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AP Chemistry 3-4-17 Place your test corrections on your desktop for collection. HW: Read 6.2-6.3 by Monday and do exercises 48, 50, 56, 60, 64, 66, 72, 74 Handouts: None Warm-up/Objective and Notes AP Chemistry 3-4-17 Objective: To master equilibrium of complex ions Warm-up #3: When a complex ions forms, what is the Lewis acid and what is the Lewis base? Answer: The metal ion is the Lewis acid accepting a pair of electrons from the ligand which is the Lewis base Qualitative Analysis This is the process of precipitating out the ions from each of the 5 groups sequentially and identifying them. This is a common type experiment in an advanced inorganic chemistry lab in college. We will come back to this at the end of today’s talk. But first some equilibrium… Equilibria Involving Complex Ions A complex ion is a charged species consisting of a metal ion (typically a transition metal) surrounded by ligands. A ligand is a Lewis base that donates a pair of electrons to an empty orbital of the metal ion (a Lewis acid) to form a dative covalent bond. Common ligands are H2O, NH3, Cl-, and CN- Coordination number The number of ligands attached to the metal ion is its coordination number. The most common coordination numbers are: 6 with examples Co(H2O)62+ and Ni(NH3)63+ 4 with examples CoCl42- and Cu(NH3)42+ 2 with example Ag(NH3)21+ Example complex ions-all with coordination number of 6 The ligand donates a pair of electrons to a small, positively charged metal cation to form the complex ion. Equilibrium Involving Complex Ions Metal ions add ligands one at a time with formation constants, K1, K2, K3 etc. Formation of Ag(NH3)2 occurs in two steps from Ag1+ and NH3: Ag1+(aq) + NH3(aq) ⇌ Ag(NH3)1+(aq) K1 = 2.1 x 103 Ag(NH3)1+(aq)+ NH3(aq) ⇌ Ag(NH3)21+(aq) K2 = 8.2 x 103 Equilibrium Involving Complex Ions Consider a solution of 100.0 mL of 2.0 M NH3 with 100.0 mL 1.0 x 10-3 M AgNO3. Before a reaction occurs, what are the major species? NH3, H2O, Ag1+ and NO31What reactions will occur in this solution? Equilibrium Involving Complex Ions What reactions will occur in this solution? Well, NH3 will react with water: NH3(aq) + H20(aq) ⇌ NH41+(aq) + OH1- Kb = 1.8 x 105 Will a significant amount of NH3 be removed and converted to NH41+? No, Kb is small so our 2.0 M NH3 conc. Won’t change significantly. Equilibrium Involving Complex Ions What are our initial conc. of Ag1+ and NH3? [Ag1+]0 = (0.100L)(1.0x10-3M)/0.200 L = 5.0 x 10-4 M [NH3]0= (0.100L)(2.0 M)/0.200 L = 1.0 M Assumptions will help us to figure out the concentration of all species at equilibrium. KEY assumption: both K1 and K2 are large (>103) so almost ALL Ag1+ will end up Ag(NH3)21+ Equilibrium Involving Complex Ions Setting up: Ag1+ + 2NH3 Ag(NH3)21+ Before 5.0 x 10-4M 1.0 M 0 After 0 1.0-2(5.0x10-4)M 5.0x10-4M Twice as much as Ag1+ is required Equilibrium Involving Complex Ions At equilibrium, the conc of Ag(NH3)21+ will equal [Ag1+]0 (5.0 x 10-4M) and the conc of NH3 will be [NH3]0 (1.0M) However, there will be small amounts of Ag(NH3)1+ and Ag1+, so those must be calculated −4 ] [𝐴𝑔 𝑁𝐻3 1+ ] [5.0×10 2 = [𝐴𝑔(𝑁𝐻3 )][𝑁𝐻3 ] [𝐴𝑔(𝑁𝐻3 )1+ ][1.0𝑀] 𝐾2 = 8.2 × 103 = Ag(NH3)1+=6.1 x 10-8M Equilibrium Involving Complex Ions Now determine [Ag1+] 𝐾1 = 2.1 × 103 = [𝐴𝑔 𝑁𝐻3 1+ ] [𝐴𝑔1+ ][𝑁𝐻3 ] = [6.1×10−8 ] [𝐴𝑔1+ ][1.0𝑀] Ag1+ = 2.9 x 10-11 M [Ag(NH3)21+] >>[Ag(NH3)1+]>>[Ag1+] Qualititative Analysis the Solubility of Complex Ions Suppose you want to identify WHICH ions precipitate out of solution in the first 4 groups or are left in solution (Group V). We can actually identify which ions are present based on the solubility of the complex ions. I will show you how you can identify which ions are present in the Group I precipitation (those that precipitate in HCl). Complex Ions and Solubility Suppose we have a solution containing one or more Ag1+, Pb2+ or Hg22+. •First we treat the solution with HCl and form the insoluble chlorides. •Next, heat and filter to remove Pb2+ ions •Then solubilized the AgCl and separate it from the possible remaining precipitated chlorides. How? Add concentrated NH3 to form the complex ions Ag(NH3)21+ which will solubilize all Ag1+ ions. Complex Ions and Solubility Quantitation First let’s write the reactions occurring in the solution for Ag. AgCl(s) ⇌ Ag+(aq) + Cl-(aq) Ksp = 1.6 x 10-10 Ag+(aq) + NH3(aq) ⇌ Ag(NH3)+(aq) K1 = 2.1 x 103 Ag(NH3)+(aq) + NH3(aq) ⇌ Ag(NH3)2+(aq) K2 = 8.2 x 103 ------------------------------------------------------------------------AgCl(s) + 2NH3(aq) ⇌ Ag(NH3)2+(aq) +Cl- Koverall = 2.8 x 10-3 Complex Ions and Solubility Koverall = 2.8 x 10-3 = [Ag(NH3)2+][Cl1-]/[NH3]2 In 10.0 M NH3, what is the solubility of AgCl(s)? Let x = molarity of Ag(NH3)2+ 2.8 x 10-3 = x2/(10.0-2x)2 2.8𝑥10−3 𝑥 = 10.0 − 2𝑥 x = 0.48 M of AgCl in 10.0 M NH3 Complex Ions and Solubility