Dissolution and Precipitation Equilibria Chapter 9 Dissolution and Precipitation Equilibria 9-1 The Nature of Solubility Equilibria 9-2 The Solubility of Ionic Solids 9-3 Precipitation and the Solubility Product 9-4 The Effects of pH on Solubility 9-5 Complex Ions and Solubility 9-6 Controlling Solubility in Qualitative Analysis 11/10/2018 OFB Chapter 9

Saturated Solution: a solution in equilibrium with excess solute e.g., NaCl solubility in grams per 100 grams water is approximately 36.0 grams = saturated solution Unsaturated Solution: contains less than the equilibrium concentration of the solute Supersaturated Solution: a solution that temporarily contains more of a solute than the equilibrium quantity 11/10/2018 OFB Chapter 9

AgF has two such changes AgF·4H20 AgF·2H20 AgF Endothermic – heat is added to a system Exothermic – heat is removed from a system Sharp changes in slope occur if water of crystallization is lost or gained by the solid that is in contact with the solution AgF has two such changes AgF·4H20 AgF·2H20 AgF 11/10/2018 OFB Chapter 9

9-2 Solubility of Salts This chapter considers only salts which are sparingly soluble or insoluble for which concentrations of saturated salts are [salt] = 0.1 Mol L-1 or less 11/10/2018 OFB Chapter 9

Solubility Product Ksp Describes a chemical equilibrium in which an excess solid salt is in equilibrium with a saturated aqueous solution of its separated ions. General equation AB (s) ↔ A+ (aq) + B- (aq) 11/10/2018 OFB Chapter 9

The Solubility of Ionic Solids The Solubility Product AgCl(s) ↔Ag+ (aq) + Cl-(aq) Ksp = = The solid AgCl, which is in excess, is understood to have a concentration of 1 mole per liter. Ksp = 1.6  10-10 at 25oC 11/10/2018 OFB Chapter 9

The Solubility of Ionic Solids The Solubility Product Ag2SO4(s) ↔2Ag+(aq) + SO42-(aq) Ksp = Fe(OH)3(s) ↔Fe+3(aq) + 3OH-1(aq) Ksp = 11/10/2018 OFB Chapter 9

The Solubility of Ionic Solids The Solubility Product Exercise 9-1 Write the Ksp equation for the dissolution of aluminum hydroxide (Al(OH)3) in water. Al(OH)3(s) ↔Al3+(aq) + 3 OH-(aq) 11/10/2018 OFB Chapter 9

The Solubility of Ionic Solids The Solubility Product TABLE 9-1contains Ksp values at 25C 11/10/2018 OFB Chapter 9

11/10/2018 OFB Chapter 9

11/10/2018 OFB Chapter 9

11/10/2018 OFB Chapter 9

The Solubility of Salts Solubility and Ksp Exercise 9-2 Determine the mass of lead(II) iodate dissolved in 2.50 L of a saturated aqueous solution of Pb(IO3)2 at 25oC. The Ksp of Pb(IO3)2 is 2.6  10-13. Pb(IO3)2(s) ↔Pb2+(aq) + 2 IO3-(aq) [y] [2y] [Pb2+][IO3-]2 = Ksp y = 4.0  10-5  [Pb(IO3)2] = [Pb2+] = y = 4.0  10-5 mol L-1  [IO3-] = 2y = 8.0  10-5 mol L-1 Gram solubility of Lead (II) iodate = (4.0  10-5 mol L-1)  (557 g mol-1) = 0.0223 g L-1  2.50 L Pb=207.2 I=126.9 O=16 Pb(IO3)2 = 557g per mole 11/10/2018 OFB Chapter 9

The Solubility of Salts Solubility and Ksp Exercise 9-3 Compute the Ksp of silver sulfate (Ag2SO4) at 25oC if its mass solubility is 8.3 g L-1. 1 Ag2SO4(s) ↔2 Ag+(aq) + 1 SO42-(aq) [y] [2y] 11/10/2018 OFB Chapter 9

The Solubility of Salts Solubility and Ksp Exercise 9-3 Compute the Ksp of silver sulfate (Ag2SO4) at 25oC if its mass solubility is 8.3 g L-1. 1 Ag2SO4(s) ↔2 Ag+(aq) + 1 SO42-(aq) [y] [2y] [y] = (8.3 g Ag2SO4 L-1)  (1 mol Ag2SO4/311.8 g) [y] = [2.66  10-2 ] mol Ag2SO4 L-1 [Ag+]2[SO42-] = Ksp 11/10/2018 OFB Chapter 9

Dissolution (Solubility) Review The Nature of Solubility Equilibria Dissolution and precipitation are reverse of each other. Dissolution (Solubility) General reaction X3Y2 (s) ↔ 3X+2 (aq) + 2Y-3 (aq) [s] [3s] [2s] Ksp = [X+2]3 [Y-3]2 = (3s)3 (2s)2 s = molar solubility expressed in moles per liter 11/10/2018 OFB Chapter 9

Q(init) = [X+2]3(init)[Y-3]2(init) Review The Nature of Solubility Equilibria Dissolution and precipitation are reverse of each other. Precipation General reaction X3Y2 (s) ↔ 3X+2 (aq) + 2Y-3 (aq) Mix [X+2] and [Y-3] Does a ppt of X3Y2 form? [X+2] [Y-3] Reaction quotient before mixing occurs: Q(init) = [X+2]3(init)[Y-3]2(init) Q > K ? Ksp = [X+2]3 [Y-3]2 11/10/2018 OFB Chapter 9

