Lecture Notes Alan D. Earhart Southeast Community College Lincoln, NE Chapter 14 Aqueous Equilibria: Acids and Bases John E. McMurry Robert C. Fay CHEMISTRY Fifth Edition Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14/2 Acid-Base Concepts: The Brønsted-Lowry Theory Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H 1+. Arrhenius Base: A substance that dissociates in water to produce hydroxide ions, OH 1-. M 1+ (aq) + OH 1- (aq)MOH(aq) H 1+ (aq) + A 1- (aq)HA(aq)

Acid-Base Concepts: The Brønsted-Lowry Theory Conjugate Acid-Base Pairs: Chemical species whose formulas differ only by one hydrogen ion, H 1+. Brønsted-Lowry Acid: A substance that can transfer hydrogen ions, H 1+. In other words, a proton donor. Brønsted-Lowry Base: A substance that can accept hydrogen ions, H 1+. In other words, a proton acceptor.

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/4 Acid-Base Concepts: The Brønsted-Lowry Theory Acid-Dissociation Equilibrium Hydronium ion = H 3 O 1+

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/5 Acid-Base Concepts: The Brønsted-Lowry Theory Base-Dissociation Equilibrium

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/6 Acid Strength and Base Strength H 3 O 1+ (aq) + A 1- (aq)HA(aq) + H 2 O(l) BaseAcidBaseAcid Weaker acid + Weaker baseStronger acid + Stronger base With equal concentrations of reactants and products, what will be the direction of reaction?

Acid Strength and Base Strength Weak Acid: An acid that is only partially dissociated in water and is thus a weak electrolyte.

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/9 Hydrated Protons and Hydronium Ions H 1+ (aq) + A 1- (aq)HA(aq) [H(H 2 O) n ] 1+ For our purposes, H 1+ is equivalent to H 3 O 1+. n = 4H 9 O 4 1+ n = 1H 3 O 1+ n = 2H 5 O 2 1+ n = 3H 7 O 3 1+ Due to high reactivity of the hydrogen ion, it is actually hydrated by one or more water molecules.

Dissociation of Water H 3 O 1+ (aq) + OH 1- (aq)2H 2 O(l) K w = [H 3 O 1+ ][OH 1- ]Ion-Product Constant for Water: Dissociation of Water: K w = (1.0 x )(1.0 x ) = 1.0 x at 25°C:[H 3 O 1+ ] = [OH 1- ] = 1.0 x M

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/11 Dissociation of Water K w = [H 3 O 1+ ][OH 1- ] = 1.0 x [OH 1- ] = [H 3 O 1+ ] 1.0 x [H 3 O 1+ ] = [OH 1- ] 1.0 x

Dissociation of Water

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/13 The pH Scale pH = -log[H 3 O 1+ ][H 3 O 1+ ] = 10 -pH pH < 7 pH > 7 pH = 7 Basic solution: Neutral solution: Acidic solution:

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/14 The pH Scale The hydronium ion concentration for lemon juice is approximately What is the pH when [H 3 O 1+ ] = M? pH = -log(0.0025) = significant figures 2 decimal places

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/15 The pH Scale Calculate the pH of an aqueous ammonia solution that has an OH 1- concentration of M. pH = -log(5.3 x ) = = 5.3 x M x [H 3 O 1+ ] == [OH 1- ] 1.0 x

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/16 The pH Scale Acid rain is a matter of serious concern because most species of fish die in waters having a pH lower than Calculate [H 3 O 1+ ] in a lake that has a pH of 4.5. [H 3 O 1+ ] = = 3 x M

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/17 Measuring pH Acid-Base Indicator: A substance that changes color in a specific pH range. Indicators exhibit pH-dependent color changes because they are weak acids and have different colors in their acid (H I n) and conjugate base ( I n 1- ) forms. H 3 O 1+ (aq) + I n 1- (aq)H I n(aq) + H 2 O(l) Color BColor A

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/18 Measuring pH

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/19 The pH in Solutions of Strong Acids and Strong Bases Since HNO 3 is a strong acid, [H 3 O 1+ ] = [HNO 3 ]. H 3 O 1+ (aq) + NO 3 1- (aq)HNO 3 (aq) + H 2 O(l) pH = -log([H 3 O 1+ ]) = -log(0.025) = % What is the pH of a M solution of HNO 3 ?

