E5 Lewis Acids and Bases (Session 1) March 19-24 Session one Pre-lab (p.151) due 1st hour discussion of E4 Lab (Parts 1and 2A) Session two Lab: Parts 2B, 3 and 4 Acids Bronsted: Acids are proton donors. Problem Compounds that do not contain are acids are acids; e.g. AlCl 3. Compounds containing cations other than + are acids! Problem: Some acids do not contain protons Example: Al 3+ (aq) = p 3! Deodorants and acid loving plant foods contain aluminum salts Lewis Acids and Bases Defines acid/base without using the word proton: Cl- + O Cl - + O Acid Base Base Acid A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S. Lewis Acids Electron deficient species ; potential electron pair acceptors. Lewis acids: + Cu 2+ Al 3+ I m deficient! Acid 1
Lewis Bases Lewis Acid-Base Reactions Electron rich species; electron pair donors. Example + + O + O Ammonia hydroxide ion water (ammine) (hydroxo) (aquo) Acid + Base Complex ion The acid reacts with the base by bonding to one or more available electron pairs on the base. The product is a complex or complex ion Lewis Acid-Base Reactions Lewis Acid-Base Reaction Net Reaction Examples + + 2 O [( 2 O)] + Lewis acid Lewis base ydronium ion Cu 2+ + 4 2 O [Cu( 2 O) 4 ] 2+ Lewis acid Lewis base Tetra aquo copper(ii)ion Metal ions are Lewis acids and BOND to electron pairs on water molecules (Lewis bases)! Metal Aquo Complex Ions The metal ions in a salt solution bond to water molecules to forms aquo complex ions. Example: Cu 2+ (aq) + SO 4 2- (aq) Part 1. Acidity of Cations Why do different cations in aqueous solution exhibit different acid strengths? Compare the p of nitrate salts containing different metal ions. [Cu( 2 O) 4 ] 2+ ydrated solid: [Cu( 2 0) 4 ]SO 4 tetra aquo copper(ii) sufate 2
Part 1. Acidity of Cations Metal Ion Acid Strength EXPERIMENT VARIABLE Nitrate salts with different metal ions EXPERIMENT CONSTANT Water to dissolve salts is from same source and of constant p (neutral or non-acidic). p of Na +, Mg 2+, and Al 3+ p comparison of chloride salts of aluminum, barium, and sodium Metal Ion Acid Strength Metal Ion Acid Strength: Al 3+ > Mg 2+ > Na + Results suggest that metal ion acidity and metal ion charge are linked Results suggest that metal ion acidity and metal ion oxidizing agent strength are linked 1A 1 1s1 IIA IIIA IVA VA VIA VIIA 3 4 Li Be 2s 1 2s 2 11 Na 3s 1 19 K 4s 1 37 Rb 5s 1 55 Cs 6s 1 87 Fr 7s 1 12 Mg 3s2 IIIB IVB VB VIB VIIB VIIIB! VIIIB IB IIB 20 Ca 4s 2 38 Sr 5s 2 56 Ba 6s 2 88 Ra 7s 2 21 22 23 24 25 26 27 28 Sc Ti V Cr Mn Fe Co Ni 3d 1 4s 2 3d 2 4s 2 3d 3 4s 2 3d 5 4s 1 3d 5 4s 2 3d 6 4s 2 3d 7 4s 2 3d 8 4s 2 39 40 41 42 43 44 45 46 Y Zr Nb Mo Tc Ru Rh Pd 4d 1 5s 2 4d 2 5s 2 4d 3 5s 2 4d 5 5s 1 4d 5 5s 2 4d 7 5s 1 4d 8 5s 1 4d 10 57 La* 5d 1 6s 2 Metal Ion Acidity Acid strength Oxidizing agent strength VIIIA 2 e 1s 2 5 6 7 8 9 10 B C N O F Ne 2s 2 2p 1 2s 2 2p 2 2s 2 2p 3 2s 2 2p 4 2s 2 2p 5 2s 2 2p 6 13 14 15 16 17 18 Al Si P S Cl Ar 3s 2 3p 1 3s 2 3p 2 3s 2 3p 3 3s 2 3p 4 3s 2 