Lewis Bases

E5 Lewis Acids and Bases (Session 1) November 5 - 11

Session one ? Pre-lab (p.151) due ? 1st hour discussion of E4 ? Lab (Parts 1and 2A)

Session two ? Lab: Parts 2B, 3 and 4

Acids

Bronsted: Acids are proton donors. Problem ? Compounds containing cations other than

H+ are acids!

DEMO

Problem: Some acids do not contain protons Example: Al3+ (aq) = pH 3!

Deodorants and acid loving plant foods contain aluminum salts

Lewis Acids and Bases

Defines acid/base without using the word proton:

H Cl-H + ???O? H

H

H +

Cl-

??O

H

Acid Base

Base Acid

A BASE DONATES unbonded ELECTRON PAIR/S. An ACID ACCEPTS ELECTRON PAIR/S .

Lewis Acids

Electron deficient species ; potential electron pair acceptors.

Lewis acids: H+ Cu2+ Al3+

"I'm deficient!"

Acid

Lewis Bases

Electron rich species; electron pair donors.

Ammonia (ammine)

hydroxide ion (hydroxo)

water__ (aquo)

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Lewis Acid-Base Reactions

Example

H+ +

H ???O? H

H ??O

H + H

Acid + Base

Complex ion

The acid reacts with the base by bonding to one 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 Reactions

Metal ion BONDED to water molecules

Metal ion surrounded by water molecules

Lewis Acid-Base Reaction Products

Net Reaction Examples

Pb2+ + 4 H2O

[Pb(H2O)4]2+

Lewis acid Lewis base Tetra aquo lead ion

Ni2+ + 6 H2O

[Ni(H2O)6]2+

Lewis acid Lewis base Hexa aquo nickel ion

DEMO

Lewis Acid-Base Reaction Products

Net Reaction Examples

H+

+ H2O

Lewis acid Lewis base

[H(H2O)]+

Hydronium ion

Cu2+ + 4 H2O

[Cu(H2O)4]2+

Lewis acid Lewis base Tetra aquo copper(II)ion

DEMO

Metal Aquo Complex Ions

The metal ions in a salt solution bond to water molecules to forms aquo complex ions.

Example: Cu2+(aq) + SO42- (aq)

[Cu(H2O)4]2+

Hydrated solid: [Cu(H20)4]SO4

DEMO

tetra aquo copper(II) sufate

Part 1. Acidity of Cations

? Compare the pH of nitrate salts containing different metal ions. 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+

2

Metal Ion Acid Strength

Metal Ion Acid Strength: __A_l_3_+_>__M_g__2+__>_N__a_+__ ? Results suggest that metal ion acidity and metal ion charge are linked ? Results suggest that metal ion acidity and metal ion oxidizing agent strength are linked

Acidity of Metal Ions

Reminder: Metal ions BOND to electron pairs on water molecules!

Metal Ion Acidity

Acid strength

1A

Oxidizing agent strength

VIIIA

1 H 1s1 IIA

2 He IIIA IVA VA VIA VIIA 1s2

34 Li Be

5 6 7 8 9 10 B C N O F Ne

2s1 2s2

2 s22 p12 s22 p2 2 s22 p32 s22 p4 2 s22 p52 s22 p6

11 12 Na Mg

13 14 15 16 17 18 Al Si P S Cl Ar

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 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 4s1 4s2 3d14 s23d24 s23d34 s23d54 s13d54 s23d64 s2 3d74 s2 3d84 s2 3d1 04s1 3d1 04s2 4 s24 p14 s24 p2 4 s24 p34 s24 p4 4 s24 p54 s24 p6

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 5s1 5s2 4d15 s24d25 s24d35 s24d55 s14d55 s24d75 s1 4d85 s1 4d1 0 4d1 05s1 4d1 05s2 5 s25 p15 s25 p2 5 s25 p35 s25 p4 5 s25 p55 s25 p6

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 6s1 6s2 5d16 s25d26 s25d36 s25d46 s25d56 s25d66 s2 5d76 s2 5d96 s1 5d1 06s1 5d1 06s2 6 s26 p16 s26 p2 6 s26 p36 s26 p4 6 s26 p56 s26 p6

8 7 8 8 8 9 1 0 4 1 0 5 1 0 6 1 0 7 1 0 8 1 0 9 + Element synthesized,

Fr Ra Ac# + + +

+ + + but no official name assigned

7s1 7s2 6d17 s26d27 s26d37 s26d47 s26d57 s26d67 s2 6d77 s2

Acidity of Metal Ion Model

Mx+ ....O H +

H + Metal ion acid strength is related to its ability to

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

Equation describing acidity of metal ions:

[X(H2O)6]2+ + H2O [X(H2O)5(OH)]+ + [H-H2O]+

Electron deficient proton/s in the bonded water 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.

Q. Complete a balanced equation to show formation

of hydronium ions:

[Al(H2O)6]3+ + HOH [Al(H2O)5(OH)]2+ + [H3O]+

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Q. Complete a balanced equation to show formation

of hydronium ions:

[Al(H2O)6]3+ + 2 HOH [Al(H2O)4(OH)2]+ + 2 [H3O]+

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 of the metal ion's element in the Periodic Table?

Part 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 Reactions

Example

? Al exists as [Al(H2O)6]3+ ? Add NaOH

DEMO

Metal Aquo Complex Ion Reactions

If reaction occurs with a charged Lewis base such as OH-, the product may be a soluble complex ion or an uncharged insoluble complex.

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

? Precipitation reactions are Lewis acid-base reactions!

Traditional net precipitation equation: Al 3+(aq) + 3OH- (aq) Al(OH)3 (s)

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Metal Aquo Complex Ion Reactions

Addition of excess hydroxide ions to the precipitated hydroxide complex results in the formation of a soluble complex ion:

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

Lewis Acid-Base Reactions ? Note that water molecules in the metal ion aquo complex are replaced with OH- ions [Al (H2O)6] + 6 OH- [Al(OH)6]3- + 6 H2O

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

Addition of Base (NaOH) Addition of acid (H+)

[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-

Lewis Acid-Base reactions are reversible equilibrium systems

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

"Wanna give me a try?"

Acid-Base + Base no reaction

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

Lewis Acid-Base Replacement Reactions

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

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