Solubility & Simultaneous Equilibria Part II: Effect of pH, Complex Ion Formation & Selective Precipitation Jespersen Chap. 18 Sec 3, 4 & 5 Dr. C. Yau Spring 2013 1

Effect of pH on Solubility Consider the following equilibrium: CaCO3 (s) Ca2+ (aq) + CO32- (aq) Why would the pH affect the solubility? Could addition of H3O+ affect the solubility? Would you expect it to affect Ca2+ or CO32-? CO32- (aq) + H3O+ (aq) HCO3- (aq) + H2O (l) HCO3- (aq) + H3O+ (aq) H2CO3 (aq) + H2O (l) H2O (l) + CO2 (g) If the cmpd contains the anion of a weak acid, addition of H3O+ (from a strong acid) increases its solubility. 2

Addition of acid to a mineral sample is a field test to identify presence of carbonates. What would one look for as a positive test for carbonates? CO32- (aq) + H3O+ (aq) HCO3- (aq) + H2O (l) HCO3- (aq) + H3O+ (aq) H2CO3 (aq) + H2O (l) H2O (l) + CO2 (g) If the addition of acid to a mineral sample produces effervescence, the sample contains carbonates.

Effect of Adding Strong Acid Write balanced eqns to explain whether addition of a strong acid would affect the solubility of the following compounds: a) lead(II) bromide b) copper(II) hydroxide c) iron(II) sulfide Write equations to explain the effect.

Example 18.8 p. 848 If a solution contains 0.10 M Cl- and 0.10 M I-, then what conc of AgNO3 will allow the separation of these halides by selective precipitation? Ksp of AgCl = 1.8x10-10 Ksp of AgI = 8.5x10-17 Pract Exer 14 & 15 p.849

Quantitative Treatment of Effect of pH on Solubility #18.52 p. 864 What is the molar solubility of Fe(OH)2 in a buffer that has a pH of 9.50? Why might pH affect the solubility? Would you expect it to be more or less soluble in pH of 9.50? Revised Ans. 4.8x10-8 M Practice Exercise 9 p. 840 and p. 864 #18.53

Selective Precipitation Consider the Ksp of AgCl and Ag2CrO4: Ksp Solubility AgCl 1.8x10-10 1.3x10-5 M Ag2CrO4 2.6x10-12 8.7x10-5 M If we add Ag+ to a soln containing 0.10 M Cl- and 0.10 M CrO42-, what will happen? Ans. Ag2CrO4 is more soluble. AgCl will precipitate out first, leaving CrO42- in solution.

Ag+ CrO42-(aq) Cl- (aq) CrO42-(aq) AgCl(s) CrO42- and Cl- can be separated by adding Ag+. Ag+ will precipitate out the Cl- as AgCl (s). The mixture is centrifuged to spin down the white AgCl ppt. The supernatant soln containing the CrO42- is then pipetted out.

Complex Ion Formation Metal ions can combine with anions or neutral molecules to form complex ions: Cu2+ + 4 H2O Cu(H2O)42+ Fe2+ + 6 CN Fe(CN)64 The H2O and CN in the above reactions are called ligands. They are acting as Lewis bases. The metal cations are acting as Lewis acids. (Metal cations are e- pair acceptors.)

Aqueous Metal Ions Are Complex Ions Water acts as a ligand, the Lewis base that forms a coordinate covalent bond with the metal. In the solvation of ionic compounds, ions are dissolved in water through ion-dipole interactions Complex ions are soluble, hence complexation is a means of dissolving some solids. Figure 17.7 The complex ion of Cu2+ and ammonia. Ammonia molecules displace water molecules from Cu(H2O)42+ (left) to give the deep blue Cu(NH3)42+ ion (right).

Writing Complex Ion Formation Eqns Write the equation for the complex ion formation of the aluminum ion with 4 hydroxide as ligands. Al3+ (aq) + 4 OH- (aq) Write the equation for the complex ion formation of the tin(IV) ion with 6 fluoride ions as ligands. Write the equation for the complex ion formation of the mercury(II) ion with 4 chloride ions as ligands. Practice: p. 865 #18.68, #18.72

Complexation & Kinst Complexes are governed by the instability constant, Kinst Ag(NH3)2+ Ag+ + 2NH3 When we reverse an equation, we invert K, thus Kform=1/Kinst. Ag+ + 2NH3 Ag(NH3)2+ Practice: p. 865 #74 ChemFAQ: Estimate the solubility of a metal salt in the presence of a ligand.

Complex Ion Formation Increases Solubility of a Salt Consider the dissolution of AgBr(s) and what happens when NH3 is added: AgBr (s) Ag+ (aq) + Br- (aq) Ag(NH3)2+ + 2 NH3 AgBr becomes more soluble.

Classical Scheme for Identifying "Group I Cations" Ksp Solubility Hg2Cl2 1.2x10-18 6.7x10-7 M AgCl 1.8x10-10 1.3x10-5 M PbCl2 1.7x10-5 1.6x10-2 M Given a soln possibly containing some combination of Hg22+, Ag+ and Pb2+ ions, you are to determine which one or ones are present.

Solubility Hg2Cl2. 6. 7x10-7 M AgCl. 1. 3x10-5 M PbCl2. 1 Solubility Hg2Cl2 6.7x10-7 M AgCl 1.3x10-5 M PbCl2 1.6x10-2 M (soluble when hot) Write a flowchart for this analysis. To unknown soln, add excess amt of 6MHCl. If a ppt forms, one or more of the cations are present. Heat the mixture to boiling and remove the supernatant. To the supernatant, add K2CrO4. If a yellow ppt forms, Pb2+ is present. To the ppt, add 6 M NH3. If a ppt remains, Hg22+ is present. To the supernatant, neutralize the NH3 with HCl. If a white ppt forms, Ag+ is present.