3p 5 3s 2 3p 6 29 Cu 3d 10 4s 1 30 Zn 3d 10 4s 2 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 4s 4p 1 4s 4p 2 4s 4p 3 4s 4p 4 4s 4p 5 4s 4p 6 47 Ag 4d 10 5s 1 48 Cd 4d 10 5s 2 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe 5s 5p 1 5s 5p 2 5s 5p 3 5s 5p 4 5s 2 5p 5 5s 5p 6 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 f Ta W Re Os Ir Pt Au g Tl Pb Bi Po At Rn 5d 2 6s 2 5d 3 6s 2 5d 4 6s 2 5d 5 6s 2 5d 6 6s 2 5d 7 6s 2 5d 9 6s 1 5d 10 6s 1 5d 10 6s 2 6s 2 6p 1 6s 2 6p 2 6s 2 6p 3 6s 2 6p 4 6s 2 6p 5 6s 2 6p 6 89 104 105 106 107 108 109 + Element synthesized, Ac # + + + + + + but no official name assigned 6d 1 7s 2 6d 2 7s 2 6d 3 7s 2 6d 4 7s 2 6d 5 7s 2 6d 6 7s 2 6d 7 7s 2 Acidity of Metal Ion Model M x+ +... O + Metal ion acid strength is related to its ability to attract and bond to electrons (oxidizing agent strength) When a metal ion bonds to a water molecule, the water molecule s polarity increases and the protons in the water molecule become more electron deficient Acidity of Metal Ion Model Equation describing acidity of metal ions: [X( 2 O) 6 ] 2+ + 2 O [X( 2 O) 5 (O)] + + [- 2 O] + Electron deficient proton/s in the bonded water molecule (of the metal aquo complex ion) are donated(bond) to an external water molecule When protons bond to water molecules, 3 O + ions are formed producing a p drop. 3
Q. Complete a balanced equation to show formation of hydronium ions: Q. Complete a balanced equation to show formation of hydronium ions: [Al( 2 O) 6 ] 3+ + O [Al( 2 O) 6 ] 3+ + 2 O [Al( 2 O) 5 (O)] 2+ + [ 3 O] + [Al( 2 O) 4 (O) 2 ] + + 2 [ 3 O] + Part 2. Complexation Reactions The reaction of a Lewis acid with a Lewis base (N 3, O -, ). Data Analysis What kinds of observations allow you to know that a complexation reaction is occurring? Are Lewis acid-base reaction results predictable from a) metal ion acid strength? b) the position of the metal ion s element in the Periodic Table? Lewis acids [Cu( 2 O) 4 ] 2+ [Ag( 2 O) 2 ] + [Zn( 2 O) 4 ] 2+ Part 2. Complexation Reactions Assigned team metal ions from Part 1 Lewis bases O - (NaO) N 3 Lewis acids bond to the best available base. - an acid does not react (bond) to any base. If a better base is available a Lewis acid will react (exchange partners)! Wanna give me a try? Acid-Base + Base no reaction Acid-Base + better Base Reaction 4
Q. The acid exchanging base partners is +. Which is the BEST base, Cl- or O? Cl- + Acid O Cl- Base Base O Lewis Acids bond to the best Lewis Base Example: + Replacement of water molecules in [Cu (2 O)4 ]2+ with ammonia molecules. [Cu (2O)4]2++ 4 N3 [Cu(N3)4]2+ + 4 2O Acid + bonds to water (best base) rather than Cl-. The bonded base-acid product = complex ion. Tetra aquo Cu(II) ion Tetra ammine Cu(II) ion Complexation Reactions Colored aquo transition metal complex ions alter color upon bonding to a different Lewis base. [Cu(2O)4]2+ [Cu (2O)4]2++ 4 N3 [Cu(N3)4]2+ + 4 2O Complex Ion Charge [Cu(N3)4]2+ Complex Ion Charge The charge on a metal complex ion = the metal ion charge if the bonded Lewis base is uncharged (e.g., 2O or N3) [Cu(2O)4]2+ [Cu(Cl)4]2- The [Cu(2O)4]2+ [Cu(N3)4]2+ charge on a metal complex ion will NOT = the metal ion charge if the bonded Lewis base is charged (e.g., Cl- or O- ) 5
