2014-08-22 16 SOLUBILITY AND PRECIPITATION EQUILIBRIA CHAPTER 16.1 The Nature of Solubility Equilibria 16.2 Ionic Equilibria between Solids and Solutions 16.3 Precipitation and the Solubility Product 16.4 The Effects of pH on Solubility 16.5 Complex Ions and Solubility General Chemistry II 733 General Chemistry II 2 1 2014-08-22 734 16.1 THE NATURE OF SOLUBILITY EQUILIBRIA General Features of Solubility Equilibria Saturation ~ Dissolution-precipitation equilibrium Recrystallization ~ Purification of solids Solvent of crystallization 2 Li+(aq) + SO42-(aq) + H2O(l) → Li2SO4H2O(s) ~ different chemical formula & mass Supersaturation ~ Slow equilibrium Fig. 16.1 Deposit of K2PtCl4 from the saturated aqueous solution as the water evaporates. General Chemistry II 735 The solubility of Ionic Solids Solubility at 25°C, AgClO4 ; 5570 g/L, AgCl; 0.0018 g/L Temperature dependence - Mostly endothermic → Solubility increases with T - CaSO4 exothermic → Solubility decreases with T Classification (at 25 °C) Soluble > 10 g/L, Slightly soluble 0.1~10 g/L, Insoluble < 0.1 g/L Fig. 16.3 Temperature dependence of solubility. General Chemistry II 4 2 2014-08-22 737 General Chemistry II 737 16.2 IONIC EQUILIBRIA BETWEEN SOLIDS AND SOLUTIONS Highly soluble salt: Nonideal solution, CsCl(s) Cs+(aq) + Cl-(aq) Fig. 16.5 The dissolution of the ionic solid CsCl in water General Chemistry II 3 2014-08-22 738 Solubility and Ksp AgCl(s) Ag+(aq) + Cl-(aq) Solubility product: Ksp = [Ag+][Cl-] = 1.610-10 at 25°C Solubility (S) of AgCl at 25°C calculated from Ksp Ksp = [Ag+][Cl-] = S2 = 1.610-10 S = 1.2610-5 M Gram solubility = (1.2610-5 mol/L) (143.3 g/mol) = 1.810-3 g/L General Chemistry II 738 General Chemistry II 8 4 2014-08-22 739 EXAMPLE 16.1 Calculation of [Ca2+] and [F-] in a saturated solution of CaF2 at 25°C: Ksp → Solubility CaF2(s) Ca2+(aq) + 2 F-(aq) Ksp = [Ca2+][F-]2 = 3.910-11 at 25°C [Ca2+] = S, [F-] = 2S Ksp = [Ca2+][F-]2 = S (2S)2 = 4S3 → S = 2.1 10-4 M Gram solubility = (2.110-4 mol/L) (78.1 g/mol) = 0.017 g/L General Chemistry II 740 EXAMPLE 16.2 Solubility (0.029 g/L) → Ksp Ag2CrO4(s) 2 Ag+(aq) + CrO42-(aq) Ksp = [Ag+]2[CrO42-] = 2.7 10-12 Gram solubility: 0.029 g/L Molar solubility: 0.029 g/L = 8.74 10-5 mol/L = S [Ag+]= 2S, [CrO42-] = S Ksp = [Ag+]2[CrO42-] = 4S3 = 2.7 10-12 → 42 % greater than the tabulated value, 1.9 10-12 General Chemistry II 5 2014-08-22 740 16.3 PRECIPITATION AND THE SOLUBILITY PRODUCT Precipitation from Solution Ksp = [Ag+][Cl-] Q0 = [Ag+]0[Cl-]0 ~ initial reaction quotient Q0 > Ksp precipitation Q0 < Ksp dissolution Fig. 16.6 A plot of precipitation and dissolution equilibrium for AgCl