Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Chapter 14 Aqueous Equilibria Acids and Bases Copyright © 2008 Pearson Prentice Hall, Inc.

Acid-Base Concepts: The Brønsted-Lowry Theory Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Acid-Base Concepts: The Brønsted-Lowry Theory Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H1+. H1+(aq) + A1-(aq) HA(aq) Arrhenius Base: A substance that dissociates in water to produce hydroxide ions, OH1-. M1+(aq) + OH1-(aq) MOH(aq) Ammonia, NH3, can’t be explained using Arrhenius theory. Copyright © 2008 Pearson Prentice Hall, Inc.

Acid-Base Concepts: The Brønsted-Lowry Theory Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Acid-Base Concepts: The Brønsted-Lowry Theory Brønsted-Lowry Acid: A substance that can transfer hydrogen ions, H1+. In other words, a proton donor. Brønsted-Lowry Base: A substance that can accept hydrogen ions, H1+. In other words, a proton acceptor. Conjugate Acid-Base Pairs: Chemical species whose formulas differ only by one hydrogen ion, H1+. Copyright © 2008 Pearson Prentice Hall, Inc.

Acid-Base Concepts: The Brønsted-Lowry Theory Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Acid-Base Concepts: The Brønsted-Lowry Theory Acid-Dissociation Equilibrium An acid-dissociation will have hydronium ion as the conjugate acid of water. Hydronium ion = H3O1+ Copyright © 2008 Pearson Prentice Hall, Inc.

Acid-Base Concepts: The Brønsted-Lowry Theory Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Acid-Base Concepts: The Brønsted-Lowry Theory Base-Dissociation Equilibrium A base-dissociation will have hydroxide ion as the conjugate base of water. Copyright © 2008 Pearson Prentice Hall, Inc.

Examples Write a balanced equation for the dissociation of each of the following Bronsted-Lowry acids in water H3O+ H3PO3

Examples Write the conjugated base for the following acids HNO3 H2SO4 Write the conjugated acids for the following bases CO32- PO33-

Acid Strength and Base Strength Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Acid Strength and Base Strength H3O1+(aq) + A1-(aq) HA(aq) + H2O(l) Acid Base Acid Base With equal concentrations of reactants and products, what will be the direction of reaction? Weaker acid + Weaker base Stronger acid + Stronger base The direction of reaction will be in the direction of the weaker acid and weaker base. Copyright © 2008 Pearson Prentice Hall, Inc.

Acid Strength and Base Strength Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Acid Strength and Base Strength Weak Acid: An acid that is only partially dissociated in water and is thus a weak electrolyte. We dealt with weak acids initially in Ch 4 (Reactions in Aqueous Solution). Copyright © 2008 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 11:50:20 PM Copyright © 2011 Pearson Prentice Hall, Inc.

Examples If you mix equal amount concentrations of reactants and products, which of the following reations proceed to the right and which proceed to the left? H2SO4(aq) + NH3(aq) NH4+(aq) + HSO4-(aq) HF(aq) + NO3-(aq) HNO3(aq) + F-(aq)

Hydrated Protons and Hydronium Ions Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Hydrated Protons and Hydronium Ions H1+(aq) + A1-(aq) HA(aq) Due to high reactivity of the hydrogen ion, it is actually hydrated by one or more water molecules. n = 4 H9O41+ n = 1 H3O1+ n = 2 H5O21+ n = 3 H7O31+ [H(H2O)n]1+ For our purposes, H1+ is equivalent to H3O1+. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases Dissociation of Water 9/20/2018 H3O1+(aq) + OH1-(aq) 2H2O(l) Dissociation of Water: Ion-Product Constant for Water: Kw = [H3O1+][OH1-] This value of Kw is only at 25 °C. at 25°C: [H3O1+] = [OH1-] = 1.0 x 10-7 M Kw = (1.0 x 10-7)(1.0 x 10-7) = 1.0 x 10-14 Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Dissociation of Water Kw = [H3O1+][OH1-] = 1.0 x 10-14 [H3O1+] = [OH1-] 1.0 x 10-14 [OH1-] = [H3O1+] 1.0 x 10-14 Copyright © 2008 Pearson Prentice Hall, Inc.

Dissociation of Water Chemistry: McMurry and Fay, 6th Edition Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 11:50:20 PM Dissociation of Water Copyright © 2011 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 The pH Scale pH = -log[H3O1+] [H3O1+] = 10 -pH Basic solution: pH > 7 Neutral solution: pH = 7 Acidic solution: pH < 7 The scale is logarithmic which means each unit change for pH corresponds to a factor of 10 change in hydronium ion concentration. Also, since pH is dependent on Kw, this scale is only good at 25 °C. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases The pH Scale 9/20/2018 The hydronium ion concentration for lemon juice is approximately 0.0025. What is the pH when [H3O1+] = 0.0025 M? 2 significant figures pH = -log(0.0025) = 2.60 2 decimal places Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 The pH Scale Calculate the pH of an aqueous ammonia solution that has an OH1- concentration of 0.0019 M. [OH1-] 1.0 x 10-14 0.0019 1.0 x 10-14 [H3O1+] = = = 5.3 x 10-12 M pH = -log(5.3 x 10-12) = 11.28 Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 The pH Scale Acid rain is a matter of serious concern because most species of fish die in waters having a pH lower than 4.5-5.0. Calculate [H3O1+] in a lake that has a pH of 4.5. [H3O1+] = 10 -4.5 = 3 x 10-5 M Only 1 significant figure in the concentration since the pH has only 1 decimal place. Copyright © 2008 Pearson Prentice Hall, Inc.

