E5 Lewis Acids and Bases (Session 1) Acids November 5 - 11 Session one Bronsted: Acids are proton donors. • Pre-lab (p.151) due • Problem • 1st hour discussion of E4 • Compounds containing cations other than • Lab (Parts 1and 2A) H+ are acids! Session two DEMO • Lab: Parts 2B, 3 and 4 Problem: Some acids do not contain protons Lewis Acids and Bases Example: Al3+ (aq) = ≈ pH 3! Defines acid/base without using the word proton: + H H H • Cl-H + • O Cl- • • •• O H H Acid Base Base Acid . A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S . Deodorants and acid loving plant foods contain aluminum salts Lewis Acids Lewis Bases . Electron rich species; electron pair donors. Electron deficient species ; potential electron pair acceptors. Lewis acids: H+ Cu2+ Al3+ “I’m deficient!” Ammonia hydroxide ion water__ (ammine) (hydroxo) (aquo) Acid 1 Lewis Acid-Base Reactions Lewis Acid-Base Reactions Example Metal ion + BONDED H H H + • to water H + • O O • • •• molecules H H Metal ion Acid + Base Complex ion surrounded by water . The acid reacts with the base by bonding to one molecules or more available electron pairs on the base. The acid-base bond is coordinate covalent. The product is a complex or complex ion Lewis Acid-Base Reaction Products Lewis Acid-Base Reaction Products Net Reaction Examples Net Reaction Examples 2+ 2+ + + Pb + 4 H2O [Pb(H2O)4] H + H2O [H(H2O)] Lewis acid Lewis base Tetra aquo lead ion Lewis acid Lewis base Hydronium ion 2+ 2+ 2+ 2+ Ni + 6 H2O [Ni(H2O)6] Cu + 4 H2O [Cu(H2O)4] Lewis acid Lewis base Hexa aquo nickel ion Lewis acid Lewis base Tetra aquo copper(II)ion DEMO DEMO Metal Aquo Complex Ions Part 1. Acidity of Cations . The metal ions in a salt solution bond to water molecules to forms aquo complex ions. • Compare the pH of nitrate salts containing different metal ions. Example: EXPERIMENT VARIABLE Cu2+(aq) + SO 2- (aq) 4 • Nitrate salts with different metal ions 2+ EXPERIMENT CONSTANT [Cu(H2O)4] • Water to dissolve salts is from same source and of constant pH (neutral or non-acidic). Hydrated solid: DEMO [Cu(H20)4]SO4 DEMO pH of Na+, Mg2+, and Al3+ tetra aquo copper(II) sufate 2 Metal Ion Acidity Metal Ion Acid Strength Acid strength 3+ 2+ + 1A VIIIA Metal Ion Acid Strength: ___________________Al > Mg > Na Oxidizing agent strength 1 2 H He 2 1s1 IIA IIIA IVA VA VIA VIIA 1s • Results suggest that metal ion acidity and 3 4 5 6 7 8 9 10 Li Be B C N O F Ne metal ion charge are linked 2s1 2s2 2s22p12s22p2 2s22p32s22p4 2s22p52s22p6 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar • Results suggest that metal ion acidity and 3s1 3s2 IIIB IVB VB VIB VIIB VIIIB ⇔ VIIIB IB IIB 3s23p13s23p2 3s23p33s23p4 3s23p53s23p6 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 metal ion oxidizing agent strength are linked K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 10 1 10 2 4s1 4s2 3d14s23d24s23d34s23d54s13d54s23d64s2 3d74s2 3d84s2 3d 4s 3d 4s 4s24p14s24p2 4s24p34s24p4 4s24p54s24p6 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 10 1 10 2 5s1 5s2 4d15s24d25s24d35s24d55s14d55s24d75s1 4d85s1 4d10 4d 5s 4d 5s 5s25p15s25p2 5s25p35s25p4 5s25p55s25p6 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La* Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 10 1 10 2 6s1 6s2 5d16s25d26s25d36s25d46s25d56s25d66s2 5d76s2 5d96s1 5d 6s 5d 6s 6s26p16s26p2 6s26p36s26p4 6s26p56s26p6 87 88 89 104 105 106 107 108 109 + Element synthesized, Fr Ra Ac# + + + + + + but no official name assigned 7s1 7s2 6d17s26d27s26d37s26d47s26d57s26d67s2 6d77s2 Acidity of Metal Ions Acidity of Metal Ion Model Reminder: ∝ + . H Metal ions x+ . BOND M ..O ∝ + to electron H pairs on . Metal ion acid strength is related to its ability to water molecules! 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 Q. Complete a balanced equation to show formation of hydronium ions: Equation describing acidity of metal ions: 2+ + + 3+ [X(H2O)6] + H2O → [X(H2O)5(OH)] + [H-H2O] [Al(H2O)6] + HOH . Electron deficient proton/s in the bonded water 2+ + [Al(H2O)5(OH)] + [H3O] molecule (of the metal aquo complex ion) are donated(bond) to an external water molecule + . When protons bond to water molecules, H3O ions are formed producing a pH drop. 3 Part 2. Complexation Reactions Q. Complete a balanced equation to show formation of hydronium ions: • The reaction of a Lewis acid with a Lewis base - (NH3, OH , …). 