Weak acids, ionization

The Br0nsted theory expands the definition of acids and bases to allow us to explain much more of solution chemistry. For example, the Brpnsted theory allows us to explain why a solution of ammonium chloride tests acidic and a solution of sodium acetate tests basic. Most of the substances that we consider acids in the Arrhenius theory are also acids in the Bronsted theory, and the same is true of bases. In both theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the ions, the weak acid, and the water. A similar statement can be made about weak bases ... 

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.)... 

Acids are classified as strong or weak, depending upon their degree of ionization in water. A weak acid ionizes in water reversibly to form HjO ions. A weak acid is a weak electrolyte, and its aqueous solution does not conduct electricity well. The dissociation reaction occurs to a very small extent usually, fewer than 1 percent of the HA molecules are ionized. The ionization of a weak acid is shown as follows ... 

Use evidence from your investigation to support the conclusion that a weak acid ionizes less than a strong acid of identical concentration. 

A substance that produces H+(aq) ions in aqueous solution. Strong acids ionize completely or almost completely in dilute aqueous solution. Weak acids ionize only slightly. 

In an aqueous solution a weak acid ionizes to a limited extent as follows HA -H H O HsO -H A-... 

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. 

B. Weak acids ionize much less than strong acids in 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. 

The second hydrogen is part of the hydrogensulfate ion, HSCV, which behaves like a weak acid, ionizing to a much smaller extent and existing in equilibrium with hydronium and sulfate ions. 

A strong acid ionizes completely in solution it exists completely as ions in solution. A weak acid ionizes only slightly and exists as an equilibrium between the molecular acid and the ions it forms in solution. 

The fu-st step is to identify all the species present in solntion that may affect its pH. Because weak acids ionize to a small extent, at eqnilibrinm the major species present are nonionized HF and some H+ and F ions. Another major species is H2O, but its very small (1.0 X 10 " ) means that water is not a significant contributor to the H+ ion concentration. Therefore, unless otherwise stated, we will always ignore the H+ ions produced by the autoionization of water. Note that we need not be concerned with the OH ions that are also present in solution. The OH concentration can be determined from Equation (15.2) after we have calculated [H+]. 

This is a qnadratic eqnation which can be solved nsing the quadratic formula (see Appendix 4). Or we can try using a shortcut to solve for x. Because HF is a weak acid and weak acids ionize only to a slight extent, we reason that x must be small compared to 0.50. Therefore we can make the approximation... 

In summary, the main steps for solving weak acid ionization problems are ... 

The extent to which a weak acid ionizes depends on the initial concentration of the acid. The more dilute the solution, the greater the percent ionization (Figure 15.4). In qualitative terms, when an acid is diluted, the number of particles (nonionized acid molecules plus ions) per unit volume is reduced. According to Le Chateher s principle (see Section 14.5), to counteract this stress (that is, the dilution), the equilibrium shifts from nonionized acid to H+ and its conjugate base to produce more particles (ions). The dependence of percent ionization on initial concentration can be illustrated by the HF case discussed on page 608 ... 

These weak acids ionize as R—COOH R—COO + H" ", where R = H . CH2 for glycine, R = H2NCH3(CH )4 for amino-n-capriotic acid, and R = CH3 for acetic acid. Glycine is the only protein-forming amino acid without a center of chirality, and amino-/t-capriotic. acid is the amino acid that is used to. treat hematological problems.]... 

It is possible in some cases to make a further approximation on Eq. (3.11). If only a small fraction of the weak acid ionizes, [H+] will be small compared with c, so that X can be neglected in the denominator. In the case of acetic acid and a gross acid concentration of 0.100 moll [H+] is approximately equal to 10 mol 1 , only about 1 % as large as c. If we can tolerate an error of about 1 %, we can neglect X compared with c/c °. We obtain... 

Weak acids ionize only slightly (usually less than 5%) in dilute aqueous solution. Some common weak acids are listed in Appendix F. Several of them and their anions are given in Table 4-6. 

Citrus fruits contain citric acid, C3HjO(COOH)3. Some ointments and powders used for medicinal purposes contain boric acid, H3BO3. These everyday uses of weak acids suggest that there is a significant difference between strong and weak acids. The difference is that strong adds ionize completely in dilute aqueous solution, whereas weak acids ionize only slightly. 

Aqueous hydrogen atom can also be regarded as a weak acid ionizing with a pK of 9.7 according to the equation... 

Note that the extent to which a weak acid ionizes depends on the initial concentration of the acid. The more dilute the solution, the greater the percent ionization (see Figure 15.4 of the text). 

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) ... 

Write the weak acid ionization and express the ionization constant in terms of the equilibrium concentrations of H, the conjugate base, and the unionized acid. First solve for x by the approximate method. If the approximate method is not valid, use the quadratic equation to solve for x. 

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