Examples Calculate the [-OH] in a solution with [H3O+] = 7.5 x 10-5M. Is this solution basic, acidic or neutral? Calculate the pH of a solution with [-OH] = 8.2 x 10-10M Calculate the concentration of H3O+ and –OH for a solution with a pH of 8.37

Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 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 (HIn) and conjugate base (In1-) forms. H3O1+(aq) + In1-(aq) HIn(aq) + H2O(l) Color A Color B Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Measuring pH Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

pH of strong acids and strong bases A strong monoprotic acds – 100% dissociated in aqueous solution (HClO4, HCl, HNO3 etc…) Contains a single dissociable proton HA(aq) + H2O(l)  H3O+(aq) + A-(aq) pH = - log H3O+ [H3O+] = [A-] = initial concentration of the acid Undissociated [HA] = 0

Strong Bases Alkali metal hydroxide, MOH Water-soluble ionic solids Exits in aqueous solution as alkali metal cations and hydroxide anions Calculate pH from [-OH] Alkaline earth metal hydroxide, M(OH)2 where M= Mg, Ca, Sr, Ba Less soluble than alkali hydroxide, therefore lower [-OH]

The pH in Solutions of Strong Acids and Strong Bases Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 The pH in Solutions of Strong Acids and Strong Bases What is the pH of a 0.025 M solution of HNO3? 100% H3O1+(aq) + NO31-(aq) HNO3(aq) + H2O(l) pH < 7 which makes sense since it’s an acid. Copyright © 2008 Pearson Prentice Hall, Inc.

The pH in Solutions of Strong Acids and Strong Bases Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 The pH in Solutions of Strong Acids and Strong Bases What is the pH of a 0.025 M solution of NaOH? Na1+(aq) + OH1-(aq) NaOH(aq) pH > 7 which makes sense since it’s a base. An alternate method is to calculate pOH from [OH1-] and then subtract that from 14.00, pKw. Copyright © 2008 Pearson Prentice Hall, Inc.

Example Calculate the pH of a solution by dissolving 0.25 g of CaO in enough water to make 0.500L of limewater Ca(OH)2 Calculate the [-OH] and pH of 1.0 M HCl

Equilibria in Solutions of Weak Acids Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Equilibria in Solutions of Weak Acids Partially ionized H3O1+(aq) + A1-(aq) HA(aq) + H2O(l) Acid-Dissociation Constant: [H3O1+][A1-] [HA] Ka = Larger Ka , stronger the acid pKa = - log Ka Ka = antilog (- Ka) = 10-pKa Copyright © 2008 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 11:50:21 PM © 2012 Pearson Education, Inc. Copyright © 2011 Pearson Prentice Hall, Inc.

Calculating Equilibrium Concentrations in Solution of Weak acids Step 1: Write the balance equation for all possible proton transfer (both acids and water) Step 2: Identify the principle reaction (the reaction that has larger Ka) Step 3: Generate an ICE table Step 4: Solve for x Step 5: Calculate pH and all other concentrations (HA, H3O+ and A-)

Example Determine the concentration of all species present (H3O+, CH3CO2-, CH3CO2H) and pH of a 0.0100 M CH3CO2H Ka = 1.8 x 10-5

Example Determine the concentration of all species present (H3O+, CH3CO2-, CH3CO2H) and pH of a 1.00 M CH3CO2H Ka = 1.8 x 10-5

Example Find the pH and [-OH] of a 0.100 HClO2 solution Ka = 1.1 x 10-2

Calculating Equilibrium Concentrations for Weak Acids Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Calculating Equilibrium Concentrations for Weak Acids Calculate the pH of a 0.10 M HCN solution. At 25 °C, Ka = 4.9 x 10-10. The assumption is that the hydronium ion produced from the dissociation is far greater than that from water. Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Percent Dissociation in Solutions of Weak Acids Chapter 14: Aqueous Equilibria: Acids and Bases 9/20/2018 Percent Dissociation in Solutions of Weak Acids [HA] initial [HA] dissociated Percent dissociation = x 100% Copyright © 2008 Pearson Prentice Hall, Inc.

Example Find the percent ionization of 0.100 M HClO2 solution (previous example) Find the percent ionization of a 2.5M HNO2 solution