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E5 Lewis Acids and Bases (Session 1)March 19 - 24

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

Acids

Bronsted: Acids are proton donors.

DEMO

Problem• Compounds that do not contain H are

acids are acids; e.g. AlCl3.

• Compounds containing cations other thanH+ are acids!

Problem: Some acids do not contain protons

Example: Al3+ (aq) = ≈ pH 3!

Deodorants and acid loving plant foods containaluminum salts

Lewis 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 Acids

Electron deficient species ; potential electronpair acceptors.

Lewis acids: H+ Cu2+ Al3+

“I’m deficient!”

Acid

2

Lewis Bases

Electron rich species; electron pair donors.

(ammine) (hydroxo) (aquo)Ammonia hydroxide ion water__

Lewis 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 product is a complex or complex ion

Lewis Acid-Base Reactions

• Metal ions are Lewis acids and BONDto electron pairs on water molecules (Lewis bases)!

Lewis Acid-Base Reaction

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 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 Cations

Why do different cations in aqueous solution exhibitdifferent acid strengths?

• Compare the pH of nitrate salts containing differentmetal ions.

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Part 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+

Metal Ion Acid Strength

pH comparison of chloride salts of aluminum, barium, and sodium

Metal 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 VIIIA

1H

1s1 IIA IIIA IVA VA VIA VIIA

2He1s2

3Li

2s1

4Be

2s2

5B

2s22p1

6C

2s22p2

7N

2s22p3

8O

2s22p4

9F

2s22p5

1 0Ne

2s22p6

1 1Na

3s1

1 2Mg

3s2 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 9K

4s1

2 0Ca

4s2

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 7Rb

5s1

3 8Sr

5s2

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 5Cs

6s1

5 6Ba

6s2

5 7

La*

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 7Fr

7s1

8 8Ra

7s2

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 Ion Model

Metal ion acid strength is related to its ability toattract and bond to electrons (oxidizing agentstrength)

∝ + 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:

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Q. Complete a balanced equation to show formationof hydronium ions:

+ [H3O]+[Al(H2O)5(OH)]2+

[Al(H2O)6]3+ + HOH

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 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 predictable

from a) metal ion acid strength? b) the position ofthe metal ion’s element in the Periodic Table?

Part 2. Complexation Reactions

Lewis acids

• [Cu(H2O)4]2+

• [Ag(H2O)2]+

• [Zn(H2O)4]2+

• Assigned team metal ions from Part 1

Lewis bases• OH- (NaOH)• NH3

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

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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.

Lewis Acids bond to the best Lewis Base

[Cu (H2O)4]2++ 4 NH3 → [Cu(NH3)4]2+ + 4 H2O

DEMO

Tetra aquo Cu(II) ion Tetra ammine Cu(II) ion

[Cu (H2O)4]2++ 4 NH3 → [Cu(NH3)4]2+ + 4 H2O

Complexation Reactions

[Cu(NH3)4]2+[Cu(H2O)4]2+

Colored aquo transition metal complex ionsalter color upon bonding to a different Lewisbase.

Complex Ion Charge

[Cu(NH3)4]2+[Cu(H2O)4]2+

The charge on a metal complex ion = themetal ion charge if the bonded Lewis base isuncharged (e.g., H2O or NH3)

Complex Ion Charge

[Cu(Cl)4]2-[Cu(H2O)4]2+

The charge on a metal complex ion willNOT = the metal ion charge if the bondedLewis base is charged (e.g., Cl- or OH- )

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Complex Ion Charge

Q. Complete the equation below:

[Cu (H2O)4]2+ + 4 Cl- → ___________________[Cu(Cl)4]2- + 4 H2O

Complex or Complex Ion?

[Cu (H2O)2(OH)2][Cu(H2O)4]2+

If the Lewis base is charged (e.g., OH-) and thereaction stoichiometry with the Lewis acid results ina product with no charge, a complex has formed

the complex will be insoluble and precipitate

+ OH-

• 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

Lewis Acid-Base Reactions

1:1 1:2 1:3 1:4 1:5 1: 6Stoichiometry of Reaction Products: Al3+ to OH-

• If a metal ion reacts with a charged Lewisbase such as OH-, the product may be a solublecomplex ion or an uncharged insoluble complexdepending on reaction stoichiometry.

DEMO

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!

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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

Add

ition

of a

cid

(H+)

Lewis 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

Complex Ions

Complex ions are the chemical basis forcolorful paint pigments.

Questions?Contact nkerner@umich.edu

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• The base NH3 bonded to Cu2+ in [Cu(NH3)4]2+ willexchange partners if a better Lewis acid than Cu(II)ions are available.

Complexation Reactions

[Cu(NH3)4]2+

DEMO: Addition of H+

Complexation 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.

• Upon addition of H+, NH3 bonds to H+ (ratherthan Cu2+).

• Cu2+ re-bonds to H2O.