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/20 The pH in Solutions of Strong Acids and Strong Bases Since NaOH is a strong base, [OH 1- ] = [NaOH]. pH = -log([H 3 O 1+ ]) = -log(4.0 x ) = What is the pH of a M solution of NaOH? Na 1+ (aq) + OH 1- (aq)NaOH(aq) = 4.0 x M x [H 3 O 1+ ] == [OH 1- ] 1.0 x

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/21 Acid-Dissociation Constant: Equilibria in Solutions of Weak Acids H 3 O 1+ (aq) + A 1- (aq)HA(aq) + H 2 O(l) [H 3 O 1+ ][A 1- ] [HA] K a =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/23 Equilibria in Solutions of Weak Acids The pH of M HF is What are the values of K a and pK a for hydrofluoric acid? H 3 O 1+ (aq) + F 1- (aq)HF(aq) + H 2 O(l) I 0.250≈00 C-x+x E xxx x = [H 3 O 1+ ] = = M

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/24 Equilibria in Solutions of Weak Acids ( )( ) = 3.51 x [HF] = x = = M [F 1- ] = [H 3 O 1+ ] = M [H 3 O 1+ ][F 1- ] [HF] K a = pK a = -log(K a ) = -log(3.51 x ) = [H 3 O 1+ ][F 1- ] [HF] =K a =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/25 Calculating Equilibrium Concentrations for Weak Acids Calculate the pH of a 0.10 M HCN solution. At 25 °C, K a = 1.4 x H 3 O 1+ (aq) + CN 1- (aq)HCN(aq) + H 2 O(l) I 0.10≈00 C-x+x E xxx [H 3 O 1+ ][CN 1- ] [HCN] K a =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/26 Calculating Equilibrium Concentrations for Weak Acids x2x ≈4.9 x = (x)(x) ( x) pH =-log([H 3 O 1+ ]) = -log(7.0 x ) = 5.15 x = [H 3 O 1+ ] = 7.0 x M

Percent Dissociation in Solutions of Weak Acids Percent dissociation = [HA] initial [HA] dissociated x 100%

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/29 Polyprotic Acids H 3 O 1+ (aq) + HCO 3 1- (aq)H 2 CO 3 (aq) + H 2 O(l) [H 3 O 1+ ] [HCO 3 1- ] [H 2 CO 3 ] = 4.3 x H 3 O 1+ (aq) + CO 3 2- (aq)HCO 3 1- (aq) + H 2 O(l) [H 3 O 1+ ] [CO 3 2- ] [HCO 3 1- ] = 5.6 x K a1 = K a2 =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/30 Polyprotic Acids

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/31 Polyprotic Acids Calculate the pH of a M H 2 CO 3 solution. At 25 °C, K a1 = 4.3 x H 3 O 1+ (aq) + HCO 3 1- (aq)H 2 CO 3 (aq) + H 2 O(l) I 0.020≈00 C-x+x E xxx [H 3 O 1+ ][HCO 3 1- ] [H 2 CO 3 ] K a1 =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/32 Polyprotic Acids x2x ≈4.3 x = (x)(x) ( x) pH =-log([H 3 O 1+ ]) = -log(9.3 x ) = 4.03 x = [H 3 O 1+ ] = 9.3 x M

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/33 Equilibria in Solutions of Weak Bases NH 4 1+ (aq) + OH 1- (aq)NH 3 (aq) + H 2 O(l) BaseAcid 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:

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/34 Equilibria in Solutions of Weak Bases

