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E5 Lewis Acids and Bases

Session oneSession onePre-lab (p.141) due at start of lab.Pre-lab (p.141) due at start of lab.First hour: Discussion of E4First hour: Discussion of E4Lab: Parts 1andLab: Parts 1and 2A2A

Session twoLab: Parts 2B, 3 and 4

Acids

Bronsted: Acids are proton donors and generatehydronium ions in aqueous solution.

DEMO

Problem• Cations other than H+ are acids that generatehydronium ions in aqueous solution!

Acids

Demo example: Al3+ in aqueous solution = ≈ pH 3!

Deodorants and acid loving plant foods contain aluminum salts

Cations other than H+ generate hydroniumions in aqueous solution!

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Lewis Definition of 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

Define acid/base without using the word proton:

Lewis Acids

Electron deficient species ; potential electronpair acceptors.

Examples: H+ Cu2+ Al3+

“I’m deficient!”

Acid

Lewis Bases

Electron rich species; electron pair donors.

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

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Lewis Acid-Base Reactions

The acid reacts with the base by bonding to one ormore available electron pairs on the base.

Acid + Base Complex ion

H H+ + ••O

• • H

H H + • O

• H

Example

The acid-base bond is a coordinate covalent bond. The product is a complex or complex ion

Lewis Acid-Base Reactions

Metal ions react with/bond to water molecules toform aquo complex ions in aqueous solution.

• Example: [Na(H2O)6]+

Aquo Complex Ions

Example: CuSO4 (s) → Cu2+(aq) + SO42- (aq)

[Cu(H2O)4]2+

When a salt dissolves, the metal ions in solution reactwith/bond to water molecules to forms aquo complex ions.

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Aquo Complex Ions

Examples

[Pb(H2O)4]2+Tetra aquo lead ion

[Ni(H2O)6]2+Hexa aquo nickel ion

[Cu(H2O)4]2+Tetra aquo copper(II)ion

[H(H2O)]+Hydronium ion

Colored aquo complex ions contain a transitionmetal ion with incompletely filled d orbitals.

Part 1. Acidity of Cations

Collect pH data for different metal aquo complexions in aqueous solution.• pH versus periodic table position of ion’s metal?• pH versus metal ion size and charge? extent of s’

• Study the reaction of metal aquo complex ions withthe Lewis base H2O to produce hydronium ions.

Metal Ion Acid Strength

The acid strength of a metal ion is dependent on itsability to attract electrons (oxidizing agent strength)

∝ + HMx+ O H

....∝ +

The water molecules bonded to metal ions in aquocomplex ions are polarized and the protons areelectron deficient. The greater the acid strength of a metal ion the morepolarized the bonded water molecules in the aquocomplex ion.

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Electron deficient proton/s in the polarized watermolecules of the metal aquo complex ion react andbond to an electron pair in another water molecule

Reactions of Metal Aquo Complex Ions with Water

Reaction of Metal Aquo Complex Ion:

[Al (H2O)6]3+ + H2O → [Al(H2O)5(OH)]2+ + [H-H2O]+

When a proton bonds to a water molecule, a H3O+ ion is formed.

Q. Complete a balanced equation to show formation of hydronium ions:

+ _____________________[Al(H2O)6]3+ + 2 HOH →

Metal Ion Acid Strength

Q. Given the reducing agent strength Na > Mg > Al,indicate the Lewis acid strength of Na+, Mg2+, and Al3+

Metal Ion Acid Strength: ___________________

DEMO

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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

Reducing agent strength: Na > Mg > Al

Acid strength: _______________

Part 2 A.Studies of complex formation with NH3 and OH -.

• Study the reactions of metal aquo complex ions withLewis bases (NH3, OH-, …).

DISCUSSION• What kinds of observations allow you to infer that a

complexation reaction is occurring?• Can you predict the reactions of a metal ion based on the

position of its element in the Periodic Table?

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

• Extent of reaction differs because of acid strength differences.

Reaction of Lewis Acids (metal ions) with Lewis Bases

•Pattern of reactivity with bases (e.g. OH- and NH3) differsdue to differences in metal ion electron configurations.

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If reaction occurs with a charged base such as OH-, theproduct may be a soluble complex ion or an unchargedinsoluble complex depending on reaction stoichiometry.

Aquo Complex Ion Reactions

Example 1: Formation of a soluble complex ion:

[Al (H2O)6]3+ + OH- → [Al(H2O)5(OH)]2+ + H2O

Aquo Complex Ion Reactions

Example 2: Formation of a 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)

Aquo Complex Ion Reactions

Example 3: Addition of more hydroxide ions to theprecipitated hydroxide complex results in formationof a soluble complex ion:

[Al (H2O)3(OH)3] + 6 OH- → [Al(OH)6]3- + 6 H2O

DEMO

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[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)

Precipitation reactions are Lewis Acid-Base reactions The reactions are reversible equilibrium systems The equilibrium point depends on the reactants (strength…)and reaction conditions (concentration…)

Add

ition

of a

cid

(H+)

• 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)

Reaction of Aquo Metal Complex Ions with NH3

[Cu(H2O)4]2+ + NH3 → light blue precipitate*

DEMO

Q. * What is the formula for the hydroxide precipitate?_________________________

Lewis acids bond to the best available base. - an acid does not react (bond) to any base.

+ Base → no reaction

“Wanna give me a try?”

Acid-Base

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If a better base is available a Lewis acid willreact (exchange partners)!

Acid-Base + better Base → Reaction

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

Answer: ______________________________

• The bonded base-acid product = complex ion.

Example 1: Replacement of water with ammonia molecules.

Aquo Complex Ion Reactions

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

DEMO

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

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Example: Replacement of water with ammonia molecules.

Aquo Complex Ion Reactions

[Cu(NH3)4]2+Tetrammine copper (II) ion

Complex Ions

Complex ions are the chemical basis for colorful paintpigments.

Aquo Complex Ion Reactions

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

Colored transition metal ions alter color uponbonding to a different Lewis base.

The charge on a complex ion = that of the metal ionwhen the metal ion is bonded to an uncharged Lewisbase.

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Example 2: Replacement of water with chloride ions. Q. Complete the equation below:

Aquo Complex Ion Reactions

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

DEMO

Tetra aquo copper(II) ion Tetra chloro copper(II) ionThe charge on the complex ion alters due to reaction with acharged base

Lewis Acid-Base Reactions

Complexes react if a better partner (acid or base) isavailable so as to form a more stable bond.

Hemoglobin is a complex of Fe that binds to/transports oxygen

Example 4: The base ammonia in the Cu(II) product(ammine complex ion) will react and bond to a better acidthan Cu(II) ions such as H+:

Aquo Complex Ion Reactions

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

DEMOQ. What will you observe upon adding acid(H+) to the deep violet-blue ammine product?

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Questions?Contact nkerner@umich.edu