Chapter 15 Lesson 2 Acid–Base Equilibria

Chapter 15 Table of Contents Copyright © Cengage Learning. All rights reserved Solutions of Acids or Bases Containing a Common Ion 15.2 Buffered Solutions 15.3 Buffering Capacity 15.4Titrations and pH Curves 15.5 Acid–Base Indicators

pH mL of Base added 7 l Strong acid with strong Base l Equivalence at pH 7

pH mL of Base added >7 l Weak acid with strong Base l Equivalence at pH >7 l When the acid is neutralized it makes a weak base 7

pH mL of acid added 7 l Strong base with strong acid l Equivalence at pH 7

pH mL of acid added <7 l Weak base with strong acid l Equivalence at pH <7 l When the base is neutralized it makes a weak acid 7

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 7 The pH Curve for the Titration of 50.0 mL of M HNO 3 with M NaOH

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 8 The pH Curve for the Titration of mL of 0.50 M NaOH with 1.0 M HCI

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 9 Concept Check Consider a solution made by mixing 0.20 mol of HC 2 H 3 O 2 (K a = 1.8 x 10 –5 ) with mol NaOH in 1.0 L of aqueous solution. What are the major species immediately upon mixing (that is, before a reaction)? HC 2 H 3 O 2, Na +, OH –, H 2 O

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 10 Let’s Think About It… HC 2 H 3 O 2 (aq) +OH –  C 2 H 3 O 2 – (aq)+ H 2 O Before0.20 mol mol0 Change–0.030 mol mol After0.17 mol mol

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 11 Steps Toward Solving for pH K a = 1.8 x 10 –5 pH = 3.99 HC 2 H 3 O 2 (aq) +H2OH2OH3O+H3O+ + C 2 H 3 O 2 - (aq) Initial M~ M Change –x+x Equilibrium – xx x

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 12 Exercise Calculate the pH of a mL solution of M acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5. pH = 2.87

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 13 The pH Curve for the Titration of 50.0 mL of M HF with 10.0 mL of M NaOH

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 14 The pH Curves for the Titrations of 50.0-mL Samples of 0.10 M Acids with Various K a Values with 0.10 M NaOH

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 15 The pH Curve for the Titration of 100.0mL of M NH 3 with 0.10 M HCl

Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 16 Marks the end point of a titration by changing color. The equivalence point is not necessarily the same as the end point (but they are ideally as close as possible).

Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 17 The Acid and Base Forms of the Indicator Phenolphthalein

Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 18 The Methyl Orange Indicator is Yellow in Basic Solution and Red in Acidic Solution

Section 15.5 Acid–Base Indicators Return to TOC 1) Usually a weak organic acid or base that has distinctly different colors in its nonionized & ionized forms. HIn (aq)  H + (aq) + In - (aq) pK HIn nonionized ionized form form

Section 15.5 Acid–Base Indicators Return to TOC Indicators Weak acids that change color when they become bases. weak acid written HIn Weak base HIn H + + In - clear red Equilibrium is controlled by pH End point - when the indicator changes color. Try to match the equivalence point

Section 15.5 Acid–Base Indicators Return to TOC Indicators Since it is an equilibrium the color change is gradual. It is noticeable when the ratio of [In - ]/[HI] or [HI]/[In - ] is 1/10 Since the Indicator is a weak acid, it has a Ka. pH the indicator changes at is. pH=pKa +log([In - ]/[HI]) = pKa +log(1/10) pH=pKa - 1 on the way up

Section 15.5 Acid–Base Indicators Return to TOC Indicators pH=pKa + log([HI]/[In - ]) = pKa + log(10) pH=pKa+1 on the way down Choose the indicator with a pKa 1 more than the pH at equivalence point if you are titrating with base. Choose the indicator with a pKa 1 less than the pH at equivalence point if you are titrating with acid.

Section 15.5 Acid–Base Indicators Return to TOC 1) 2) 3)

Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 24 Useful pH Ranges for Several Common Indicators

Section 15.4 Titrations and pH Curves Return to TOC P.228 Kjeldahl Nitrogen Analysis : developed in 1883, the analysis remains one of the most widely used methods for determining nitrogen in organic substances such as protein, cereal, and flour.

Section 15.4 Titrations and pH Curves Return to TOC Melamine contaminated milk