AgCl(s) ↔Ag+ (aq) + Cl-(aq) Precipitation from Solution: Does a solid ppt form? AgCl(s) ↔Ag+ (aq) + Cl-(aq) Q (init) = [Ag+] (init) [Cl-] (init)= Reaction quotient Ksp = [Ag+][Cl-] If Q > Ksp then the solid precipitates 11/10/2018 OFB Chapter 9

Precipitation and the Solubility Product Precipitation from Solution Exercise 9-4: The Ksp of thallium (I) iodate is 3.1  10-6 at 25oC. Suppose that 555 mL of a 0.0022 M solution of TlNO3 is mixed with 445 mL of a 0.0022 M solution of NaIO3. Does TlIO3 precipitate at equilibrium? Evaluate : Reaction quotient before mixing occurs: Q(init) = [Tl+](init)[IO3-](init) If Q(init) > Ksp, solid TlIO3 precipitates until Q = Ksp If Q(init) < Ksp, no solid TlIO3 can appear. [Tl+] [IO3-] Q > Ksp Solid ppt Q < Ksp No ppt 11/10/2018 OFB Chapter 9

Tl(IO3) (s) ↔Tl+(aq) + IO3-(aq) Exercise 9-4 The Ksp of thallium(I) iodate is 3.1  10-6 at 25oC. Suppose that 555 mL of a 0.0022 M solution of TlNO3 is mixed with 445 mL of a 0.0022 M solution of NaIO3. Does TlIO3 precipitate at equilibrium? Tl(IO3) (s) ↔Tl+(aq) + IO3-(aq) [Tl+](init) = (0.0022 mol L-1)(555 mL/1000 mL) = 0.0012 mol L-1 [IO3-](init) = (0.0022 mol L-1)(445 mL/1000 mL) = 0.00098 mol L-1 Q(init) = [Tl+](init)[IO3-](init) = (0.0012)(0.00098) = 1.17  10-6 Q(init) ? Ksp = 1.17  10-6 < 3.1  10-6 solid TlIO3 does NOT precipitate! Because Q(init) < Ksp, 11/10/2018 OFB Chapter 9

Precipitation and the Solubility Product The Common Ion Effect If a solution and a solid salt to be dissolved in it have an ion in common, then the solubility of the salt is depressed. 11/10/2018 OFB Chapter 9

TlIO3(s) ↔ Tl+(aq) + IO3-(aq) The Common Ion Effect Exercise 9-6 The Ksp of thallium(I) iodate (TlO3) is 3.1  10-6 at 25oC. Determine the molar solubility of TlIO3 in 0.050 mol L-1 KIO3 at 25oC. TlIO3(s) ↔ Tl+(aq) + IO3-(aq) [Tl+] (mol L-1) [IO3-] (mol L-1) Initial concentration Equilibrium concentration Change in concentration [Tl+][IO3-] = Ksp Assume s is small s = [TlIO3]= 6.2 × 10-5 mol L-1 which is depressed 28 times relative to the 1.76 x 10-3 conc. without the common ion 11/10/2018 OFB Chapter 9

The Effects of pH on Solubility Solubility of Hydroxides Many solids dissolve more readily in more acidic solutions Zn(OH)2(s) ↔Zn2+(aq) + 2 OH-(aq) [Zn2+][OH-]2 = Ksp = 4.5  10-17 If pH decreases (or made more acidic), the [OH-] decreases. In order to maintain Ksp the [Zn2+] must increase and consequently more solid Zn(OH)2 dissolves. 11/10/2018 OFB Chapter 9

The Effects of pH on Solubility Solubility of Hydroxides Exercise 9-7 Estimate the molar solubility of Fe(OH)3 in a solution that is buffered to a pH of 2.9. In pure water: [Fe3+] = y [OH-] = 3y y(3y)3 = 27y4 = Ksp = 1.1  10-36 y = 4.5  10-10 mol L-1 = [Fe3+] = [Fe(OH)3]= [OH-] = 3y = 1.3  10-9 mol L-1 pOH = 8.87 (and pH = 5.13) In pure water, Fe(OH)3 is 5 x 10 6 less soluble than at pH = 2.9 11/10/2018 OFB Chapter 9

9-7 The Effects of pH on Solubility Solubilities of Hydroxides Solubility of Salts and Weak Bases Selective Precipitation of Ions Metal Sulfides Somewhat more complicated due to other competing reactions. E.g., MS + H2O ↔ M2+ + OH- + HS- (Metal Sulfide) But as before solubility of Metal Sulfides increase as pH decreases Ksp = [M2+][OH-][HS-] As pH decreases (or made more acidic), the [OH-] decreases. In order to maintain Ksp the [M2+] must increase and consequently more solid Metal Sulfide dissolves. 11/10/2018 OFB Chapter 9

Examples / Exercises 9-1, Ksp calculations 9-2, Ksp calculations 9-4, ppt Q ? Ksp 9-5, Equilibrium concentrations 9-6, Common Ion effect 9-7 Effect of pH of on solubility 11/10/2018 OFB Chapter 9