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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]. A/Prof Adam Bridgeman (Series 1) Dr Feike Dijkstra (Series 2) Weeks 8 – 13

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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] A/Prof Adam Bridgeman (Series 1) Dr FeikeDijkstra (Series 2) Weeks 8 – 13 Office Hours: Monday 2-3, Friday 1-2 Room: 543a e-mail:adam.bridgeman@sydney.edu.au e-mail:feike.dijkstra@sydney.edu.au

  3. Summary of Last Lecture • Solubility Equilibria I • Saturated solutions contains solid salts in equilibrium with the maximum amount of the ions in solution that is possible • The solubility product (Ksp) is the equilibrium constant for this equilibrium situation • The solubility product (Ksp) gives the maximum concentrations of the ions in the solution and the maximum amount of solid that will dissolve • The ionic product (Qsp) has the same form has the solubility product and is used to test whether more solid will dissolve or precipitation will occur • If Qsp < Ksp, more ions can enter solution and more solid can dissolve • If Qsp > Ksp, precipitation must occur

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

  5. Reminder: Solubility Equilibria PbCl2(s) Pb2+(aq) + 2Cl-(aq) Ksp = [Pb2+(aq)][Cl-(aq)]2 solubility product constant • If Pb2+(aq) and Cl-(aq) are present in the same solution at the same time, their concentrations can never be so large that [Pb2+(aq)][Cl-(aq)]2 is bigger than Ksp

  6. Solubility PbCl2(s) Pb2+(aq) + 2Cl-(aq) Ksp = [Pb2+(aq)][Cl-(aq)]2 solubility product constant • If x mol of PbCl2(s) dissolves in 1 L of solution, then • [Pb2+(aq)] = x mol L-1 and [Cl-] = 2x mol L-1 • Ksp = [Pb2+(aq)][Cl-(aq)]2 = (x)(2x)2 = 4x3 • x = (Ksp/4)1/3 = solubility • As Ksp = 1.6 x 10-5, • solubility = (1.6 x 10-5/4)1/3 = 0.016 mol L-1

  7. The Common Ion Effect and Solubility PbCl2(s) Pb2+(aq) + 2Cl-(aq) Ksp = [Pb2+(aq)][Cl-(aq)]2 solubility product constant • If NaCl is added to the solution, [Cl-(aq)] increases so • equilibrium shifts to the left: less PbCl2 dissolves • [Pb2+(aq)][Cl-(aq)]2+ must remain constant so [Pb2+(aq)] decreases

  8. The Common Ion Effect and Solubility PbCl2(s) Pb2+(aq) + 2Cl-(aq) Ksp = [Pb2+(aq)][Cl-(aq)]2 solubility product constant • If NaCl is added to the solution, to give [Cl-(aq)] = 0.2 M • If x mol of PbCl2(s) dissolve in 1 L of solution, then • [Pb2+(aq)] = x mol L-1 and [Cl-] = 0.2 mol L-1 • Ksp = [Pb2+(aq)][Cl-(aq)]2 = (x)(0.2)2 = 0.04x • As Ksp = 1.6 x 10-5, • solubility = 1.6 x 10-5/0.04 = 0.0004 mol L-1

  9. Solubility and pH • Because of the common ion effect, solubility will be pH dependent if dissolution involves H+ and OH- • e.g. Mg(OH)2(s) Mg2+(aq) + 2OH-(aq) • In this case solubility will be higher at low pH values.

  10. Separation of Cations • Fe3+ and Zn2+ can be separated using the pH dependence of the solubilities of their salts. • For example, Fe(OH)3 and Zn(OH)2 can be separated at a pH of 4.76 (achieved by with a buffer). • Ksp {Fe(OH)3} = 1.0 x 10-38 = [Fe3+][OH-]3 • pOH = 14.00 - 4.76 so [OH-] = 10-9.24 M • [Fe3+] = 1.0 x 10-38/(10-9.24)3 M = 5.2 x 10-11 M Fe(OH)3 is highly insoluble at this pH.

  11. Separation of Cations • Ksp {Zn(OH)2} = 1.0 x 10-15= [Zn2+][OH-]2 • [OH-] = 10-9.24 M • [Zn2+] = 1.0 x 10-15/(10-9.24)2 M = 3.0 x 103 M Fe(OH)3 is highly insoluble at this pH. Zn(OH)2 is highly soluble at this pH

  12. Summary: Solubility EquilibriaII • Learning Outcomes - you should now be able to: • Complete the worksheet • Apply solubility equilibria(qualitative and quantitative) • Use ionic product to determine solubility • Apply the common ion effect • Answer review problems 10.49 - 10.75 in Blackman • Next lecture: • Complexes

  13. Practice Examples • 2. The Ksp for scandium(III) hydroxide is 2 x 10. What is the solubility of Sc(OH)3 (in mol L-1) of a solution buffered at 6.7?

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