3+ [Al(H2O)6] + 2 HOH Data Analysis • What kinds of observations allow you to know that a complexation reaction is occurring? [Al(H O) (OH) ]+ + 2 [H O]+ 2 4 2 3 • 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 Example Metal aquo complex ions 3+ • • Al exists as [Al(H2O)6] • Formula for metal aquo complex ion is provided • Add NaOH Lewis bases • OH- (NaOH) • NH3 DEMO Metal Aquo Complex Ion Reactions Metal Aquo Complex Ion Reactions . If reaction occurs with a charged Lewis base Formation of an insoluble complex: such as OH-, the product may be a soluble complex ion or an uncharged insoluble complex. 3+ - [Al (H2O)6] + 3OH → [Al(H2O)3(OH)3] + 3H2O Formation of a soluble complex ion: 3+ - 2+ [Al (H2O)6] + OH → [Al(H2O)5(OH)] + H2O • Precipitation reactions are Lewis acid-base reactions! Traditional net precipitation equation: 3+ - Al (aq) + 3OH (aq) → Al(OH)3 (s) 4 Metal Aquo Complex Ion Reactions Lewis Acid-Base Reactions Addition of excess hydroxide ions to the • Note that water molecules in the metal ion precipitated hydroxide complex results in the aquo complex are replaced with OH- ions formation of a soluble complex ion: - 3- [Al (H2O)6] + 6 OH → [Al(OH)6] + 6 H2O - 3- [Al (H2O)3(OH)3] + 3OH → [Al(OH)6] + 3 H2O 1:1 1:2 1:3 1:4 1:5 1: 6 Stoichiometry of Reaction Products: Al3+ to OH- [Al(H O) ]3+ ) 2 6 . Lewis acids bond to the best available ↓↑ 2+ base. - an acid does not react (bond) to any base. [Al(H2O)5(OH)] NaOH ↓↑ (H+) ( + [Al(H2O)4(OH)2] ↓↑ acid “Wanna give me a try?” Base Base [Al(H2O)3(OH)3] of of of ↓↑ - [Al(H2O)2(OH)4] ↓↑ 2- [Al(H2O) (OH)5] Addition Addition Addition ↓↑ 3- [Al(OH)6] . Lewis Acid-Base reactions are reversible Acid-Base + Base → no reaction equilibrium systems . If a better base is available a Lewis acid will Lewis Acid-Base Replacement Reactions react (exchange partners)! . Complexes react if a better partner (acid or base) is available so as to form a more stable bond. Acid-Base + better Base → Reaction Hemoglobin is a complex of Fe that binds to/transports oxygen 5 + Q. The acid exchanging base partners is H . Aquo Complex Ion Replacement Reactions Which is the BEST base, Cl- or HOH? Example: 2+ Replacement of water molecules in [Cu (H2O)4] + with ammonia molecules. H H H • Cl-H + • O Cl- O • • •• [Cu (H O) ]2++ 4 NH → [Cu(NH ) ]2+ + 4 H O H H 2 4 3 3 4 2 Acid Base Base Acid DEMO H+ 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 Aquo Complex Ion Replacement Reactions Aquo Complex Ion Replacement Reactions Example: 2+ Net reaction: Replacement of water molecules in [Cu (H2O)4] 2+ 2+ with ammonia molecules. [Cu(H2O)4] + 4 NH3 → [Cu(NH3)4] + 4 H2O 2+ 2+ [Cu (H2O)4] + 4 NH3 → [Cu(NH3)4] + 4 H2O DEMO [Cu(H2O)4 ](NO3)2 + 4 NH3 → [Cu(NH3)4](NO3)2 + 4 H2O Tetra aquo Cu(II) nitrate Tetra ammine Cu(II)nitrate Tetra aquo Cu(II) ion Tetra ammine Cu(II) ion Aquo Complex Ion Replacement Reactions Aquo Complex Ion Replacement Reactions • The base NH bonded to Cu2+ in [Cu(NH ) ]2+ will 2+ 2+ 3 3 4 [Cu(H2O)4] + 4 NH3 → [Cu(NH3)4] + 4 H2O exchange partners if a better Lewis acid than Cu(II) ions are available. + + H + + DEMO: Addition of H NH4 2+ + + [Cu(NH3)4] • Upon addition of H , NH3 bonds to H (rather than Cu2+). 2+ • Cu re-bonds to H2O. 6 Reactions with NH3 Aquo Complex Ion Reactions • If a precipitate forms upon addition of NH3(aq), the metal aquo complex ion is reacting with the SMALL . Colored transition metal complex ions alter - color upon bonding to a different Lewis base. AMOUNT of OH ions present in NH3(aq): + - NH3(aq) + HOH(l) NH4 (aq) + OH (aq) DEMO 2+ Addition of NH3 to Cu resulting in a copper [Cu(H O) ]2+ [Cu(NH ) ]2+ hydroxide ppt. 2 4 3 4 2+ [Cu(H2O)4] + 2 OH- [Cu (H2O)2(OH)2] + 2 H2O Complex Ions Aquo Complex Ion Reactions 2+ [Cu(NH ) ]2+ [Cu(H2O)4] 3 4 .