Weak acids ionization constants

The strength of a weak acid is measured by its acid ionization constant,. This equilibrium constant can be calculated from the measured pH of the solution, as illustrated in Example. ... 

In these equations, HA symbolizes a weak acid and A symbolizes the anion of the weak acid. The calculations are beyond our scope. However, we can correlate the value of the equilibrium constant for a weak acid ionization, Ka, with the position of the titration curve. The weaker the acid, the smaller the IQ and the higher the level of the initial steady increase. Figure 5.2 shows a family of curves representing several acids at a concentration of 0.10 M titrated with a strong base. The curves for HC1 and acetic acid (represented as HAc) are shown, as well as two curves for two acids even weaker than acetic acid. (The IQ s are indicated.)... 

The ammonia can be distilled into an excess of standard sulfuric acid, and the excess determined by back titration. Preferably, the distilled ammonia can be absorbed into a solution of boric acid or other weak acid. Direct titration of the boric acid distillate with standard acid, using bromophenol blue, with a blank determination gives excellent results. As a routine method for determining traces of nitrogen, Milner and Zahner distilled the ammonia into a dilute boric acid solution. Urban suggested an aqueous solution of p-hydroxybenzoic acid (ionization constant 2.9 X 10 ) as an ammonia-absorbing solution. 

A strong acid is a substance that reacts completely with water, so that the acid ionization constant defined in Eq. (10) or (11) is effectively infinite. This situation can only be achieved if the conjugate base of the strong acid is very weak. A weak acid will be characterized by an acid ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (8) favors the existence of unreacted free acid. 

Write the acid-ionization constant expression for the weak acid H2SO3. 

The acid-ionization constant reflects the strength of a weak acid and the strength of the acid s conjugate base. 

Propanoic acid, C2H5COOH, is a weak acid. Write the expression defining its acid-ionization constant. 

Hydrazoic acid, HN3, is a weak acid. A 0.01 M solution of hydrazoic acid contained a concentration of 0.0005 M of the N3 ion. Find the acid-ionization constant of hydrazoic acid. 

Acid ionization constants As you have learned, a weak acid produces an equilibrium mixture of molecules and ions in aqueous solution. Thus, the equilibrium constant, provides a quantitative measure of the degree of ionization of the acid. Consider hydrocyanic acid (HCN), a deadly poison with applications in the steel industry and in the processing of metal ores. See Figure 19-9. The ionization equation and equilibrium constant expression for hydrocyanic acid are... 

Consider a general case in which the initial concentration of a weak acid HA is called c, and the initial concentration of its conjugate base (which for simplicity is assumed to come from the salt NaA) is Cb. In solution there will be five dissolved species HA, A , Na, HaO, and OH . It is necessary to write down and solve five independent equations that relate the equilibrium concentrations of these species to the initial concentrations and Cb and to the acid ionization constant of HA. The first equation is simply... 

Table 15.2 lists a number of organic acids with pfC values in aqueous solution between 0 (strong acid) and 14 (weak acid). Almost all of them are carboxylic acids with the characteristic — COOH functional group. Acid ionization constants... 

Arsenic acid (H3ASO4) is a weak triprotic acid. Given the three acid ionization constants from Table 15.2 and an initial concentration of arsenic acid (before ionization) of 0.1000 M, calculate the equilibrium concentrations of H3ASO4, H2ASO2, HAs04, As04, and H30. ... 

We have discussed strong acids and strong bases. There are relatively few of these. Weak acids are much more numerous than strong acids. For this reason you were asked to learn the list of common strong acids (see Table 18-1). You may assume that nearly all other acids you encounter in this text will be weak acids. Table 18-4 contains names, formulas, ionization constants, and pK values for a few common weak acids Appendix F contains a longer list of values. Weak acids ionize only slightly in dilute aqueous solution. Our classification of acids as strong or weak is based on the extent to which they ionize in dilute aqueous solution. 

A weak acid ionizes only slightly in solution, perhaps only to a few percent, and in solution between the molecular acid is in equilibrium with its ions. There are thousands of known weak acids and five of the more common ones are listed below with the equation for their equilibrium in solution. The equilibrium constant for the ionization of weak acids is symbolized Ka, and is called the acid-ionization constant. 

WEAK ACIDS AND ACID IONIZATION CONSTANTS Review Questions... 

Acids ionize and produce hydrogen ions in aqueous solution. The strength of an acid depends on how completely it ionizes. Strong acids undergo essentially complete ionization, whereas weak acids are only partly ionized at equilibrium. Like other reversible reactions, the ionization of a weak acid can be represented by an equilibrium constant expression. The value of this expression, called the acid ionization constant, K, is a measure of the strength of a weak acid. 

The rccison for the increased activity of protic acid-Lewis acid systems is ascribed to the enhancement of the protic acid acidity by the Lewis acid, (Table 2) 31). Whereas 100% HF has a value of —10.2, the acid system of HF +7% BF3 has an Ho value of —16.8, six orders of magnitude greater. The Hammett acidity function. Ho, is evaluated by the use of indicators, B, which are very weak bases and for which the acid ionization constants, Ka, of the conjugate acids, BH+, have been determined (usually spectrophotometrically), Eq. (9) ... 

In this reaction, the protons from the weak acid are transferred to the water molecules. The acid-ionization constant (AT) of this reaction is shown below ... 

An essential part of Arrhenius s theory of ionization was the application of the law of mass-action to describe the state of ionic equilibrium. Thus, acetic acid (CH3COOH) is a weak acid which ionizes in water to give some hydrogen ions (H ) and some acetate anions (CHsCOO"). The product of the concentration of the ions (which is [H" "] [CHsGOO"]) always bears a fixed ratio to the concentration of the neutral molecules [CH3COOH]. This ratio is called the acidic ionization constant ( a) more simply the ionization constant. Thus ... 

The state of ionization of weak bases also can be described by acidic ionization constants. For example, ammonia is a weak base which can take up hydrogen ions to form ammonium ions. This is, of course, equivalent to thinking of the ammonium ion (NH4 ) as a weak acid which ionizes in water to give some hydrogen ions (H" ") and some molecules of ammonia (NH3). Thus ... 

Most acidic substances are weak acids and therefore only partially ionized in aqueous solution ( FIGURE 16.9). We can use the equilibrium constant for the ionization reaction to express the extent to which a weak acid ionizes. If we represent a general weak acid as HA, we can write the equation for its ionization in either of the following ways, depending on whether the hydrated proton is represented as H.30 aq) or H aq) ... 

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