Analytical chemistry chem 3811 chapter 11
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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 11. DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university. CHAPTER 11 POLYPROTIC ACIDS AND BASES. POLYPROTIC ACIDS. - Have more than one acidic proton Examples

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 11

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 11

DR. AUGUSTINE OFORI AGYEMAN

Assistant professor of chemistry

Department of natural sciences

Clayton state university


CHAPTER 11

POLYPROTIC ACIDS AND BASES


POLYPROTIC ACIDS

- Have more than one acidic proton

Examples

phosphoric acid, carbonic acid, amino acids


POLYPROTIC ACIDS

Tooth Decay

- Bacteria on teeth metabolize sugar into lactic acid

CH3CH(OH)CO2H

- Tooth enamel contains hydroxyapatite (calcium hydroxyphosphate)

Ca10(PO4)6(OH)2

- Tooth decay is the result of reaction between

lactic acid and hydroxyapatite to produce phosphoric acid

Ca10(PO4)6(OH)2 + 14H+↔ 10Ca2+ + 6H2PO4- + 2H2O


POLYPROTIC ACIDS

Erosion of limestone or marble (calcium carbonate)

- Calcite (calcium carbonate) is soluble in acidic solutions

(insoluble in neutral or basic solutions)

- Calcite dissolves in acid rain causing erosion on buildings

CaCO3(s) ↔ Ca2+ + CO32-

CO32- + H+↔ HCO3-

Acid Rain

- SO2, NOx, and CO2 combine with water vapor and sunlight

to produce sulfuric acid, nitric acid, and carbonic acid


POLYPROTIC ACIDS

Amino Acids

- Building blocks of proteins

- Have acidic carboxylic acid group and basic amino group

- The acidic proton resides on the N of the amino group

- Have positive site (amino group) and negative site (acid group)

- Called zwitterion

- Both groups are protonated at low pH and depotonated at high pH


DIPROTIC SYSTEMS

- Contain two acidic protons

H2A ↔ HA- + H+(Ka1)

HA- ↔ A2- + H+(Ka2)

- Acid dissociation constants: Ka1 > Ka2

A2- + H2O ↔ HA- + OH-(Kb1)

HA- + H2O ↔ H2A + OH-(Kb2)

- Base association constants: Kb1 > Kb2


DIPROTIC SYSTEMS

H2A ↔ HA- + H+(Ka1)

+

HA- + H2O ↔ H2A + OH- (Kb2)

=

H2O ↔ H+ + OH-

Ka1 x Kb2 = Kw

Ka2 x Kb1 = Kw


DIPROTIC SYSTEMS

Ka1>>>> Ka2

- A solution of a diprotic acid behaves like a solution of a

monoprotic acid with Ka = Ka1

Kb1 >>>> Kb2

- The fully basic form of a diprotic acid can be

considered as monobasic with Kb = Kb1


DIPROTIC SYSTEMS

The Intermediate Form

- Is both an acid and a base

- Can donate or accept a proton

- Called amphiprotic


TRIPROTIC SYSTEMS

Ka1 x Kb3 = Kw

Ka2 x Kb2 = Kw

Ka3 x Kb1 = Kw

First Intermediate (H2A-)

Second Intermediate (HA2-)


PREDOMINANT SPECIES

- From the Henderson-hasselbalch equation

- pH changes by 1 if the ratio changes by a factor of 10

pH = pKa + 1 if [A-]/[HA] = 10

pH = pKa - 1 if [A-]/[HA] = 0.10


PREDOMINANT SPECIES

Monoprotic Systems

[A-] = [HA] when pH = pKa

A- is the predominant form when pH > pKa

HA is the predominant form when pH < pKa


PREDOMINANT SPECIES

Diprotic Systems

There are two pKa values

[H2A] = [HA-] when pH = pKa1

[HA-] = [A2-] when pH = pKa2

H2A is the predominant form when pH < pKa1

HA- is the predominant form when pKa1 < pH < pKa2

A2- is the predominant form when pH > pKa2


PREDOMINANT SPECIES

Triprotic Systems

There are three pKa values

[H3A] = [H2A-] when pH = pKa1

[H2A-] = [HA2-] when pH = pKa2

[HA2-] = [A3-] when pH = pKa3

H3A is the predominant form when pH < pKa1

H2A- is the predominant form when pKa1 < pH < pKa2

HA2- is the predominant form when pKa2 < pH < pKa3

A3- is the predominant form when pH > pKa3


TITRATION CURVES

Diprotic acids (two equivalence points)

pH

H2A/HA-

HA-/A2-

Excess OH-

pKa2

pKa1

volume of OH- added


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