1. Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
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2. 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

3. 14.11 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
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. Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
Polyprotic Acids
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5. 14.12 Equilibria in Solutions of Weak Bases
Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
14.12 Equilibria in Solutions of Weak Bases
BH1+(aq) + OH1-(aq)
B(aq) + H2O(l)
Base
Acid
Acid
Base
[BH1+][OH1-]
[B]
Kb =
Base-Dissociation Constant:
NH41+(aq) + OH1-(aq)
NH3(aq) + H2O(l)
[NH41+][OH1-]
[NH3]
Kb =
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6. Equilibria in Solutions of Weak Bases
Chemistry: McMurry and Fay, 6th Edition
Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018 4:44:26 AM
Equilibria in Solutions of Weak Bases
Copyright © 2011 Pearson Prentice Hall, Inc.

7. Equilibria in Solutions of Weak Bases
Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
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.
While the initial hydroxide ion concentration is not 0, it’s insignificant when compared to that which comes from the dissociation of NH3.
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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. 14.13 Relation Between Ka and Kb
Chapter 14: Aqueous Equilibria: Acids and Bases
14.13 Relation Between Ka and Kb
4/14/2018
H3O1+(aq) + NH3(aq)
NH41+(aq) + H2O(l) Ka
NH41+(aq) + OH1-(aq)
NH3(aq) + H2O(l) Kb
H3O1+(aq) + OH1-(aq)
2H2O(l) 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
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10. Relation Between Ka and Kb
Chapter 14: Aqueous Equilibria: Acids and Bases
Relation Between Ka and Kb
4/14/2018
Ka x Kb = Kw
conjugate acid-base pair Kb Kw Ka Kw
Ka =
Kb =
pKa + pKb = pKw = 14.00
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11. Examples Calculate Kb for CN- (Ka = 4.9 x 10-10)
Ka for HOCl = 3.5 x 10-8, find Kb and pKb

12. 14.14 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. Neutral Salt A salt of a strong base and a strong acid. E.g NaCl
Neutral cation + neutral anion  neutral salt
Na+ Cl- NaCl
Na+(aq) + H2O(l)  NR
Cl –(aq) + H2O(l)  NR

15. Basic Salts A salt of a strong base and a weak acid. E.g NaCN
Neutral cation + basic anion  basic salt
Na CN- NaCN
Na+(aq) + H2O(l)  NR
CN-(aq) + H2O(l)  HCN(aq) + -OH(aq)

16. Acidic Salts A salt of a weak base and a strong acid. E.g NH4Cl
Acidic cation + neutral anion  Acidic salt
NH Cl NH4Cl
NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq)
Cl –(aq) + H2O(l)  NR

17. Acid-Base Properties of Salts
A salt of a weak base and a weak acid
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).

18. 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)

19. Acid-Base Properties of Salts
Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
Acid-Base Properties of Salts
Salts That Yield Acidic Solutions
Hydrated cations of small, highly charged metal ions, such as Al3+.
The cations listed previously are slightly acidic even though we ignore them at this level. Acidity increases as the charge increases and also as the ion size decreases.
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20. Acid-Base Properties of Salts
Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
Acid-Base Properties of Salts
Copyright © 2008 Pearson Prentice Hall, Inc.

21. Example
Predict the acidity of this salt solution and calculate the pH of a 0.10M solution of sodium fluoride (NaF) at 25oC.
Ka = 7.1 x 10-4

22. Example
Find the pH of a 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

23. Example
What is the pH of a 0.30M solution for methylammonium chloride, CH3NH3Cl?
Kb = 4.4 x 10-4

24. Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
14.16 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
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25. Lewis Acids and Bases Lewis Base Lewis Acid
Chemistry: McMurry and Fay, 6th Edition
Chapter 14: Aqueous Equilibria: Acids and Bases
Lewis Acids and Bases
4/14/2018 4:44:26 AM
Lewis Base
Lewis Acid
Copyright © 2011 Pearson Prentice Hall, Inc.

26. Chapter 14: Aqueous Equilibria: Acids and Bases
4/14/2018
Lewis Acids and Bases
Copyright © 2008 Pearson Prentice Hall, Inc.

27. Examples
For each of the following reactions, identify the Lewis acid and the Lewis base
CO2(g) OH(aq) HCO3-(aq)
B(OH)3(aq) + H2O(l) B(OH)4-(aq) + H+ (aq)