1 E5 Lewis Acids and Bases (Session 1) November 5 - 11 Session one Session one • Pre-lab (p.151) due Pre-lab (p.151) due • 1st hour discussion of E4 1st hour discussion of E4 • Lab (Parts 1and Lab (Parts 1and 2A) 2A) Session two • Lab: Parts 2B, 3 and 4 Acids Bronsted: Acids are proton donors. DEMO Problem • Compounds containing cations other than H + are acids! Problem: Some acids do not contain protons Example: Al 3+ (aq) = ≈ pH 3! Deodorants and acid loving plant foods contain aluminum salts Lewis Acids and Bases A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S . Acid Base Base Acid H Cl-H + • • O • • H Cl - H H + • O • H Defines acid/base without using the word proton: Lewis Acids Electron deficient species ; potential electron pair acceptors. Lewis acids: H + Cu 2+ Al 3+ “I’m deficient!” Acid Lewis Bases Electron rich species; electron pair donors. (ammine) (hydroxo) (aquo) Ammonia hydroxide ion water __
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E5 Lewis Acids and Bases (Session 1)November 5 - 11
E5 Lewis Acids and Bases (Session 1)November 5 - 11
Session oneSession one
•• Pre-lab (p.151) duePre-lab (p.151) due
•• 1st hour discussion of E41st hour discussion of E4
•• Lab (Parts 1andLab (Parts 1and 2A)2A)
Session two
• Lab: Parts 2B, 3 and 4
AcidsAcids
Bronsted: Acids are proton donors.
DEMO
Problem• Compounds containing cations other than
H+ are acids!
Problem: Some acids do not contain protonsProblem: Some acids do not contain protons
Example: Al3+ (aq) = ≈ pH 3!
Deodorants and acid loving plant foods containaluminum salts
Lewis Acids and BasesLewis Acids and Bases
A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S .
Acid Base Base Acid
HCl-H + ••O
• • H
Cl- H H +
• O
• H
Defines acid/base without using the word proton:
Lewis AcidsLewis Acids
Electron deficient species ; potential electronpair acceptors.
Lewis acids: H+ Cu2+ Al3+
“I’m deficient!”
Acid
Lewis BasesLewis Bases
Electron rich species; electron pair donors.
(ammine) (hydroxo) (aquo)
Ammonia hydroxide ion water__
2
Lewis Acid-Base ReactionsLewis Acid-Base Reactions
The acid reacts with the base by bonding to oneor more available electron pairs on the base.
Acid + Base Complex ion
H H+ + ••O
• • H
H H +
• O
• H
Example
The acid-base bond is coordinate covalent. The product is a complex or complex ion
Lewis Acid-Base ReactionsLewis Acid-Base Reactions
Metal ionsurroundedby watermolecules
Metal ionBONDEDto watermolecules
Lewis Acid-Base Reaction ProductsLewis Acid-Base Reaction Products
Net Reaction Examples
Ni2+ + 6 H2O [Ni(H2O)6]2+
Lewis acid Lewis base Hexa aquo nickel ion
Pb2+ + 4 H2O [Pb(H2O)4]2+
Lewis acid Lewis base Tetra aquo lead ion
DEMO
Lewis Acid-Base Reaction ProductsLewis Acid-Base Reaction Products
Net Reaction Examples
Cu2+ + 4 H2O [Cu(H2O)4]2+
Lewis acid Lewis base Tetra aquo copper(II)ion
H+ + H2O [H(H2O)]+ Lewis acid Lewis base Hydronium ion
DEMO
Metal Aquo Complex IonsMetal Aquo Complex Ions
Example:Cu2+(aq) + SO4
2- (aq)
[Cu(H2O)4]2+
The metal ions in a salt solution bond to watermolecules to forms aquo complex ions.
Hydrated solid:DEMO
[Cu(H20)4]SO4
tetra aquo copper(II) sufate
Part 1. Acidity of CationsPart 1. Acidity of Cations
EXPERIMENT VARIABLE• Nitrate salts with different metal ions EXPERIMENT CONSTANT• Water to dissolve salts is from same source and of
constant pH (neutral or non-acidic).
DEMO pH of Na+, Mg2+, and Al3+
• Compare the pH of nitrate salts containing differentmetal ions.
3
Metal Ion Acid StrengthMetal Ion Acid Strength
Metal Ion Acid Strength: ___________________Al3+ > Mg2+ > Na+
• Results suggest that metal ion acidity andmetal ion charge are linked
• Results suggest that metal ion acidity andmetal ion oxidizing agent strength are linked
Acidity of Metal Ion ModelAcidity of Metal Ion Model
Metal ion acid strength is related to its ability toattract and bond to electrons (oxidizing agent strength)
∝ + HMx+ O H
....∝ +
When a metal ion bonds to a water molecule, thewater molecule’s polarity increases and the protons inthe water molecule become more electron deficient
Electron deficient proton/s in the bonded watermolecule (of the metal aquo complex ion) aredonated(bond) to an external water molecule
Acidity of Metal Ion Model
[X(H2O)6]2+ + H2O → [X(H2O)5(OH)]+ + [H-H2O]+
When protons bond to water molecules, H3O+
ions are formed producing a pH drop.
