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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 . CHEM1002 [Part 2]. Dr Michela Simone Weeks 8 – 13 Office Hours: Monday 3-5, Friday 4-5

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  1. 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

  2. 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

  3. 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

  4. Solubility Equilibria • Lecture 10: • Solubility • Blackman Chapter 10, Sections 10.4 • Lecture 11: • Common ion effect • Blackman Chapter 10, Sections 10.4

  5. What Determines Solubility? Solvent Dissolution Precipitation Solute • Dissolution • break cation-anion ionic bonds vsmake new ion – water bonds

  6. 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: harderfor 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

  7. 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 × 10-4M 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

  8. 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!

  9. 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

  10. 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 = Ksponly at equilibrium.

  11. 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 QvsK inequilibrium discussion in CHEM1001

  12. Supersaturated Solutions a b c • Sodium acetate • Superaturated solution. • After seed crystal is added crystallisation starts. • Most of the precipitate has formed.

  13. 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.

  14. Summary: Solubility Equilibria I • Learning Outcomes - you should now be able to: • 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: • The common ion effect

  15. Practice Examples • 2. The Ksp for Cd(OH)2 is 5.9 × 10–15 at 25 oC. What is the solubility of Cd(OH)2? • (a) 7.7 × 10–8 mol L–1 • (b) 5.4 × 10–8mol L–1 • (c) 1.1 × 10–5mol L–1 • (d) 5.9 × 10–15mol L–1 • (e) 1.8 × 10–5mol L–1 Cd(OH)2(s) Cd2+(aq) + 2OH–(aq)

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