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.

Acid-Base Concepts: The Brønsted-Lowry Theory Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/2 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.

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

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

Examples Write a balanced equation for the dissociation of each of the following Bronsted-Lowry acids in water H 3 O + H 3 PO 3

Examples Write the conjugated base for the following acids HNO 3 H 2 SO 4 Write the conjugated acids for the following bases CO 3 2- PO3 3-

Acid Strength and Base Strength Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/8 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.

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? H 2 SO 4 (aq) + NH 3 (aq) NH 4 + (aq) + HSO 4 - (aq) HCO 3 - (aq) + SO 4 2- (aq) HSO 4 - (aq) + CO 3 2- (aq)

Hydrated Protons and Hydronium Ions Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/12 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

Dissociation of Water Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/14 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

The pH Scale Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/16 pH = -log[H 3 O 1+ ][H 3 O 1+ ] = 10 -pH pH < 7 pH > 7 pH = 7 Basic solution: Neutral solution: Acidic solution:

The pH Scale Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/17 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

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

The pH Scale Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/19 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

Examples Calculate the [ - OH] in a solution with [H 3 O + ] = 7.5 x M. Is this solution basic, acidic or neutral? Calculate the pH of a solution with [ - OH] = 8.2 x M Calculate the concentration of H 3 O + and – OH for a solution with a pH of 8.37

Measuring pH Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/21 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

Measuring pH Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/22

pH of strong acids and strong bases A strong monoprotic acds – 100% dissociated in aqueous solution (HClO 4, HCl, HNO 3 etc…) Contains a single dissociable proton HA(aq) + H 2 O(l)  H 3 O + (aq) + A - (aq) pH = - log H 3 O + [H 3 O + ] = [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 Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/25 H 3 O 1+ (aq) + NO 3 1- (aq)HNO 3 (aq) + H 2 O(l) 100% What is the pH of a M solution of HNO 3 ?

The pH in Solutions of Strong Acids and Strong Bases Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/26 What is the pH of a M solution of NaOH? Na 1+ (aq) + OH 1- (aq)NaOH(aq)

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 Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/28 Acid-Dissociation Constant: H 3 O 1+ (aq) + A 1- (aq)HA(aq) + H 2 O(l) [H 3 O 1+ ][A 1- ] [HA] K a = Partially ionized pK a = - log K a K a = antilog (- K a ) = 10 -pKa Larger K a, stronger the acid

Equilibria in Solutions of Weak Acids Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/30 The pH of M HF is What are the values of K a and pK a for hydrofluoric acid?

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, H 3 O + and A-)

Example Determine the concentration of all species present (H 3 O +, CH 3 CO 2 -, CH 3 CO 2 H) and pH of a M CH 3 CO 2 H K a = 1.8 x 10 -5

Example Determine the concentration of all species present (H 3 O +, CH 3 CO 2 -, CH 3 CO 2 H) and pH of a 1.00 M CH 3 CO 2 H K a = 1.8 x 10 -5

Example Find the pH and [ - OH] of a HClO 2 solution K a = 1.1 x 10 -2

Calculating Equilibrium Concentrations for Weak Acids Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/35 Calculate the pH of a 0.10 M HCN solution. At 25 °C, K a = 4.9 x

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

Example Find the percent ionization of M HClO 2 solution (previous example) Find the percent ionization of a 2.5M HNO 2 solution