Chapter 14 Aqueous Equilibria: Acids and Bases

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

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 HCO 3 - (aq) + H 2 O(l) H 3 O + ( aq) + CO 3 2- (aq) K a2 = 4.8 x

Polyprotic Acids

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:

Equilibria in Solutions of Weak Bases

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)

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

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 M solution of morphine is 9.5, what is the value of K b ?

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 )(1.8 x ) = 1.0 x K a x K b =

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

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

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

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

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)

Acidic Salts A salt of a weak base and a strong acid. E.g NH 4 Cl Acidic cation + neutral anion  Acidic salt NH 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

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).

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 , K a for HCN = 4.9 x )

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

Acid-Base Properties of Salts

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

Example Find the pH of a 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

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

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

Lewis Acids and Bases Lewis Base Lewis Acid

Lewis Acids and Bases

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)