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 3811 chapter 11

ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 11

DR. AUGUSTINE OFORI AGYEMAN

Assistant professor of chemistry

Department of natural sciences

Clayton state university


Analytical chemistry chem 3811 chapter 11

CHAPTER 11

POLYPROTIC ACIDS AND BASES


Analytical chemistry chem 3811 chapter 11

POLYPROTIC ACIDS

- Have more than one acidic proton

Examples

phosphoric acid, carbonic acid, amino acids


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

DIPROTIC SYSTEMS

H2A ↔ HA- + H+(Ka1)

+

HA- + H2O ↔ H2A + OH- (Kb2)

=

H2O ↔ H+ + OH-

Ka1 x Kb2 = Kw

Ka2 x Kb1 = Kw


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

DIPROTIC SYSTEMS

The Intermediate Form

- Is both an acid and a base

- Can donate or accept a proton

- Called amphiprotic


Analytical chemistry chem 3811 chapter 11

TRIPROTIC SYSTEMS

Ka1 x Kb3 = Kw

Ka2 x Kb2 = Kw

Ka3 x Kb1 = Kw

First Intermediate (H2A-)

Second Intermediate (HA2-)


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

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


Analytical chemistry chem 3811 chapter 11

TITRATION CURVES

Diprotic acids (two equivalence points)

pH

H2A/HA-

HA-/A2-

Excess OH-

pKa2

pKa1

volume of OH- added


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