(as on 19.08.2014) on 15th meeting of JCM-III Level Council (OFB
OFB Chapter 9 lecture notes - Georgia Institute of...
Transcript of OFB Chapter 9 lecture notes - Georgia Institute of...
2/17/2004 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
2/17/2004 OFB Chapter 9 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 soluteSupersaturated Solution: a solution that temporarily contains more of a solute than the equilibrium quantity
2/17/2004 OFB Chapter 9 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 crystallizationis lost or gained by the solid that is in contact with the solution
2/17/2004 OFB Chapter 9 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
2/17/2004 OFB Chapter 9 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)
2/17/2004 OFB Chapter 9 6
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.
2/17/2004 OFB Chapter 9 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 =
2/17/2004 OFB Chapter 9 8
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)
2/17/2004 OFB Chapter 9 9
The Solubility of Ionic SolidsThe Solubility Product
TABLE 9-1contains Ksp values at 25C
2/17/2004 OFB Chapter 9 10
2/17/2004 OFB Chapter 9 11
2/17/2004 OFB Chapter 9 12
2/17/2004 OFB Chapter 9 13
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
2/17/2004 OFB Chapter 9 14
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]
MassMolar x molesmassMassMolar
massmoles
=
=
spK Find 4.solubilitymolar in expressedexpression mequilibriu Find 3.
solubilitymolar Find 2.solubilitymass Given 1.
2/17/2004 OFB Chapter 9 15
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]
[Ag+]2[SO42-] = Ksp
[y] = (8.3 g Ag2SO4 L-1) × (1 mol Ag2SO4/311.8 g)
[y] = [2.66 × 10-2 ] mol Ag2SO4 L-1
2/17/2004 OFB Chapter 9 16
Review
The Nature of Solubility Equilibria
Dissolution and precipitation are reverse of each other.
General reaction
X3Y2 (s) ↔ 3X+2 (aq) + 2Y-3 (aq)
Ksp =
Dissolution (Solubility)
[X+2]3 [Y-3]2
[s] [3s] [2s]
= (3s)3 (2s)2
s = molar solubility expressed in moles per liter
2/17/2004 OFB Chapter 9 17
Review
The Nature of Solubility Equilibria
Dissolution and precipitation are reverse of each other.
General reaction
X3Y2 (s) ↔ 3X+2 (aq) + 2Y-3 (aq)
Ksp =
Precipation
[X+2]3 [Y-3]2
[Y-3][X+2]Mix [X+2] and [Y-3]
Does a ppt of X3Y2 form?
Reaction quotient before mixing occurs:
Q(init) = [X+2]3(init)[Y-3]2
(init) Q > K ?
2/17/2004 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?
2/17/2004 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
2/17/2004 OFB Chapter 9 20
[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
Because Q(init) < Ksp, solid TlIO3 does NOTprecipitate!
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?
Q(init) ? Ksp = 1.17 × 10-6 < 3.1 × 10-6
Tl(IO3) (s) ↔Tl+(aq) + IO3-(aq)
2/17/2004 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.
2/17/2004 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 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
TlIO3(s) ↔ Tl+(aq) + IO3-(aq)
2/17/2004 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.
2/17/2004 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
2/17/2004 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)
2/17/2004 OFB Chapter 9 26
• Examples / Exercises– 9-1, Ksp calculations– 9-2, Ksp calculations– 9-3, Ksp calculations– 9-4, ppt Q ? Ksp– 9-5, Equilibrium concentrations– 9-6, Common Ion effect– 9-7 Effect of pH of on solubility
• Problems– 16, 19, 23, 30, 39, 41, 42