Complex Ion Charge Complex or Complex Ion? If the Lewis base is charged (e.g., O - ) and the reaction stoichiometry with the Lewis acid results in a product with no charge, a complex has formed the complex will be insoluble and precipitate Q. Complete the equation below: [Cu ( 2 O) 4 ] 2+ + 4 Cl - [Cu(Cl) 4 ] 2- + 4 2 O [Cu( 2 O) 4 ] 2+ + O - [Cu ( 2 O) 2 (O) 2 ] If a precipitate forms upon addition of N 3 (aq), the metal aquo complex ion is reacting with the SMALL AMOUNT of O - ions present in N 3 (aq): N 3 (aq) + O(l) Reactions with N 3 N 4 + (aq) + O - (aq) Lewis Acid-Base Reactions If a metal ion reacts with a charged Lewis base such as O -, the product may be a soluble complex ion or an uncharged insoluble complex depending on reaction stoichiometry. Addition of N 3 to Cu 2+ resulting in a copper hydroxide ppt. [Cu( 2 O) 4 ] 2+ + 2 O- [Cu ( 2 O) 2 (O) 2 ] + 2 2 O 1:1 1:2 1:3 1:4 1:5 1: 6 Stoichiometry of Reaction Products: Al 3+ to O - Metal Aquo Complex Ion Reactions Formation of a soluble complex ion: [Al ( 2 O) 6 ] 3+ + O - [Al( 2 O) 5 (O)] 2+ + 2 O Metal Aquo Complex Ion Reactions Formation of an insoluble complex: [Al ( 2 O) 6 ] 3+ + 3O - [Al( 2 O) 3 (O) 3 ] + 3 2 O Precipitation reactions are Lewis acid-base reactions! Traditional net precipitation equation: Al 3+ (aq) + 3O - (aq) Al(O) 3 (s) 6
Metal Aquo Complex Ion Reactions Addition of excess hydroxide ions to the precipitated hydroxide complex results in the formation of a soluble complex ion: [Al ( 2 O) 3 (O) 3 ] + 3O - [Al(O) 6 ] 3- + 3 2 O Lewis Acid-Base Reactions Note that water molecules in the metal ion aquo complex are replaced with O - ions [Al ( 2 O) 6 ] + 6 O - [Al(O) 6 ] 3- + 6 2 O 1:1 1:2 1:3 1:4 1:5 1: 6 Stoichiometry of Reaction Products: Al 3+ to O - Addition of Base (NaO ) [Al( 2 O) 6 ] 3+ [Al( 2 O) 5 (O)] 2+ [Al( 2 O) 4 (O) 2 ] + [Al( 2 O) 3 (O) 3 ] [Al( 2 O) 2 (O) 4 ] - [Al( 2 O) (O) 5 ] 2- [Al(O) 6 ] 3- Addition of acid (+) Lewis Acid-Base Replacement Reactions Complexes react if a better partner (acid or base) is available so as to form a more stable bond. Lewis Acid-Base reactions are reversible equilibrium systems emoglobin is a complex of Fe that binds to/transports oxygen Complex Ions Complex ions are the chemical basis for colorful paint pigments. Questions? Contact nkerner@umich.edu 7
Complexation Reactions The base N 3 bonded to Cu 2+ in [Cu(N 3 ) 4 ] 2+ will exchange partners if a better Lewis acid than Cu(II) ions are available. : Addition of + Complexation Reactions [Cu( 2 O) 4 ] 2+ + 4 N 3 [Cu(N 3 ) 4 ] 2+ + 4 2 O + + N 4 + [Cu(N 3 ) 4 ] 2+ Upon addition of +, N 3 bonds to + (rather than Cu 2+ ). Cu 2+ re-bonds to 2 O. Upon addition of +, N 3 bonds to + (rather than Cu 2+ ). Cu 2+ re-bonds to 2 O. 8