent strongly addic but weakly basic compared with water. These two characteristics by themselves would cause gladal acetic add to have a relatively high autoprotolysis constant. Nevertheless, owing to the low dielectric constant (6.13), the autoprotolysis constant (p sh = 14.45) turns out to be about the same as that of water. ... 

Although acid-base titrations in alcohol-water mixtures have been studied extensively, we do not consider them in detail since titration curves and indicator equilibria in ethanol-water and methanol-water mixtures can be calculated in the same way as in water. Values of the autoprotolysis constants of the mixtures, are close to for mixtures containing only a moderate amount of alcohol. On the other hand, even a trace of water in ethanol causes a large increase in J SH According to Gutbezahl and Grunwald, pXsH is 14.33, 14.88, 15.91, and 19.5 for ethanol-water mixtures containing 20, 50, 80, and 100 wt % ethanol. 

The extent of the autoprotolysis is a measure of both the acidic and basic strengths of the solvent and is given by the autoprotolysis constant or ionic product for example, for water K p = [H3O+] [OH ] = lO" (25°) and for sulfuric acid Kpp = [H3SOi+] [HSO4-] = 1.7 X 10- (10°). The autoprotolysis constant of sulfuric acid is greater than that for any other solvent that has been studied. Such a large value implies that, in spite of its very high acidity, sulfuric acid must also be appreciably basic. 

The pH concept is most commonly used for dilute aqueous media however, a similar formalism can be extended to other systems. The extent of the pH scale, which in aqueous media can be described as 14 units, depends on the autoprotolysis constant of the amphiprotic solvent, so that the equivalent range, e.g., in methanol, equals 16.7 units, in sulfuric acid 2.9 units, and in acetic acid 14.5 units. In such solvents, as in water, the pH of neutrality corresponds to the middle of this range. Such reasoning cannot be extended to protophilic (e.g., pyridine, ethers), and aprotic (e.g., hydrocarbons) solvents, for which the logan+ scale is from one or both sides, respectively, unlimited. 

Values of the autoprotolysis constants of sulfuric acid and some other amphiprotic solvents, as determined by measuring the electric conductivity of the solvent and some of its solutions, are given in Table 12-1. 

H2SO4.Z2H2O, are known with = 1, 2, 3, 4 (mps 8.5", -39.5". -36.4" and -28.3% respectively). Other compounds in the H2O/SO3 system are H2S2O7 (mp 36") and H2S4O13 (mp 4"). Anhydrous H2SO4 is a remarkable compound with an unusually high dielectric constant, and a very high electrical conductivity which results from the ionic self-dissociation (autoprotolysis) of the compound coupled with a proton-switch mechanism for the rapid... 

Now we come to a very important point that will be the basis of much of the discussion in this chapter and the next. Because Kw is an equilibrium constant. the product of the concentrations ofHjO+ and OH ions is always equal to Kw. We can increase the concentration of H30+ ions by adding acid, and the concentration of OH ions will immediately respond by decreasing to preserve the value of Kk. Alternatively, we can increase the concentration of OH ions by adding base, and the concentration of H30 ions will decrease correspondingly. The autoprotolysis equilibrium links the concentrations of H30+ and OH" ions rather like a seesaw when one goes up, the other must go down (Fig. 10.10). 

Since the acid itself serves as the solvent, Ohx is practically constant and can be included in K. This only applies as long as the autoprotolysis of the concentrated acid is very slight. If HF is used as the acid then this condition is probably fulfilled since, according to Fredenhagen (1939 Fredenhagen and Cadenbach, 1930), anhydrous HF does not dissociate significantly. On the other hand, in the case of sulphuric acid the dissociation of the acid itself has to be taken into account so that we here have to deal with two interrelated equilibria. The equilibrium (5) then has to be formulated as... 

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