1Ag+(aq) + 2 H2O(l) Ag(H2O)2+(aq) Complex Ion Formationtransition metals tend to be good Lewis acidsthey often bond to one or more H2O molecules to form a hydrated ionH2O is the Lewis base, donating electron pairs to form coordinate covalent bondsAg+(aq) + 2 H2O(l) Ag(H2O)2+(aq)ions that form by combining a cation with several anions or neutral molecules are called complex ionse.g., Ag(H2O)2+the attached ions or molecules are called ligandse.g., H2O
2Complex Ion Equilibria if a ligand is added to a solution that forms a stronger bond than the current ligand, it will replace the current ligandAg(H2O)2+(aq) + 2 NH3(aq) Ag(NH3)2+(aq) + 2 H2O(l)generally H2O is not included, since its complex ion is always present in aqueous solutionAg+(aq) + 2 NH3(aq) Ag(NH3)2+(aq)
3Ag+(aq) + 2 NH3(aq) Ag(NH3)2+(aq) Formation Constantthe reaction between an ion and ligands to form a complex ion is called a complex ion formation reactionAg+(aq) + 2 NH3(aq) Ag(NH3)2+(aq)the equilibrium constant for the formation reaction is called the formation constant, Kf
4The stepwise exchange of NH3 for H2O in M(H2O)42+. M(H2O)3(NH3)2+3NH3M(NH3)42+
6The Effect of Complex Ion Formation on Solubility In general: the solubility of an ionic compound containing a metal cation, that forms a complex ion, increases in the presence of aqueous ligands
7Kf = [Ag(NH3)2+] [Ag+][NH3]2 COMPLEX ION EQUILIBRIATransition metal Ions form coordinate covalent bonds withmolecules or anions having a lone pair of e-.AgCl(s) Ag+ + Cl Ksp = 1.82 x 10-10Ag NH3 Ag(NH3) Kf = 1.7 x 107AgCl + 2NH3 Ag(NH3) Cl Keq = Ksp x KfComplex Ion: Ag(NH3)2+ which bonds like: H3N:Ag:NH3metal = Lewis acidligand = Lewis baseKf = [Ag(NH3)2+] [Ag+][NH3]2adding NH3 to a solution in equilibrium with AgCl(s) increases the solubility of Ag+
9Cu2+(aq) + 4 NH3(aq) Cu(NH3)22+(aq) Ex – mL of 1.5 x 10-3 M Cu(NO3)2 is mixed with mL of 0.20 M NH3. What is the [Cu2+] at equilibrium?Write the formation reaction and Kf expression.Look up Kf valueDetermine the concentration of ions in the diluted solutionsCu2+(aq) + 4 NH3(aq) Cu(NH3)22+(aq)
10Cu2+(aq) + 4 NH3(aq) Cu(NH3)22+(aq) Ex – mL of 1.5 x 10-3 M Cu(NO3)2 is mixed with mL of 0.20 M NH3. What is the [Cu2+] at equilibrium?Create an ICE table. Since Kf is large, assume all the Cu2+ is converted into complex ion, then the system returns to equilibriumCu2+(aq) + 4 NH3(aq) Cu(NH3)22+(aq)[Cu2+][NH3][Cu(NH3)22+]Initial6.7E-40.11Change-≈6.7E-4-4(6.7E-4)+ 6.7E-4Equilibriumx
11Cu2+(aq) + 4 NH3(aq) Cu(NH3)22+(aq) Ex – mL of 1.5 x 10-3 M Cu(NO3)2 is mixed with mL of 0.20 M NH3. What is the [Cu2+] at equilibrium?Cu2+(aq) + 4 NH3(aq) Cu(NH3)22+(aq)Substitute in and solve for xconfirm the “x is small” approximation[Cu2+][NH3][Cu(NH3)22+]Initial6.7E-40.11Change-≈6.7E-4-4(6.7E-4)+ 6.7E-4Equilibriumxsince 2.7 x << 6.7 x 10-4, the approximation is valid
12Sample Problem 2Calculating the Effect of Complex-Ion Formationon SolubilityPROBLEM:In black-and-white film developing, excess AgBr is removed from the film negative by “hypo”, an aqueous solution of sodium thiosulfate (Na2S2O3), through formation of the complex ion Ag(S2O3)23-. Calculate the solubility of AgBr in (a) H2O; (b) 1.0M hypo. Kf of Ag(S2O3)23- is 4.7x1013 and Ksp AgBr is 5.0x10-13.PLAN:
13Practice Problems on Complex Ion Formation Q 1. Calculate [Ag+] present in a solution at equilibrium when concentrated NH3 is added to a M solution of AgNO3 to give an equilibrium concentration of [NH3] = 0.20M.Q2. Silver chloride usually does not ppt in solution of 1.0 M NH3. However AgBr has a smaller Ksp. Will AgBr ppt form a solution containing M AgNO3, M NaBr and 1.0 M NH3? Ksp = 5.0 x 10-13Q3. Calculate the molar solubility of AgBr in 1.0M NH3?
14Solubility of Amphoteric Metal Hydroxides many metal hydroxides are insolubleall metal hydroxides become more soluble in acidic solutionshifting the equilibrium to the right by removing OH−some metal hydroxides also become more soluble in basic solutionacting as a Lewis base forming a complex ionsubstances that behave as both an acid and base are said to be amphotericsome cations that form amphoteric hydroxides include Al3+, Cr3+, Zn2+, Pb2+, and Sb2+
15Amphoteric ComplexesMost MOH and MO compounds are insoluble in water but some will dissolve in a strong acid or base. Al3+, Cr3+, Zn2+, Sn2+, Sn4+, and Pb2+ all form amphoteric complexes with water.Al(H2O) OH- ⇆ Al(H2O)5(OH)2+ + H2OAl(H2O)5(OH)2+ + OH- ⇆ Al(H2O)4(OH)2+ + H2OAl(H2O)4(OH)2+ + OH- ⇆ Al(H2O)3(OH)3 + H2OAl(H2O)3(OH)3 + OH- ⇆ Al(H2O)2(OH)4- + H2O
17Qualitative Analysisan analytical scheme that utilizes selective precipitation to identify the ions present in a solution is called a qualitative analysis schemewet chemistrya sample containing several ions is subjected to the addition of several precipitating agentsaddition of each reagent causes one of the ions present to precipitate out
18Selective Precipitation a solution containing several different cations can often be separated by addition of a reagent that will form an insoluble salt with one of the ions, but not the othersa successful reagent can precipitate with more than one of the cations, as long as their Ksp values are significantly different
19Sample Problem 3Separating Ions by Selective PrecipitationPROBLEM:A solution consists of 0.20M MgCl2 and 0.10M CuCl2. Calculate the [OH-] that would separate the metal ions as their hydroxides. Ksp of Mg(OH)2= is 6.3x10-10; Ksp of Cu(OH)2 is 2.2x10-20.