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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 11

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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  1. ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 11 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

  2. CHAPTER 11 POLYPROTIC ACIDS AND BASES

  3. POLYPROTIC ACIDS - Have more than one acidic proton Examples phosphoric acid, carbonic acid, amino acids

  4. 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

  5. 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

  6. 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

  7. 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

  8. DIPROTIC SYSTEMS H2A ↔ HA- + H+ (Ka1) + HA- + H2O ↔ H2A + OH- (Kb2) = H2O ↔ H+ + OH- Ka1 x Kb2 = Kw Ka2 x Kb1 = Kw

  9. 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

  10. DIPROTIC SYSTEMS The Intermediate Form - Is both an acid and a base - Can donate or accept a proton - Called amphiprotic

  11. TRIPROTIC SYSTEMS Ka1 x Kb3 = Kw Ka2 x Kb2 = Kw Ka3 x Kb1 = Kw First Intermediate (H2A-) Second Intermediate (HA2-)

  12. 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

  13. 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

  14. 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

  15. 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

  16. TITRATION CURVES Diprotic acids (two equivalence points) pH H2A/HA- HA-/A2- Excess OH- pKa2 pKa1 volume of OH- added

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