Acetic acid, autoprotolysis

BOROHYDRIDE REDUCTION ACETAZOLAMIDE Acetic acid, autoprotolysis constant, AUTOPROTOLYSIS ACETOACETATE DECARBOXYLASE Acetoacetate decarboxylase reduction, BOROHYDRIDE REDUCTION ACETOLACTATE SYNTHASE Acetone,... 

In aqueous solutions, whenever protonation equilibria are involved, the autoprotolysis of water needs to be incorporated into the model. Thus, for an acetic acid titration the model comprises two equilibria... 

Organic solvents can also be classified according to their ability to accept or transfer protons (i.e., their acid-base behavior) [20,21]. Amphiprotic solvents possess donor as well as acceptor capabilities and can undergo autoprotolysis. They can be subdivided into neutral solvents that possess approximately equal donor and acceptor capabilities (water and alcohols), acidic solvents with predominantly proton donor properties (acetic acid, formic acid), and basic solvents with primarily proton acceptor characteristics (formamide, N-methylformamide, and N,N-dimethylformamide). Aprotic solvents are not capable of autoprotolysis but may be able to accept protons (ACN, DMSO, propylene carbonate). Inert solvents (hexane) neither accept nor donate protons nor are they capable of autoprotolysis. 

Protic solvents have a reactive H+, and all protic solvents undergo autoprotolysis. An example is acetic acid ... 

Strategy Acetic acid is a weak acid consequently, we expect the molarity of H30+ ions to be less than 0.10 moI-L-1 and, therefore, its pH to be greater than 1.0. To find the actual value, we set up an equilibrium table S with the initial molarity of acid equal to 0.10 mol-L 1 and allow the molarity of acid to decrease by x mol-L1 to reach equilibrium. Assume that the presence of acid dominates the pH and therefore that the autoprotolysis of 5 water need not be considered. We assume x is less than about 5% of the ini-j rial molarity of acid and simplify the expression for the equilibrium constant f by ignoring x relative to the initial molarity of the acid. This assumption i must be verified at the end of the calculation. 

Reactions (i) to (iv) represent dissociations of acids, reaction (v) is the common reaction, called neutralization , of strong acids with strong bases, reaction (vi) describes the neutralization reaction between acetic acid and ammonia which takes place in the absence of water, reactions (vii) to (ix) represent hydrolysis reactions, while reaction (x), which is the same as reaction (v) but in the opposite direction, describes the dissociation (or, more properly, the autoprotolysis) of water. Some of these reactions will be discussed in more detail in subsequent chapters. 

Formic add is more addic than acetic acid, but has a high dielectric constant (62). The autoprotolysis constant is so large (p = 6.2) that formic acid is relatively useless as a titration medium. Similarly, sulfuric add has a high autoprotolysis constant (p. sH = 3.85) and a high dielectric constant. 

Glacial acetic acid, though like water in being classed as amphiprotic, represents a solvent type distinctly different from water in that it is a much weaker base. This weak basicity makes it a useful solvent for the titration of weakly basic substances. As mentioned in Section 4-3, the autoprotolysis constant has a p T h value of 14.45 for the reaction... 

That is, when acetic acid concentration is 1.0 x lO"" M or lower, autoprotolysis of water must be taken into account. 

The constants of several solvents are listed in Table III.3.7. Some examples of autoprotolysis are the reactions of water, methanol, acetic acid, and liquid ammonia ... 

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. 

The autoprotolysis constant of formic acidH2,ii3 ig in the order of magnitude of while that of acetic acid is similar to that of liquid water. The cations formed are called formacidium and acetacidium ions respectively. 

Table I. Over-all Dissociation Constants of Acids, Bases, and Salts in Glacial Acetic Acid (-log autoprotolysis constant of glacial acetic acid, pKg = 14.45). ...

Ethylenediamine (en), NH2C2H4NH2, a strongly basic substance, may be considered to represent solvents that are weakly acidic compared with water. Ethylenediamine is therefore useful as a solvent for the titration of weakly acidic substances. It is a leveling solvent for adds whose ionization constants are larger than about 10 in water thus acetic add and hydrochloric acid are leveled to about equal strength. The titrant base normally used in en is sodium ethanolamine. The autoprotolysis constant of en is 5 x 10" for the equilibrium... 

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