1. Chapter 14 Aqueous Equilibria: Acids and Bases

2. 14.11 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

3. 14.11 Polyprotic Acids Diprotic acid solutions contain a mixture of acids: H 2 A, HA, H 2 O ◦ Strongest acid – HA  Principle reaction – dissociation of H 2 A  All of H 3 O + come from the first ionization H 2 CO 3 (aq) + H 2 O(l) H 3 O + ( aq) + HCO 3 - (aq) K a1 = 4.3 x 10 -7 HCO 3 - (aq) + H 2 O(l) H 3 O + ( aq) + CO 3 2- (aq) K a2 = 4.8 x 10 -11

4. Polyprotic Acids

5. 14.12 Equilibria in Solutions of Weak Bases NH 4 1+ (aq) + OH 1- (aq)NH 3 (aq) + H 2 O(l) Base Acid Base [NH 4 1+ ][OH 1- ] [NH 3 ] K b = BH 1+ (aq) + OH 1- (aq)B(aq) + H 2 O(l) [BH 1+ ][OH 1- ] [B] K b = Base-Dissociation Constant:

6. Equilibria in Solutions of Weak Bases

7. Examples Write a balanced dissociation equation for the following base. Then write the K b expression C 6 H 5 NH 2 (aq) NH 2 OH(aq)

8. Equilibria in Solutions of Weak Bases Calculate the [ - OH] and pH of a 0.40 M NH 3 solution. At 25 °C, K b = 1.8 x 10 -5.

9. Example Morphine (C 17 H 19 NO 3 ), 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 K b ?

10. 14.13 Relation Between K a and K b H 3 O 1+ (aq) + OH 1- (aq)2H 2 O(l) H 3 O 1+ (aq) + NH 3 (aq)NH 4 1+ (aq) + H 2 O(l) NH 4 1+ (aq) + OH 1- (aq)NH 3 (aq) + H 2 O(l) [NH 4 1+ ][OH 1- ] [NH 3 ] [H 3 O 1+ ][NH 3 ] [NH 4 1+ ] x KwKw KaKa KbKb = [H 3 O 1+ ][OH 1- ] = K w = (5.6 x 10 -10 )(1.8 x 10 -5 ) = 1.0 x 10 -14 K a x K b =

11. Relation Between K a and K b pK a + pK b = pK w = 14.00 K a x K b = K w Kb =Kb = KaKa KwKw K a = KbKb KwKw conjugate acid-base pair

12. Examples Calculate K b for CN - (Ka = 4.9 x 10 -10 ) K a for HOCl = 3.5 x 10 -8, find K b and pK b

13. 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  Acidic solution ◦ Weak acid + Strong Base  Basis solution

15. Neutral Salt A salt of a strong base and a strong acid. E.g NaCl Neutral cation + neutral anion  neutral salt Na + Cl - NaCl Hydrolysis equations: Na + (aq) + H 2 O(l)  NR Cl – (aq) + H 2 O(l)  NR

16. Basic Salts A salt of a strong base and a weak acid. E.g NaCN Neutral cation + basic anion  basic salt Na + CN - NaCN Hydrolysis equations: Na + (aq) + H 2 O(l)  NR CN-(aq) + H 2 O(l)  HCN(aq) + - OH(aq)

17. Acidic Salts A salt of a weak base and a strong acid. E.g NH 4 Cl Acidic cation + neutral anion  Acidic salt NH 4 + + Cl - NH 4 Cl Hydrolysis equations: NH 4 + (aq) + H 2 O(l)  NH 3 (aq) + H 3 O + (aq) Cl – (aq) + H 2 O(l)  NR

18. Acid-Base Properties of Salts A salt of a weak base and a weak acid Acidic cation + basic anion  (50 :50 mixture) must compare K a and K b K a > K b : The solution will contain an excess of H 3 O 1+ ions (pH < 7). K a 7). K a = K b : The solution will contain approximately equal concentrations of H 3 O 1+ and OH 1- ions (pH ≈ 7).

19. Examples Calculate K a for the cation, and K b for the anion in an aqueous NH 4 CN solution. Is the solution acidic, basic or neutral? Write the hydrolysis reaction of the salt (K b for NH 3 = 1.8 x 10 - 5, K a for HCN = 4.9 x 10 -10 )

20. Acid-Base Properties of Salts Salts That Yield Acidic Solutions Hydrated cations of small, highly charged metal ions, such as Al 3+.

21. Acid-Base Properties of Salts

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

23. Example Find the pH of a 0.100 M NaCHO 2 solution. The salt completely dissociate into Na + (aq) and CHO 2 - (aq) and Na + ion has no acid or base properties. K a (HCHO 2 )= 1.8 x 10 -4

24. Example What is the pH of a 0.30M solution for methylammonium chloride, CH 3 NH 3 Cl? K b = 4.4 x 10 -4

25. 14.16 Lewis Acids and Bases Lewis Base: An electron-pair donor. All Lewis bases are Bronsted-Lowry 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

26. Lewis Acids and Bases Lewis Base Lewis Acid

27. Lewis Acids and Bases

28. Examples For each of the following reactions, identify the Lewis acid and the Lewis base ◦ CO 2 (g) + - OH(aq) HCO 3 - (aq) ◦ B(OH) 3 (aq) + H 2 O(l) B(OH) 4 - (aq) + H + (aq)