1

Polyprotic acids

H3PO4 + H2O = H2PO4- + H3O+ pKa1 = 2.1

H2PO4- + H2O = HPO4

2- + H3O+ pKa1 = 7.4

HPO42- + H2O = PO4

3- + H3O+ pKa1 = 12.7

Since pKa values are generally well-separated, only 1 or 2 species will be present at significant concentration

2

Common acidsHNO3 NO3

Nitric acid Nitrate

HNO2 NO2

Nitrous acid Nitrite

H3PO4 PO43

Phosphoric acid Phosphate

H3PO3 HPO32

Phosphorous acid Phosphite

H2SO4 SO42

Sulfuric acid Sulfate

H2SO3 SO32

Sulfurous acid Sulfite

HClO4 ClO4

Perchloric acid Perchlorate

HClO3 ClO3

Chloric acid Chlorate

HClO2 ClO2

Chlorous acid Chlorite

HOCl OCl

Hypochlorous acid Hypochlorite

3

AnhydridesEx: H2O + SO3 = H2SO4

anhydride acid form

Acidic

SO3 / H2SO4

“P2O5” / H3PO4

CO2/H2CO3

Basic

Na2O / NaOH

Amphoteric

Al2O3 / Al(OH)3

4

Trends in acidity

5

Pauling’s rules for pKa‘s of oxoacids

1. Write formula as MOp(OH)q

2. pKa 8 – 5p

3. Each succeeding deprotonation increases the pKa by 5

Ex: rewrite HNO3 as NO2(OH)

p = 2; pKa 8 – 5(2) 2 (exptl value is 1.4)

Ex: rewrite H3PO4 as PO(OH)3

p = 1; pKa1 8 – 5(1) 3 (exptl value is 2.1)

pKa2 8 (exptl value is 7.4)

pKa3 13 (exptl value is 12.7)

6

pKa values

p Pauling pKa

calcn exptl

Cl(OH) 0 8 7.5

ClO(OH)1 3 2.0

ClO2(OH) 2 2 1.2

ClO3(OH) 3 7 ≈ 10

HlO4 + 2H2O H5IO6

7

Acid/base chemistry of complexes

Aqueous chemistry:

Fe(NO3)3 [Fe(OH2)6]3+(aq) + 3 NO3(aq)

2 [Fe(OH2)6]3+ (aq) = [Fe2(OH2)10OH]5+ (aq) + H3O+(aq)

Hexaaquairon(III), pKa ~ 3

H2O

dimer

8

Lewis acids and bases

A + :B = A:B

LA LB complex

LA = e pr acceptor; LB = e pr donor

Lewis definition is more general than BL definition, does not require aqueous or protic solvent

Ex: W + 6 :CO = [W(CO)6]

BCl3 + :OEt2 = BCl3:OEt2

Fe3+(g) + 6 :OH2 → [Fe(OH2)6]3+

9

log K and ligand type