in water. The slope of the path toward equilibrium represented by red or blue arrow is 1. General Chemistry II 742 EXAMPLE 16.4 [Ag+]0 = 0.0015 M, [Cl-]0 = 5.010-6 M Equilibrium concentrations? Cl- is the limiting reactant → complete precipitation first Remaining [Ag+] = 0.0015 5.0 10-6 0.0015 M AgCl(s) Ag+(aq) + Cl-(aq) ---------------------------------------------------------------------Initial 0.0015 0 Change +y +y --------------- -----Equilibrium 0.0015 + y y ---------------------------------------------------------------------Ksp = 1.60 10-10 = (0.0015 + y) y 0.0015 y y = [Cl-] = 1.1 10-7 M, [Ag+] = 0.0015 M General Chemistry II 6 2014-08-22 742 The Common-Ion Effect ~ Solubility decreases in the presence of a common ion AgCl NaCl or AgNO3 Solubility of AgCl(s) in 1.00 L of 0.100 M NaCl solution [Ag+]NaCl = S, [Cl-]NaCl = 0.100 + S Ksp = 1.60 10-10 = [Ag+] NaCl [Cl-] NaCl = S (0.100 + S) 0.100 S (S < Swater =1.3 10-5 << 0.100) [Ag+] NaCl = S = 1.60 10-9 M [Cl-] NaCl = 0.100 M [Ag ]H 2O [Ag ]H2O 1.3 105 M 1.3 105 8.1103 9 [Ag ] 1.6 10 0.1M NaCl General Chemistry II Fig. 16.7 Common-ion effect for the solubility of AgCl in AgNO3 solution and in NaCl solution. 744 16.4 THE EFFECTS OF pH ON SOLUBILITY CaCO3(s) + H3O+(aq) Ca2+(aq) + HCO3-(aq) + H2O(l) Fig. 16.8 Damage due to increased acidity from air pollution. On the east pier of Stanford White's Washington Square Arch is Herma A. MacNeil's Washington in War (1916) (Washington Square Park in the Greenwich Village neighborhood of Lower Manhattan in New York City) General Chemistry II 7 2014-08-22 744 Solubility of Hydroxides Zn(OH)2(s) Zn2+(aq) + 2 OH-(aq) Ksp = [Zn2+][OH-]2 = 4.5 10-17 In acidic solution, [OH-] decreases. → reaction goes to the right EXAMPLE 16.6 Comparison of solubilities of Zn(OH)2(s) in pure water and in a buffer with pH 6.00. In pure water, [Zn2+] = S, [OH-] = 2S Ksp = S(2S)2 S = [Zn2+] = 2.2 10-6 M, [OH-] = 2S = 4.5 10-6 M, pH = 8.65 In a pH = 6.00 buffer, [OH-] = 1.0 10-8 M (fixed). [Zn2+] = Ksp / [OH-]2 = 0.45 M Metal hydroxides are basic → more soluble in acidic solution General Chemistry II 746 Solubility of Salts of Bases CaF2(s) Ca2+(aq) + 2 F-(aq), Ksp = 3.9 10-11 - Solubility of CaF2(s) at low pH : F-(aq) + H3O+(aq) HF(aq) + H2O(l), K = 2.9 103 more soluble in acidic solution (large K) [H3O+] → [F-] → more CaF2(s) dissolves (Le Chatelier) - Solubility of AgCl(s) at low pH : AgCl(s) Ag+(aq) + Cl-(aq) - Even in acidic solution, Cl-(aq) + H3O+(aq) HCl(aq) + H2O(l) → negligible effect of pH on the solubility of AgCl General Chemistry II 8 2014-08-22 746 16.5 COMPLEX IONS AND SOLUBILITY Complex-Ion Equilibria