Acetic acid dissociation reaction

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. 

Let us now consider the reaction of acetic acid dissociation shown in Eq. 8.37. By using the electrochemical potential of ions, the equilibrium of the reaction is expressed in Eq. 8.42 ... 

There are a few things that you need to pay attention to as you set up this problem. First, the sodium need not be written in the equation because it doesn t do anything (it s a spectator). Second, unlike the previous weak acid problems, these problems aren t starting out with no products. The common ion in this reaction is the acetate ion, which is a product of the acetic acid dissociation. When you set up your chart, you need to include all amounts of all substances present at the start of the reaction. We re going to omit water in our chart because it is not part of the equilibrium expression. 

Notice the arrow that is used in the equation in Figure 10.7. It points in both directions, indicating that the reaction is reversible. In other words, the products of the reaction also react to produce the original reactants. In this reaction, molecules of acetic acid dissociate just as quickly and as often as the dissociated ions re-associate to produce acetic acid molecules. (Figure 10.8 will help you visualize what happens.)... 

The double arrow in the acetic acid ionization reaction indicates that the dissociation is incomplete, that is, a certain amount of undissociated acetic acid exists in solution with the dissociated acetic add. Acetic acid is called a weak add because it ionizes only to a limited extent in water. HCl and HNO3 are strong acids because they are, for all practical purposes, completely ionized in water. The relative strengths of acids and bases are discussed in detail in Section 11.2. 

Assume no acetic acid dissociation in the aqueous phase and that is unaffected hy these reactions ... 

The first mention of polythiophenes as potential conducting polymers can be found in the literature in 1967. A. G. Davies and coauthors investigated the acid catalyzed polymerization of furan, pyrrole, and thiophene hetero-cycles. Although this polymerization reaction was known before, several new and interesting facts could be elucidated. Besides the confirmation of the cyclic structure of the polymer units (instead of ring opened), the electric conductivity was investigated. In contrast to the already known types of electronically conducting polymers (polyaniline and polypyrrole, see Chapter 1), the conductivity of all polyheterocycles in Armour s paper, isolated as trichloro- or trifluoroacetates (triflate), is ionic. In the case of the polythiophene, ion pairs formed by the reaction of polythiophene with trifluoro acetic acid dissociated in methylene chloride solution to the protonated polymer and triflate anions. The polythiophene triflate decomposed at relatively low temperatures (60°C-80°C). ... 

Fig. 3. Plots of log K for acid dissociation reactions as functions of temperature at Psat, and for acetic and benzoic acids at elevated pressure labeled in kilobars. Symbols represent experimental data, but curves are calculated with the revised HKF equations of state using data and parameters from Shock (1993a)...

Note that the concentration of H2O is omitted from the expression because its value is so large that it is unaffected by the dissociation reaction.The magnitude of provides information about the relative strength of a weak acid, with a smaller corresponding to a weaker acid. The ammonium ion, for example, with a Ka of 5.70 X 10 °, is a weaker acid than acetic acid. 

Tabulating Values for K and Kb A useful observation about acids and bases is that the strength of a base is inversely proportional to the strength of its conjugate acid. Consider, for example, the dissociation reactions of acetic acid and acetate. 

The equilibrium position for any reaction is defined by a fixed equilibrium constant, not by a fixed combination of concentrations for the reactants and products. This is easily appreciated by examining the equilibrium constant expression for the dissociation of acetic acid. 

When acetic acid, which is a weak acid, is placed in water, the dissociation reaction... 

Iron(III) bromide [10031-26-2], FeBr, is obtained by reaction of iron or inon(II) bromide with bromine at 170—200°C. The material is purified by sublimation ia a bromine atmosphere. The stmcture of inoa(III) bromide is analogous to that of inon(III) chloride. FeBr is less stable thermally than FeCl, as would be expected from the observation that Br is a stronger reductant than CF. Dissociation to inon(II) bromide and bromine is complete at ca 200°C. The hygroscopic, dark red, rhombic crystals of inon(III) bromide are readily soluble ia water, alcohol, ether, and acetic acid and are slightly soluble ia Hquid ammonia. Several hydrated species and a large number of adducts are known. Solutions of inon(III) bromide decompose to inon(II) bromide and bromine on boiling. Iron(III) bromide is used as a catalyst for the bromination of aromatic compounds. 

Now the dissociation of acetic acid can be regarded as an acid-base reaction. The acid CHjCOOH transfers a proton to the base H20 forming the acid H30+ and the base CH3COO-. The reaction (54) now takes the form... 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

The concept of using the base 10 logarithm to express the magnitude is a widespread practice today. Equilibrium constants of chemical reactions are often noted or compared as pK values where pK = — log 10 (magnitude of equilibrium constant). For example, the extent of dissociation of acetic acid, the acid in vinegar, is quantified by an equilibrium constant of 1.8 x 10-5. Here, then, pK = — log,o (1.8 x 1(T5) = 4.74. 

B Since acetic acid is a weak acid, it is not dissociated completely in aqueous solution (except at infinite dilution) it is misleading to write it in ionic form. The products of this reaction are the gas carbon dioxide, the covalent compound water, and the ionic solute calcium acetate. Only the latter exists as ions in aqueous solution. 

---