ofb chapter 9 lecture notes -...
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6/17/2003 OFB Chapter 9 1
Chapter 9Dissolution and Precipitation
Equilibria
9-1 The Nature of Solubility Equilibria
9-2 The Solubility of Ionic Solids9-3 Precipitation and the
Solubility Product9-4 The Effects of pH on
Solubility9-5 Complex Ions and Solubility9-6 Controlling Solubility in
Qualitative Analysis
6/17/2003 OFB Chapter 9 2
The Nature of Solubility Equilibria
Recrystallization:
Saturated solution:
Dissolution and precipitation are reverse of each other.
A solution begins to deposit a compound when it is brought to the point of saturation with respect to that compound.
A solution in which a dissolution-precipitation (solubility) equilibrium exists between the solid substance and its dissolved form.
Solvate: In dissolution, the solute-solvent attractions I2(CCl4) replace the solute-solute interactions I2(s)formerly present in the solid, and the molecules go off into the bulk of the solvent.
I2(s) ↔I2(CCl4)
6/17/2003 OFB Chapter 9 3
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 soluteSupersaturated Solution: a solution that temporarily contains more of a solute than the equilibrium quantity
6/17/2003 OFB Chapter 9 5
The Nature of Solubility Equilibria
[I2]CCl4= K
The molarity of a saturated solution is a constant.
Le Chatelier’s principle:
I2(s) ↔I2(CCl4)
6/17/2003 OFB Chapter 9 6
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
6/17/2003 OFB Chapter 9 7
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)
6/17/2003 OFB Chapter 9 8
The Solubility of Ionic Solids
The Solubility Product
AgCl(s) ↔Ag+ (aq) + Cl-(aq)
= 1.6 × 10-10 at 25oC
=Ksp =
Ksp
The solid AgCl, which is in excess, is understood to have a concentration of
1 mole per liter.
6/17/2003 OFB Chapter 9 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 =
6/17/2003 OFB Chapter 9 10
The Solubility of Ionic SolidsThe 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)
6/17/2003 OFB Chapter 9 11
The Solubility of Ionic SolidsThe Solubility Product
TABLE 9-1contains Ksp values at 25C
6/17/2003 OFB Chapter 9 15
The Solubility of SaltsSolubility and Ksp
Exercise 9-2Determine 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.
Gram solubility of
Lead (II) iodate
Pb(IO3)2(s) ↔Pb2+(aq) + 2 IO3-(aq)
[y] [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
= (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
6/17/2003 OFB Chapter 9 16
The Solubility of SaltsSolubility 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]
6/17/2003 OFB Chapter 9 17
A common problem is to calculate if a precipitate will form at equilibrium
(Chapter 7) Reaction quotient before mixing occurs:
Q(init) = [A+](init)[B-](init)
If Q(init) < Ksp, no solid AB can appear.
If Q(init) > Ksp, solid AB precipitates until
Q = Ksp
AB (s) ↔ A+ + B-
6/17/2003 OFB Chapter 9 18
AgCl(s) ↔Ag+ (aq) + Cl-(aq)
Ksp = [Ag+][Cl-]If Q > Ksp then the solid precipitates
Q (init) = [Ag+] (init) [Cl-] (init)= Reaction quotient
Precipitation from Solution: Does a solid ppt form?
6/17/2003 OFB Chapter 9 19
Precipitation and the Solubility Product
Precipitation from SolutionExercise 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 TlNO3is 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, no solid TlIO3 can appear.
If Q(init) > Ksp, solid TlIO3 precipitates until Q = Ksp
[Tl+]
[IO3-]
Q > Ksp
Solid ppt
Q < Ksp
No ppt
6/17/2003 OFB Chapter 9 20
Exercise 9-4The Ksp of thallium(I) iodate is 3.1 × 10-6 at 25oC. Suppose that 555 mL of a 0.0022 M solution of TlNO3is mixed with 445 mL of a 0.0022 M solution of NaIO3. Does TlIO3 precipitate at equilibrium?
6/17/2003 OFB Chapter 9 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.
6/17/2003 OFB Chapter 9 22
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.
[Tl+] (mol L-1) [IO3-] (mol L-1)
Initial concentration
Equilibrium concentration
Change in concentration
[Tl+][IO3-] = Ksp
6/17/2003 OFB Chapter 9 23
The Effects of pH on Solubility
Solubility of Hydroxides
Zn(OH)2(s) ↔Zn2+(aq) + 2 OH-(aq)
[Zn2+][OH-]2 = Ksp = 4.5 × 10-17
Many solids dissolve more readily in more acidic solutions
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.
6/17/2003 OFB Chapter 9 24
The Effects of pH on SolubilitySolubility of Hydroxides
Exercise 9-7Estimate the molar solubility of Fe(OH)3 in a solution that is buffered to a pH of 2.9.
In pure water:
[OH-] = 3y = 1.3 × 10-9 mol L-1
pOH = 8.87 (and pH = 5.13)
[Fe3+] = y [OH-] = 3yy(3y)3 = 27y4 = Ksp = 1.1 × 10-36
y = 4.5 × 10-10 mol L-1 = [Fe3+] = [Fe(OH)3]=
In pure water, Fe(OH)3 is 5 x 10 6 less soluble than at pH = 2.9
6/17/2003 OFB Chapter 9 25
9-7 The Effects of pH on Solubility
• Solubilities of Hydroxides• Solubility of Salts and Weak Bases• Selective Precipitation of Ions• Metal Sulfides
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.
Somewhat more complicated due to other competing reactions. E.g.,
MS + H2O ↔ M2+ + OH- + HS-
(Metal Sulfide)