Ag+(aq) + NH3(aq) Ag(NH3)+(aq) K1 = [Ag(NH3) +] / ([Ag+][NH3]) = 2.1 103 Ag(NH3)+(aq) + NH3(aq) Ag(NH3)2+(aq) K2 = [Ag(NH3)2+] / ([Ag(NH3)+][NH3]) = 8.2 103 Ag+(aq) + 2 NH3(aq) Ag(NH3)2+(aq) Kf = K1 K2 = [Ag(NH3)2+] / ([Ag+][NH3]2) = 1.7 107 Kf : Formation constant General Chemistry II 747 General Chemistry II 9 2014-08-22 EXAMPLE 16.7 0.100 mol of AgNO3 dissolved in 1.00 L of 1.00 M NH3 747 [Ag+] and [Ag(NH3)+] at equilibrium ? Assume that Ag+ is present as Ag(NH3)2+. [Ag(NH3)2+]0 = 0.100 M; [NH3] 0 = 1.00 M – (2 0.100) M = 0.80 M Ag(NH3)2+(aq) Ag(NH3)+(aq) + NH3(aq), K–2 = K2–1 Ag(NH3)+(aq) Ag+(aq) + NH3(aq), K–1 = K1–1 Ag(NH3)2+(aq) Ag(NH3)+(aq) + NH3(aq) -----------------------------------------------------------------------------------------Initial 0.100 0 0.80 Change –y +y +y ----------------------------Equilibrium 0.100 – y y 0.80 + y -----------------------------------------------------------------------------------------K 2 = 1 [Ag NH3 ][NH3 ] y 0.80 + y 1 = = = + 0.10 y 8.2 103 K2 [Ag NH3 2 ] K 1 = + 1 [Ag+ ][NH3 ] [Ag ] 0.80 1 = = = + 5 K 1 [Ag NH3 ] 1.5 10 2.1 103 + y = [Ag(NH3)+] = 1.5 10–5 M [Ag+] = 9 10–9 M << [Ag(NH3)2+] 0.100 M General Chemistry II 749 Formation of coordination of complexes → increases solubilities AgBr(s) Ag+(aq) + Br–(aq) Ksp = 7.7 10–13 AgBr(s) + 2 S2O32– (aq) Ag(S2O3)23– (aq) + Br–(aq) thiosulfate ion, S2O32– sulfate ion, SO42– Fig. 16.12 Effect of complex ion formation on solubility. AgBr in thiosulfate solution and in pure water. General Chemistry II 10 2014-08-22 749 EXAMPLE 16.8 Solubility of AgBr in 1.00 M aqueous solution of NH3? AgBr(s) + 2 NH3(aq) Ag(NH3)2+ (aq) + Br–(aq), K AgBr(s) Ag+(aq) + Br–(aq), Ksp = 7.7 10–13 Ag+(aq) + 2 NH3(aq) Ag(NH3)2+(aq), Kf = 1.7 107 K = KspKf = 1.3 10–5 Solubility of AgBr: S S = [Br–] [Ag(NH3)2+] → [NH3] = 1.00 – 2S K = S2 [Ag(NH3 )2+ ][Br ] 1.3 105 2 2 [NH3 ] 1.00 - 2S S = 3.6 10–3 = [Br–] [Ag(NH3)2+] [Ag+] = Ksp/ [Br–] = Ksp/ S = 2.1 10–10 << [Ag(NH3)2+] General Chemistry II 749 Re-dissolving by forming complex ions The opposite of common ion effects Hg2+(aq) + 2I–(aq) HgI2 (s) HgI2(s) + I–(aq) HgI3–(aq) HgI3–(aq) + I–(aq) HgI4–(aq) Fig. 16.13 "Orange Tornado" General Chemistry II 11 2014-08-22 750 Separation of cations In a strongly basic solution of Mg2+ and Zn2+ Mg(OH)2(s) Zn(OH)42–(aq) In a strongly basic solution of Al3+ and Fe3+ Fe(OH)3(s) Al(OH)42–(aq) Fig.16.14 AlCl3(s) + H2O(l) Chemistry Al(OH)4–(aq) General II + HCl(aq) Fig. 16.15 Solubility of Zn(OH)2 in acid, water, base. 10 Problem Sets For Chapter 16, 14, 22, 30, 34, 44, 48, 66, 74, 76, 82 General Chemistry II 12
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