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1
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
1A VIIIA1H1s1 IIA IIIA IVA VA VIA VIIA
2He1s2
3Li2s1
4Be2s2
5B
2s22p1
6C
2s22p2
7N
2s22p3
8O
2s22p4
9F
2s22p5
1 0Ne
2s22p6
1 1Na3s1
1 2Mg3s2 IIIB IVB VB VIB VIIB VIIIB ⇔ VIIIB IB IIB
1 3Al
3s23p1
1 4Si
3s23p2
1 5P
3s23p3
1 6S
3s23p4
1 7Cl
3s23p5
1 8Ar
3s23p6
1 9K4s1
2 0Ca4s2
2 1Sc
3d14s2
2 2Ti
3d24s2
2 3V
3d34s2
2 4Cr
3d54s1
2 5Mn
3d54s2
2 6Fe
3d64s2
2 7Co
3d74s2
2 8Ni
3d84s2
2 9Cu
3d1 04s1
3 0Zn
3d1 04s2
3 1Ga
4s24p1
3 2Ge
4s24p2
3 3As
4s24p3
3 4Se
4s24p4
3 5Br
4s24p5
3 6Kr
4s24p6
3 7Rb5s1
3 8Sr5s2
3 9Y
4d15s2
4 0Zr
4d25s2
4 1Nb
4d35s2
4 2Mo
4d55s1
4 3Tc
4d55s2
4 4Ru
4d75s1
4 5Rh
4d85s1
4 6Pd
4d10
4 7Ag
4d1 05s1
4 8Cd
4d1 05s2
4 9In
5s25p1
5 0Sn
5s25p2
5 1Sb
5s25p3
5 2Te
5s25p4
5 3I
5s25p5
5 4Xe
5s25p6
5 5Cs6s1
5 6Ba6s2
5 7La*
5d16s2
7 2Hf
5d26s2
7 3Ta
5d36s2
7 4W
5d46s2
7 5Re
5d56s2
7 6Os
5d66s2
7 7Ir
5d76s2
7 8Pt
5d96s1
7 9Au
5d1 06s1
8 0Hg
5d1 06s2
8 1Tl
6s26p1
8 2Pb
6s26p2
8 3Bi
6s26p3
8 4Po
6s26p4
8 5At
6s26p5
8 6Rn
6s26p6
8 7Fr7s1
8 8Ra7s2
8 9Ac#
6d17s2
1 0 4 +
6d27s2
1 0 5 +6d37s2
1 0 6 +6d47s2
1 0 7 +6d57s2
1 0 8 +6d67s2
1 0 9 +6d77s2
+ Element synthesized,but no official name assigned
Metal Ion Acidity
Acid strength
Oxidizing agent strength
Acidity of Metal IonsAcidity of Metal Ions
Reminder:Metal ionsBONDto electronpairs onwatermolecules!
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]
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
Aquo Complex Ion Replacement Reactions
Net reaction:[Cu(H2O)4]
2++ 4 NH3 → [Cu(NH3)4]2+ + 4 H2O
Tetra aquo Cu(II) nitrate
[Cu(H2O)4 ](NO3)2 + 4 NH3 → [Cu(NH3)4](NO3)2 + 4 H2O
Tetra ammine Cu(II)nitrate
• The base NH3 bonded to Cu2+ in [Cu(NH3)4]2+ willexchange partners if a better Lewis acid than Cu(II)ions are available.
Aquo Complex Ion Replacement Reactions
[Cu(NH3)4]2+
DEMO: Addition of H+
Aquo Complex Ion Replacement ReactionsAquo Complex Ion Replacement Reactions
[Cu(H2O)4]2++ 4 NH3 → [Cu(NH3)4]
2+ + 4 H2O
+ H+
NH4+
• Upon addition of H+, NH3 bonds to H+ (ratherthan Cu2+).
• Cu2+ re-bonds to H2O.
7
• If a precipitate forms upon addition of NH3(aq), themetal aquo complex ion is reacting with the SMALLAMOUNT of OH- ions present in NH3(aq):
NH3(aq) + HOH(l) NH4+ (aq) + OH- (aq)
Reactions with NH3
DEMO
Addition of NH3 to Cu2+
resulting in a copperhydroxide ppt.
[Cu(H2O)4]2+ + 2 OH- [Cu (H2O)2(OH)2] + 2 H2O
Aquo Complex Ion ReactionsAquo Complex Ion Reactions
[Cu(NH3)4]2+[Cu(H2O)4]
2+
Colored transition metal complex ions altercolor upon bonding to a different Lewis base.
Complex IonsComplex Ions
Complex ions are the chemical basis forcolorful paint pigments.
Aquo Complex Ion ReactionsAquo Complex Ion Reactions
[Cu(NH3)4]2+[Cu(H2O)4]
2+
The charge on a aquo metal complex ionwill = the metal ion charge if the bondedLewis base is uncharged (e.g., H2O or NH3)
Example:Replacement of neutral water molecules in [Cu(H2O)4]
2+
with a charged Lewis base - chloride ions.
Aquo Complex Ion Replacement Reactions
DEMO
[Cu(Cl)4]2-[Cu(H2O)4]2+
Q. Complete the equation below:
Aquo Complex Ion Replacement Reactions
[Cu (H2O)4]2+ + 4 Cl- → ___________________[Cu(Cl)4]
2- + 4 H2O
The charge on the complex ion alters due toreplacement of a neutral Lewis base (water) with acharged Lewis base
8
Questions?Contact [email protected]