Hydrolysis of Salts Quiz

PDF page 798: Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt's constituent ions. For example, dissolving ammonium chloride in water results in its dissociation, as described by the equation + − NH Cl(s) ⇌ NH (aq) + Cl (aq) 4 4 The ammonium ion is the conjugate acid of the weak base ammonia, NH3, and so it will undergo acid ionization (or acid hydrolysis): + + NH (aq) + H O(l) ⇌ H O (aq) + NH (aq) 4 2 3 3 Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt’s constituent ions. For example, dissolving the ammonium chloride in water results in its dissociation, as described by the equation + − NH Cl(s) ⇌ NH (aq) + Cl (aq) 4 4 The ammonium ion is the conjugate acid of the base ammonia, NH3; its acid ionization (or acid hydrolysis) reaction is represented by + + NH (aq) + H O(l) ⇌ H O (aq) + NH (aq) K a = K w /K 4 2 3 3 b Since ammonia is a weak base, Kb is measurable and Ka > 0 (ammonium ion is a weak acid). The chloride ion is the conjugate base of hydrochloric acid, and so its base ionization (or base hydrolysis) reaction is represented by − − Cl (aq) + H O(l) ⇌ HCl(aq) + OH (aq) K = Kw /Ka 2 b Since HCl is a strong acid, Ka is immeasurably large and Kb ≈ 0 (chloride ions don’t undergo appreciable hydrolysis).

PDF page 798: Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt's constituent ions. For example, dissolving ammonium chloride in water results in its dissociation, as described by the equation + − NH Cl(s) ⇌ NH (aq) + Cl (aq) 4 4 The ammonium ion is the conjugate acid of the weak base ammonia, NH3, and so it will undergo acid ionization (or acid hydrolysis): + + NH (aq) + H O(l) ⇌ H O (aq) + NH (aq) 4 2 3 3 Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt’s constituent ions. For example, dissolving the ammonium chloride in water results in its dissociation, as described by the equation + − NH Cl(s) ⇌ NH (aq) + Cl (aq) 4 4 The ammonium ion is the conjugate acid of the base ammonia, NH3; its acid ionization (or acid hydrolysis) reaction is represented by + + NH (aq) + H O(l) ⇌ H O (aq) + NH (aq) K a = K w /K 4 2 3 3 b Since ammonia is a weak base, Kb is measurable and Ka > 0 (ammonium ion is a weak acid). The chloride ion is the conjugate base of hydrochloric acid, and so its base ionization (or base hydrolysis) reaction is represented by − − Cl (aq) + H O(l) ⇌ HCl(aq) + OH (aq) K = Kw /Ka 2 b Since HCl is a strong acid, Ka is immeasurably large and Kb ≈ 0 (chloride ions don’t undergo appreciable hydrolysis).

PDF page 799: Solution The Ka for anilinium ion is derived from the Kb for its conjugate base, aniline (see Appendix H): −14 K −5 1.0 × 10 w =

PDF page 799: pH = − log[H O ] = −log(0.0023) = 2.64

PDF page 799: As another example, consider dissolving sodium acetate in water: − NaCH CO (s) Na + (aq) + CH CO (aq) 3 2 3 2 The sodium ion does not undergo appreciable acid or base ionization and has no effect on the solution pH. This may seem obvious from the ion's formula, which indicates no hydrogen or oxygen atoms, but some dissolved metal ions function as weak acids, as addressed later in this section. − The acetate ion, CH CO , is the conjugate base of acetic acid, CH3CO2H, and so its base ionization (or base 3 2 hydrolysis) reaction is represented by − CH CO (aq) + H O(l) ⇌ CH CO H(aq) + OH − (aq) K = Kw /Ka 2 3 2 2 3 b Because acetic acid is a weak acid, its Ka is measurable and Kb > 0 (acetate ion is a weak base).

PDF page 799: As another example, consider dissolving sodium acetate in water: − NaCH CO (s) Na + (aq) + CH CO (aq) 3 2 3 2 The sodium ion does not undergo appreciable acid or base ionization and has no effect on the solution pH. This may seem obvious from the ion's formula, which indicates no hydrogen or oxygen atoms, but some dissolved metal ions function as weak acids, as addressed later in this section. − The acetate ion, CH CO , is the conjugate base of acetic acid, CH3CO2H, and so its base ionization (or base 3 2 hydrolysis) reaction is represented by − CH CO (aq) + H O(l) ⇌ CH CO H(aq) + OH − (aq) K = Kw /Ka 2 3 2 2 3 b Because acetic acid is a weak acid, its Ka is measurable and Kb > 0 (acetate ion is a weak base).

PDF page 800: Dissolving sodium acetate in water yields a solution of inert cations (Na ) and weak base anions (CH CO ), 3 2 resulting in a basic solution.

PDF page 800: Some salts are composed of both acidic and basic ions, and so the pH of their solutions will depend on the relative strengths of these two species. Likewise, some salts contain a single ion that is amphiprotic, and so the relative strengths of this ion’s acid and base character will determine its effect on solution pH. For both types of salts, a comparison of the Ka and Kb values allows prediction of the solution’s acid-base status, as illustrated in the following example exercise.

PDF page 801: (a) The K cation is inert and will not affect pH. The bromide ion is the conjugate base of a strong acid, and

PDF page 801: so it is of negligible base strength (no appreciable base ionization). The solution is neutral. − +

PDF page 801: amphiprotic. The Ka of HCO is 4.7 × 10 ,and its Kb is 3 −7 4.3 × 10 Since Kb >> Ka, the solution is basic.

PDF page 801: HF). Comparing the two ionization constants: Ka of NH is 5.6 × 10 and the Kb of F is 1.4 × 10 ,

PDF page 801: 4 so the solution is acidic, since Ka > Kb. Check Your Learning Determine whether aqueous solutions of the following salts are acidic, basic, or neutral:

PDF page 801: Unlike the group 1 and 2 metal ions of the preceding examples (Na , Ca , etc.), some metal ions function as acids in aqueous solutions. These ions are not just loosely solvated by water molecules when dissolved, instead they are covalently bonded to a fixed number of water molecules to yield a complex ion (see chapter on coordination chemistry). As an example, the dissolution of aluminum nitrate in water is typically represented as 3 − Al(NO )(s) ⇌ Al + (aq) + 3NO (aq) 3 3 However, the aluminum(III) ion actually reacts with six water molecules to form a stable complex ion, and so the more explicit representation of the dissolution process is 3+ −

PDF page 801: Unlike the group 1 and 2 metal ions of the preceding examples (Na , Ca , etc.), some metal ions function as acids in aqueous solutions. These ions are not just loosely solvated by water molecules when dissolved, instead they are covalently bonded to a fixed number of water molecules to yield a complex ion (see chapter on coordination chemistry). As an example, the dissolution of aluminum nitrate in water is typically represented as 3 − Al(NO )(s) ⇌ Al + (aq) + 3NO (aq) 3 3 However, the aluminum(III) ion actually reacts with six water molecules to form a stable complex ion, and so the more explicit representation of the dissolution process is 3+ −

PDF page 801: Al(NO ) (s) + 6H O(l) ⇌ Al(H O) (aq) + 3NO (aq)