Equation describing acidity of metal ions:
Q. Complete a balanced equation to show formationof hydronium ions:Q. Complete a balanced equation to show formationof hydronium ions:
+ [H3O]+[Al(H2O)5(OH)]2+
[Al(H2O)6]3+ + HOH
4
Q. Complete a balanced equation to show formationof hydronium ions:Q. Complete a balanced equation to show formationof hydronium ions:
+ 2 [H3O]+[Al(H2O)4(OH)2]+
[Al(H2O)6]3+ + 2 HOH
Part 2. Complexation ReactionsPart 2. Complexation Reactions
• The reaction of a Lewis acid with a Lewis base(NH3, OH-, …).
Data Analysis• What kinds of observations allow you to know
that a complexation reaction is occurring?
• Are Lewis acid-base reaction results predictablefrom a) metal ion acid strength? b) the position ofthe metal ion’s element in the Periodic Table?
Part 2 Lewis Acid-Base ReactionsPart 2 Lewis Acid-Base Reactions
Lewis acids
• Metal aquo complex ions• Formula for metal aquo complex ion is provided
Lewis bases• OH- (NaOH)• NH3
Part 2 Lewis Acid-Base ReactionsPart 2 Lewis Acid-Base Reactions
Example
• Al exists as [Al(H2O)6]3+
• Add NaOH
DEMO
If reaction occurs with a charged Lewis basesuch as OH-, the product may be a solublecomplex ion or an uncharged insoluble complex.
Metal Aquo Complex Ion Reactions
Formation of a soluble complex ion:
[Al (H2O)6]3+ + OH- → [Al(H2O)5(OH)]2+ + H2O
Metal Aquo Complex Ion Reactions
Formation of an insoluble complex:
[Al (H2O)6]3+ + 3OH- → [Al(H2O)3(OH)3] + 3H2O
Traditional net precipitation equation: Al 3+(aq) + 3OH- (aq) → Al(OH)3 (s)
• Precipitation reactions are Lewis acid-base reactions!
5
Metal Aquo Complex Ion Reactions
Addition of excess hydroxide ions to theprecipitated hydroxide complex results in theformation of a soluble complex ion:
[Al (H2O)3(OH)3] + 3OH- → [Al(OH)6]3- + 3 H2O
Lewis Acid-Base Reactions
1:1 1:2 1:3 1:4 1:5 1: 6Stoichiometry of Reaction Products: Al3+ to OH-
• Note that water molecules in the metal ionaquo complex are replaced with OH- ions
[Al (H2O)6] + 6 OH- → [Al(OH)6]3- + 6 H2O
[Al(H2O)6]3+
↓↑ [Al(H2O)5(OH)]2+
↓↑ [Al(H2O)4(OH)2] +
↓↑[Al(H2O)3(OH)3]
↓↑ [Al(H2O)2(OH)4]-
↓↑ [Al(H2O) (OH)5] 2-
↓↑ [Al(OH)6]3-
Add
ition
of B
ase
(NaO
H)
Lewis Acid-Base reactions are reversibleequilibrium systems
Ad
dit
ion
of
acid
(H
+)
Lewis acids bond to the best availablebase. - an acid does not react (bond) to any base.
+ Base → no reaction
“Wanna give me a try?”
Acid-Base
If a better base is available a Lewis acid willreact (exchange partners)!
Acid-Base + better Base → Reaction
Lewis Acid-Base Replacement ReactionsLewis Acid-Base Replacement Reactions
Complexes react if a better partner (acid or base)is available so as to form a more stable bond.
Hemoglobin is a complex of Fe that binds to/transports oxygen
6
Q. The acid exchanging base partners is H+. Which is the BEST base, Cl- or HOH?Q. The acid exchanging base partners is H+. Which is the BEST base, Cl- or HOH?
Acid Base Base Acid
HCl-H + ••O
• • H
Cl- H H +
• O
• H
H+ bonds to water (best base) rather than Cl-.
• The bonded base-acid product = complex ion.
Example:Replacement of water molecules in [Cu (H2O)4]
2+
with ammonia molecules.
Aquo Complex Ion Replacement Reactions
[Cu (H2O)4]2++ 4 NH3 → [Cu(NH3)4]
2+ + 4 H2O
DEMO
Tetra aquo Cu(II) ion Tetra ammine Cu(II) ion
Example:Replacement of water molecules in [Cu (H2O)4]