1. 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
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2. 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
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3. 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
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4. 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
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5. 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)
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6. 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  at 25oC
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7. 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 =
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8. 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)
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9. The Solubility of Ionic Solids
The Solubility Product
TABLE 9-1contains Ksp values at 25C
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13. 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 
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)
= g L-1
 2.50 L
Pb=207.2
I=126.9 O=16
Pb(IO3)2 = 557g per mole
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14. 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]
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15. 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
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16. 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
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17. 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
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18. 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
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19. 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 M solution of TlNO3 is mixed with 445 mL of a 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
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20. 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 M solution of TlNO3 is mixed with 445 mL of a M solution of NaIO3. Does TlIO3 precipitate at equilibrium?
Tl(IO3) (s) ↔Tl+(aq) + IO3-(aq)
[Tl+](init) = ( mol L-1)(555 mL/1000 mL)
= mol L-1
[IO3-](init) = ( mol L-1)(445 mL/1000 mL)
= mol L-1
Q(init) = [Tl+](init)[IO3-](init)
= (0.0012)( )
= 1.17  10-6
Q(init) ? Ksp
= 1.17  10-6 < 3.1  10-6
solid TlIO3 does NOT precipitate!
Because Q(init) < Ksp,
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21. 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.
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22. 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 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 conc. without the common ion
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23. 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.
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24. 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  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
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25. 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.
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26. 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
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