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Polyprotic Acids

Polyprotic Acids. Acids that contains more than one dissociable proton Dissociate in a stepwise manner Each dissociation step has its own Ka Stepwise dissociation constants decreases in the order Ka 1 > Ka 2 > Ka 3 More difficult to remove a positively charge proton from negative ion.

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Polyprotic Acids

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  1. Polyprotic Acids • Acids that contains more than one dissociable proton • Dissociate in a stepwise manner • Each dissociation step has its own Ka • Stepwise dissociation constants decreases in the order Ka1 > Ka2 > Ka3 • More difficult to remove a positively charge proton from negative ion

  2. Polyprotic Acids • Diprotic acid solutions contain a mixture of acids: H2A, HA, H2O • Strongest acid – HA • Principle reaction – dissociation of H2A • All of H3O+ come from the first ionization H2SO4(aq) + H2O(l)  H3O+(aq) + HSO4-(aq) HSO4-(aq) + H2O(l) H3O+(aq) + SO42-

  3. Polyprotic acids • H2CO3(aq) + H2O(l) H3O+( aq) + HCO3-(aq) Ka1 = 4.3 x 10-7 HCO3-(aq) + H2O(l) H3O+( aq) + CO32-(aq) Ka2 = 4.8 x 10-11

  4. Polyprotic Acids

  5. [BH1+][OH1-] NH3(aq) + H2O(l) B(aq) + H2O(l) NH41+(aq) + OH1-(aq) BH1+(aq) + OH1-(aq) Kb = [B] [NH41+][OH1-] [NH3] Equilibria in Solutions of Weak Bases Base Acid Acid Base Base-Dissociation Constant: Kb =

  6. Equilibria in Solutions of Weak Bases

  7. Equilibria in Solutions of Weak Bases Calculate the [-OH] and pH of a 0.40 M NH3 solution. At 25 °C, Kb = 1.8 x 10-5.

  8. Example • Morphine (C17H19NO3), a narcotic used in painkillers, is a weak organic base. If the pH of a 7.0 x 10-4 M solution of morphine is 9.5, what is the value of Kb?

  9. NH41+(aq) + H2O(l) NH3(aq) + H2O(l) H3O1+(aq) + NH3(aq) NH41+(aq) + OH1-(aq) 2H2O(l) H3O1+(aq) + OH1-(aq) Relation Between Ka and Kb Ka Kb Kw [H3O1+][NH3] [NH41+][OH1-] Ka x Kb = x = [H3O1+][OH1-] = Kw [NH41+] [NH3] = (5.6 x 10-10)(1.8 x 10-5) = 1.0 x 10-14

  10. Kw Kw Ka Kb Relation Between Ka and Kb Ka x Kb = Kw conjugate acid-base pair Ka = Kb= pKa + pKb = pKw = 14.00

  11. Example • Find the pH of a 0.100 M NaCHO2 solution. The salt completely dissociate into Na+(aq) and CHO2-(aq) and Na+ ion has no acid or base properties. Ka (HCHO2)= 1.8 x 10-4

  12. Example • What is the pH of 0.10M sodium nicotinate at 25oC? The Ka for nicotinic acid is 1.4 x 10-5.

  13. Acid-Base Properties of Salts • pH of a salt solution is determined by the acid-base properties of the consistuent cations and anions • In an acid-base reaction, the influence of the stronger partner is predominant • Strong acid + Strong Base  Neutral solution • Strong acid + Weak Base  Basis solution • Weak acid + Strong Base  Acidic solution

  14. Acid-Base Properties of Salts • Acidic cation + neutral anion  Acidic salt NH4+ + Cl- NH4Cl • Neutral cation + neutral anion  neutral salt Na+ Cl- NaCl • Neutral cation + basic anion  basic salt Na+ CN- NaCN

  15. Acid-Base Properties of Salts • Acidic cation + basic anion  (50 :50 mixture) must compare Ka and Kb • Ka > Kb: The solution will contain an excess of H3O1+ ions (pH < 7). • Ka < Kb: The solution will contain an excess of OH1- ions (pH > 7). • Ka = Kb: The solution will contain approximately equal concentrations of H3O1+ and OH1- ions (pH ≈ 7).

  16. Acid-Base Properties of Salts Salts That Yield Acidic Solutions Hydrated cations of small, highly charged metal ions, such as Al3+.

  17. Acid-Base Properties of Salts

  18. Examples • Classify each of the following salt solution as acidic basic or neutral. Write a hydrolysis equation for each ion. • KBr • NaNO2 • NH4Br

  19. Example • Calculate the pH of a 0.10M solution of sodium fluoride (NaF) at 25oC. Ka = 7.1 x 10-4

  20. Examples • Calculate Ka for the cation, and Kb for the anion in an aqueous NH4CN solution. Is the solution acidic, basic or neutral? Write the hydrolysis reaction of the salt (Kb for NH3 = 1.8 x 10-5, Ka for HCN = 4.9 x 10-10)

  21. Example • Predict whether 0.35M NH4Br solution is acidic, basic or neutral. Calculate its pH. Kb = 1.8 x10-5

  22. Lewis Acids and Bases • Lewis Acid: An electron-pair acceptor. • Include cations and neutral molecule having vacant valence orbitals that can accept a share in a pair of electrons from a Lewis Base • Lewis Base: An electron-pair donor. • All Lewis bases are Bronsted-Lowry bases

  23. Lewis Acids and Bases

  24. Lewis Acids and Bases

  25. Lewis Acids and Bases

  26. Examples • For each of the following reactions, identify the Lewis acid and the Lewis base • CO2(g) + -OH(aq)  HCO3-(aq) • AlCl3(aq) + Cl-(aq)  AlCl4-(aq)

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