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/35 Equilibria in Solutions of Weak Bases Calculate the pH of a 0.40 M NH 3 solution. At 25 °C, K b = 1.8 x [NH 4 1+ ][OH 1- ] [NH 3 ] K b = NH 4 1+ (aq) + OH 1- (aq)NH 3 (aq) + H 2 O(l) I 0.400≈0 C-x+x E xxx

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/36 Equilibria in Solutions of Weak Bases x2x ≈1.8 x = (x)(x) ( x) pH =-log([H 3 O 1+ ]) = -log(3.7 x ) = x = [OH 1- ] = M = 3.7 x M x [H 3 O 1+ ] =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/37 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 =

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/38 Relation Between K a and K b pK a + pK b = pK w = K a x K b = K w K b = KaKa KwKw K a = KbKb KwKw conjugate acid-base pair

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/39 Acid-Base Properties of Salts Cations from strong bases: Alkali metal cations of group 1a (Li 1+, Na 1+, K 1+ ) Alkaline earth metal cations of group 2a (Ca 2+, Sr 2+, Ba 2+ ), except for Be 2+ Anions from strong monoprotic acids: Cl 1-, Br 1-, I 1-, NO 3 1-, and ClO 4 1- The following ions do not react appreciably with water to produce either H 3 O 1+ or OH 1- ions: Salts That Yield Neutral Solutions

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/40 Acid-Base Properties of Salts Salts That Yield Acidic Solutions Ammonium ion (NH 4 1+ ) is the conjugate acid of the weak base ammonia (NH 3 ) while chloride ion (Cl 1- ) is neither acidic nor basic. H 3 O 1+ (aq) + NH 3 (aq)NH 4 1+ (aq) + H 2 O(l) Salts such as NH 4 Cl that are derived from a weak base (NH 3 ) and a strong acid (HCl) yield acidic solutions.

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

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/43 Acid-Base Properties of Salts Salts That Yield Basic Solutions Cyanide ion (CN 1- ) is the conjugate base of the weak acid hydrocyanic acid (HCN) while sodium ion (Na 1+ ) is neither acidic nor basic. HCN(aq) + OH 1- (aq)CN 1- (aq) + H 2 O(l) Salts such as NaCN that are derived from a strong base (NaOH) and a weak acid (HCN) yield basic solutions.

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/44 Acid-Base Properties of Salts Salts That Contain Acidic Cations and Basic Anions The pH of an ammonium carbonate solution, (NH 4 ) 2 CO 3, depends on the relative acid strength of the cation and the relative base strength of the anion. Is it acidic or basic?

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/45 Acid-Base Properties of Salts Salts That Contain Acidic Cations and Basic Anions HCO 3 1- (aq) + OH 1- (aq)CO 3 2- (aq) + H 2 O(l) KbKb H 3 O 1+ (aq) + NH 3 (aq)NH 4 1+ (aq) + H 2 O(l) KaKa (NH 4 ) 2 CO 3 : Three possibilities: 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).

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/46 Acid-Base Properties of Salts Salts That Contain Acidic Cations and Basic Anions HCO 3 1- (aq) + OH 1- (aq)CO 3 2- (aq) + H 2 O(l) KbKb H 3 O 1+ (aq) + NH 3 (aq)NH 4 1+ (aq) + H 2 O(l) KaKa (NH 4 ) 2 CO 3 : = 1.8 x x x K b for CO 3 2- = K a for HCO 3 1- KwKw = = 5.6 x x x K a for NH 4 1+ = K b for NH 3 KwKw = Basic, K a < K b

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/47 Acid-Base Properties of Salts

Factors That Affect Acid Strength Bond Strength

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/49 Factors That Affect Acid Strength Bond Polarity

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/50 Factors That Affect Acid Strength Oxoacids

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/51 Factors That Affect Acid Strength Oxoacids

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/52 Lewis Acids and Bases Lewis Base: An electron-pair donor. Lewis Acid: An electron-pair acceptor.

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 14/53 Lewis Acids and Bases Lewis Base: An electron-pair donor. Lewis Acid: An electron-pair acceptor.