Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, 2007 (John Wiley)
ISBN: 9 78047081 0866
e
CHEM1002 [Part 2]
Dr Michela Simone
Weeks 8 – 13
Office Hours: Monday 3-5, Friday 4-5
Room:
412A (or 416)
Phone:
93512830
e-mail:
michela.simone@sydney.edu.au
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Summary of Last Lecture
Crystal structures II
•
•
•
•
•
Face centred cubic : close packing with 74% efficiency,
coordination number of 12 and 4 atoms per unit cell
Hexagonal close packing : close packing with 74% packing
efficiency and coordination number of 12
Body centred cubic : not closed packed with 68% packing
efficiency, coordination number of 8 and 2 atoms per unit cell
Simple cubic : not close packed with 52% packing efficiency
and coordination number of 6
Even close packed structures have ‘holes’ (interstitial sites)
and ionic structures can be considered to be based on close
packed anions with cations in some or all of these sites
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Solubility Equilibria
Lecture 10:
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Solubility
•
Blackman Chapter 10, Sections 10.4
Lecture 11:
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Common ion effect
•
Blackman Chapter 10, Sections 10.4
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What Determines Solubility?
Solvent
Dissolution
Precipitation
Solute
• Dissolution
 break cation-anion ionic bonds vs make new ion – water bonds
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Dissolving Ionic Compounds in Water
Mn+(g) + Xn-(g)
lattice enthalpy
hydration enthalpy
enthalpy of solution
MX(s)
•
•
•
Mn+(aq) + Xn-(aq)
To dissolve a salt:
 need to break up lattice: harder
for small ions and for those with higher charges
 makes new ion-water bonds: more beneficial for
small ions and for those with higher charges
both increase with
ion charge and
decrease with ion
size!
Very difficult to guess what will be insoluble and what wiil be soluble!
See E2 experiment
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Solubility and Saturated Solutions
•
When dissolving a solid in solution, equilibrium means that as much
solid has dissolved as possible
 the solution is said to be saturated
 the solubility is the maximum amount that will dissolve
•
Solubility of magnesium(II) hydroxide is 9.63 mg L-1 or 1.65 × 10-4 M
Mg(OH)2(s)
•
Mg2+(aq) + 2OH-(aq)
By convention,
 if solubility > 0.1 M then compound is soluble
 if solubility < 1.0 × 10-3 M then compound is insoluble
 intermediate cases are said to be moderately soluble
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•
Definition: Solubility Equilibria
Solubility equilibria are always presented as dissolution
processes as in this example:
PbCl2(s)
•
Pb2+(aq) + 2Cl-(aq)
Since the concentration of a pure solid is
taken to be constant, the equilibrium
constant is:
Ksp = [Pb2+(aq)][Cl-(aq)]2
solubility product constant
Note that a high value of Ksp corresponds to a high solubility!
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Determine Solubility from Ksp (I)
PbCl2(s)
Pb2+(aq) + 2Cl-(aq)
• Imagine that someone dumps a truckload of lead(II) chloride
into a lake. What is the concentration of Pb2+(aq) ions in the
lake (assuming that a saturated solution results).
Ksp = [Pb2+(aq)][Cl-(aq)]2 = 1.6 x 10-5
• From chemical equation, [Cl-(aq)] = 2 x [Pb2+(aq)]
Ksp = [Pb2+(aq)](2 x [Pb2+(aq)])2 = 4 x [Pb2+(aq)]3
4[Pb2+(aq)]3 = 1.6 x 10-5 M3

[Pb2+(aq)] = 0.016 M
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The Ionic Product
• Important: Ksp is defined for a saturated solution in
equilibrium with some solid
• When the system is NOT at equilibrium
Define: Q = reaction quotient = ionic product
For PbCl2, Q = [Pb2+(aq)][Cl-(aq)]2
• Note: Q, the ionic product is a non-equilibrium parameter!
 Q = Ksp only at equilibrium.
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Ionic Product (Q) vs Solubility Product (Ksp)
For ionic compounds:
(a)
when Q < Ksp, further dissolution can occur.
(b)
when Q > Ksp, the solution is supersaturated,
precipitation will occur.
(c)
when Q = Ksp, the solution is saturated - the system
is at equilibrium.
Exactly the same as Q vs K in
equilibrium discussion in CHEM1001
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Supersaturated Solutions
a
b
c
Sodium acetate
A) Superaturated solution.
B) After seed crystal is added crystallisation starts.
C) Most of the precipitate has formed.
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Determine Solubility from Ksp (II)
PbCl2(s)
Pb2+(aq) + 2Cl-(aq)
Ksp = 1.6 x 10-5
• What happens if 1.0 tonne of PbCl2 is dumped into a lake
containing 50 cubic metres of water. Would all the PbCl2
dissolve?
• 1 tonne = (1 x106 g)/(278.11 g mol-1) = 3.6 x 103 mol
• volume = 50 m3 = 50 x 103 L
• [Pb2+(aq) = (3.6 x 103 mol)/(50 x 103 L) = 7.2 x 10-2 mol L-1
Q = [Pb2+(aq)][Cl-(aq)]2 = [Pb2+(aq)](2 x [Pb2+(aq)])2
= 4 x (7.2 x 10-2)3 mol3 L-3
= 1.5 x 10-3 mol3 L-3
Q > Ksp so it would not all dissolve.
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Summary: Solubility Equilibria I
Learning Outcomes - you should now be able to:
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•
•
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Complete the worksheet
Apply solubility equilibria (qualitative and
quantitative)
Use ionic product to determine solubility
Answer review problems 10.49 - 10.75 in Blackman
Next lecture:
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The common ion effect
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Practice Examples
1.What is the solubility product constant expression for Ag3PO4?
(a) Ksp = [Ag+][PO43–]
(b) Ksp = [Ag+][PO43–]3
(c) Ksp = [Ag+]3[PO43–]
(d) Ksp = 1/[Ag+][PO43–]
(e) Ksp = 3[Ag+][PO43–]
2. The Ksp for Cd(OH)2 is 5.9 × 10–15 at 25 oC. What is the solubility of Cd(OH)2?
Cd(OH)2(s)
Cd2+(aq) + 2OH–(aq)
(a) 7.7 × 10–8 mol L–1
(b) 5.4 × 10–8 mol L–1
(c) 1.1 × 10–5 mol L–1
(d) 5.9 × 10–15 mol L–1
(e) 1.8 × 10–5 mol L–1
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