E5 Lewis Acids and Bases (Session 1) November 5-11 Acids Bronsted: Acids are proton donors. Session one Pre-lab (p.151) due 1st hour discussion of E4 Lab (Parts 1and 2A) Problem Compounds containing cations other than + are acids! Session two Lab: Parts 2B, 3 and 4 Lewis Acids and Bases Problem: Some acids do not contain protons Defines acid/base without using the word proton: Example: Al3+ (aq) = p 3! Cl- + Acid O Cl- Base Base O + Acid A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S. Deodorants and acid loving plant foods contain aluminum salts Lewis Bases Lewis Acids Electron rich species; electron pair donors. Electron deficient species ; potential electron pair acceptors. Lewis acids: + I m deficient! Cu2+ Al3+ Ammonia hydroxide ion (ammine) (hydroxo) water (aquo) Acid 1
Example + + Lewis Acid-Base Reactions O Acid + Base + Complex ion The acid reacts with the base by bonding to one or more available electron pairs on the base. O The acid-base bond is coordinate covalent. The product is a complex or complex ion Metal ion BONDED to water molecules Metal ion surrounded by water molecules Lewis Acid-Base Reactions Lewis Acid-Base Reaction Products Net Reaction Examples Pb 2+ + 4 2 O [Pb( 2 O) 4 ] 2+ Lewis acid Lewis base Tetra aquo lead ion Lewis Acid-Base Reaction Products Net Reaction Examples + + 2 O [( 2 O)] + Lewis acid Lewis base ydronium ion Ni 2+ + 6 2 O [Ni( 2 O) 6 ] 2+ Lewis acid Lewis base exa aquo nickel ion Cu 2+ + 4 2 O [Cu( 2 O) 4 ] 2+ Lewis acid Lewis base Tetra aquo copper(ii)ion 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) [Cu( 2 O) 4 ] 2+ ydrated solid: [Cu( 2 0) 4 ]SO 4 tetra aquo copper(ii) sufate Part 1. Acidity of Cations Compare the p of nitrate salts containing different metal ions. 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+ 2
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 Ions Acidity of Metal Ion Model Reminder: Metal ions BOND to electron pairs on water molecules! 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] + Q. Complete a balanced equation to show formation of hydronium ions: [Al( 2 O) 6 ] 3+ + O Electron deficient proton/s in the bonded water molecule (of the metal aquo complex ion) are donated(bond) to an external water molecule [Al( 2 O) 5 (O)] 2+ + [ 3 O] + 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: [Al( 2 O) 6 ] 3+ + 2 O [Al( + 2 [ 3 O] + 2 O) 4 (O) 2 ] + 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? Part 2 Lewis Acid-Base Reactions Part 2 Lewis Acid-Base Reactions Lewis acids Metal aquo complex ions Formula for metal aquo complex ion is provided Lewis bases O - (NaO) N 3 Example Al exists as [Al( 2 O) 6 ] 3+ Add NaO Metal Aquo Complex Ion Reactions If reaction occurs with a charged Lewis base such as O -, the product may be a soluble complex ion or an uncharged insoluble complex. 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) 4
Addition of Base (NaO ) 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 - [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 acids bond to the best available base. - an acid does not react (bond) to any base. Wanna give me a try? Lewis Acid-Base reactions are reversible equilibrium systems Acid-Base + Base no reaction If a better base is available a Lewis acid will react (exchange partners)! Lewis Acid-Base Replacement Reactions Complexes react if a better partner (acid or base) is available so as to form a more stable bond. Acid-Base + better Base Reaction emoglobin is a complex of Fe that binds to/transports oxygen 5
Q. The acid exchanging base partners is +. Which is the BEST base, Cl - or O? Cl- + O Cl - + O Example: Replacement of water molecules in [Cu ( 2 O) 4 ] 2+ with ammonia molecules. [Cu ( 2 O) 4 ] 2+ + 4 N 3 [Cu(N 3 ) 4 ] 2+ + 4 2 O Acid Base Base 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 Example: Replacement of water molecules in [Cu ( 2 O) 4 ] 2+ with ammonia molecules. Net reaction: [Cu( 2 O) 4 ] 2+ + 4 N 3 [Cu(N 3 ) 4 ] 2+ + 4 2 O [Cu ( 2 O) 4 ] 2+ + 4 N 3 [Cu(N 3 ) 4 ] 2+ + 4 2 O Tetra aquo Cu(II) ion Tetra ammine Cu(II) ion [Cu( 2 O) 4 ](NO 3 ) 2 + 4 N 3 [Cu(N 3 ) 4 ](NO 3 ) 2 + 4 2 O Tetra aquo Cu(II) nitrate Tetra ammine Cu(II)nitrate 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 + [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. 6
Reactions with N3 Aquo Complex Ion Reactions If a precipitate forms upon addition of N3(aq), the metal aquo complex ion is reacting with the SMALL AMOUNT of O- ions present in N3(aq): N3(aq) + O(l) Colored transition metal complex ions alter color upon bonding to a different Lewis base. N4+ (aq) + O- (aq) Addition of N 3 to Cu 2+ resulting in a copper hydroxide ppt. [Cu( 2O) 4]2+ + 2 O- [Cu(2O)4]2+ [Cu(N3)4]2+ [Cu ( 2O)2(O)2] + 2 2O Complex Ions Aquo Complex Ion Reactions [Cu(2O)4]2+ [Cu(N3)4]2+ The Complex ions are the chemical basis for colorful paint pigments. charge on a aquo metal complex ion will = the metal ion charge if the bonded Lewis base is uncharged (e.g., 2O or N3) Example: Q. Complete the equation below: Replacement of neutral water molecules in [Cu(2O)4]2+ with a charged Lewis base - chloride ions. 2- + 4 O [Cu (2O)4]2+ + 4 Cl- [Cu(Cl) 4] 2 [Cu(2O)4]2+ [Cu(Cl)4]2- The charge on the complex ion alters due to replacement of a neutral Lewis base (water) with a charged Lewis base 7
Questions? Contact nkerner@umich.edu 8