Chapter 15 Acid–Base Equilibria

Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 2 Key Points about Buffered Solutions Buffered Solution – resists a change in pH. They are composed of :  a weak acid and its conjugate base such as HC 2 H 3 O 2 and NaC 2 H 3 O 2  or a weak base and its conjugate acid such as NH 3 and NH 4 Cl.

Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 3 Buffers

Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 4 Henderson–Hasselbalch Equation A - = Base HA = Acid pK a = - log K a For buffers that have the same ratio [A – ] / [HA], pH = pK a.

Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 5 Exercise What is the pH of a buffer solution that is 0.45 M acetic acid (HC 2 H 3 O 2 ) and 0.85 M sodium acetate (NaC 2 H 3 O 2 )? The K a for acetic acid is 1.8 × 10 –5. pH = 5.02

Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 6 Buffered Solution Characteristics Buffers contain relatively large amounts of weak acid and corresponding conjugate base. Added H + reacts with the conjugate base. Added OH  reacts with the weak acid.

Section 15.3 The MoleBuffering Capacity Return to TOC Copyright © Cengage Learning. All rights reserved 7 The amount of protons or hydroxide ions the buffer can absorb without a significant change in pH. Determined by the magnitudes of [HA] and [A – ]. A buffer with large capacity contains large concentrations of the buffering components.

Section 15.3 The MoleBuffering Capacity Return to TOC Copyright © Cengage Learning. All rights reserved 8 Optimal buffering occurs when [HA] is equal to [A – ]. It is for this condition that the ratio [A – ] / [HA] is most resistant to change when H + or OH – is added to the buffered solution.

Section 15.3 The MoleBuffering Capacity Return to TOC Copyright © Cengage Learning. All rights reserved 9 Choosing a Buffer pK a of the weak acid to be used in the buffer should be as close as possible to the desired pH.

Section 15.3 The MoleBuffering Capacity Return to TOC Example A chemist needs a solution buffered at pH = Which of the following acids (and their Na+ salts) would make the best buffer? A. chloroacetic acid (K a = 1.35x10 -3 ) B. propanoic acid (K a = 1.30x10 -5 ) C. benzoic acid (K a = 6.40x10 -5 ) benzoic acid Copyright © Cengage Learning. All rights reserved 10

Section 15.3 The MoleBuffering Capacity Return to TOC Indirect Buffers 1.What is the pH of a solution made by mixing mol NaOH and mol HC 2 H 3 O 2 in a 1.0 L solution? 4.14 Copyright © Cengage Learning. All rights reserved 11

Section 15.3 The MoleBuffering Capacity Return to TOC Example Are the following solutions buffers? A mol HC 2 H 3 O 2 and mol NaC 2 H 3 O 2 yes, direct buffer (weak acid and CB) B mol HCl and mol NaCl no, strong acid and CB C mol HNO 2 and mol NaOH yes, indirect buffer and strong base is LR D mol HNO 2 and mol NaOH no, indirect buffer and strong base is not LR Copyright © Cengage Learning. All rights reserved 12

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 13 Titration Curve Plotting the pH of the solution being analyzed as a function of the amount of titrant added. Equivalence Point – point in the titration when moles acid = moles of base Endpoint – color change

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 14 Neutralization of a Strong Acid with a Strong Base

Section 15.4 Titrations and pH Curves Return to TOC Strong Acid-Strong Base 1)50.00 ml of M HCl is titrated with M NaOH. What is the pH before any NaOH is added? 0.00 B) ml of NaOH 0.48 C) ml of NaOH 4.0 Copyright © Cengage Learning. All rights reserved 15

Section 15.4 Titrations and pH Curves Return to TOC Strong Acid- Strong Base D) ml of NaOH (equivalence pt) 7.0 E) ml of NaOH 10.0 F) ml of NaOH 13.3 Copyright © Cengage Learning. All rights reserved 16

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 17 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 18 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 19 Weak Acid–Strong Base 1)50.00 ml of M HC 2 H 3 O 2 is titrated with 1.000M NaOH. What is the pH when 0.00 ml of NaOH are added? 2.37 B) ml of NaOH (midpoint) 4.74 C) ml of NaOH (equivalence pt) 9.22 D) ml of NaOH 13.3

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 20 The pH Curve for the Titration of 50.0 mL of M HC 2 H 3 O 2 with M NaOH

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 21 Concept Check Calculate the pH of a solution made by mixing 0.20 mol HC 2 H 3 O 2 (K a = 1.8 x 10 –5 ) with mol NaOH in 1.0 L of aqueous solution.

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 22 BUFFER 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

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 24 Concept Check Calculate the pH of a solution made by mixing mL of a M solution of acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5, and 50.0 mL of a 0.10 M NaOH solution. pH = 4.74

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 25 Concept Check Calculate the pH of a solution at the equivalence point when mL of a M solution of acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5, is titrated with a 0.10 M NaOH solution. pH = 8.72

Section 15.4 Titrations and pH Curves Return to TOC Weak Base- Strong Acid 1)50.00 ml of NH 3 is titrated with M HCl. What is the pH when no HCl has been added? B) ml HCl (midpoint) 9.25 C) ml HCl (endpoint) 4.78 D) 75.0 ml HCl 0.70 Copyright © Cengage Learning. All rights reserved 26

Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 27 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 28 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 29 The Acid and Base Forms of the Indicator Phenolphthalein

Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 30 The Methyl Orange Indicator is Yellow in Basic Solution and Red in Acidic Solution

Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 31 Useful pH Ranges for Several Common Indicators