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Chapter 15 Acid–Base Equilibria. Chapter 15 Table of Contents Copyright © Cengage Learning. All rights reserved 2 15.1Solutions of Acids or Bases Containing.

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Presentation on theme: "Chapter 15 Acid–Base Equilibria. Chapter 15 Table of Contents Copyright © Cengage Learning. All rights reserved 2 15.1Solutions of Acids or Bases Containing."— Presentation transcript:

1 Chapter 15 Acid–Base Equilibria

2 Chapter 15 Table of Contents Copyright © Cengage Learning. All rights reserved 2 15.1Solutions 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

3 Section 15.1 Solutions of Acids or Bases Containing a Common Ion Return to TOC Copyright © Cengage Learning. All rights reserved 3 Common Ion Effect Shift in equilibrium position that occurs because of the addition of an ion already involved in the equilibrium reaction. An application of Le Châtelier’s principle.

4 Section 15.1 Solutions of Acids or Bases Containing a Common Ion Return to TOC Copyright © Cengage Learning. All rights reserved 4 Example HCN(aq) + H 2 O(l) H 3 O + (aq) + CN - (aq) Addition of NaCN will shift the equilibrium to the left because of the addition of CN -, which is already involved in the equilibrium reaction. A solution of HCN and NaCN is less acidic than a solution of HCN alone.

5 Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 5 Key Points about Buffered Solutions Buffered Solution – resists a change in pH. They are weak acids or bases containing a common ion. After addition of strong acid or base, deal with stoichiometry first, then the equilibrium.

6 Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 6 Henderson–Hasselbalch Equation For a particular buffering system (conjugate acid–base pair), all solutions that have the same ratio [A – ] / [HA] will have the same pH.

7 Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 7 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

8 Section 15.2 Atomic MassesBuffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 8

9 Section 15.3 The MoleBuffering Capacity Return to TOC Copyright © Cengage Learning. All rights reserved 9 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.

10 Section 15.3 The MoleBuffering Capacity Return to TOC The amount of H + or OH - that buffered solution can absorb without a significant change in pH Buffer capacity measures how well a solution resists changes in pH when acid or base is added. The greater the buffer capacity, the less the pH changes.

11 Section 15.3 The MoleBuffering Capacity Return to TOC 2) Magnitudes of [HA] and [A - ]  the capacity of a buffered soln. Ex : soln A : 5.00 M HOAc + 5.00 M NaOAc soln B : 0.05 M HOAc + 0.05 M NaOAc pH change when 0.01 mol of HCl(g) is added

12 Section 15.3 The MoleBuffering Capacity Return to TOC 3) [A - ] / [HA] ratio  the pH of a buffered soln.

13 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 0.200 M HNO 3 with 0.100 M NaOH

14 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 14 The pH Curve for the Titration of 100.0 mL of 0.50 M NaOH with 1.0 M HCI

15 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 15 Concept Check Consider a solution made by mixing 0.10 mol of HCN (K a = 6.2 x 10 –10 ) with 0.040 mol NaOH in 1.0 L of aqueous solution. What are the major species immediately upon mixing (that is, before a reaction)? HCN, Na +, OH –, H 2 O

16 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 16 Let’s Think About It… K = 1.8 x 10 9 HC 2 H 3 O 2 (aq) +OH –  C 2 H 3 O 2 – (aq)+ H 2 O Before0.20 mol 0.030 mol0 Change–0.030 mol +0.030 mol After0.17 mol 0 0.030 mol

17 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 17 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 0.170 M~00.030 M Change –x+x Equilibrium 0.170 – xx0.030 + x

18 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 18 Exercise Calculate the pH of a 100.0 mL solution of 0.100 M acetic acid (HC 2 H 3 O 2 ), which has a K a value of 1.8 x 10 –5. pH = 2.87

19 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 19 Concept Check Calculate the pH of a solution made by mixing 100.0 mL of a 0.100 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

20 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 20 Concept Check Calculate the pH of a solution at the equivalence point when 100.0 mL of a 0.100 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

21 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 21 The pH Curve for the Titration of 50.0 mL of 0.100 M HC 2 H 3 O 2 with 0.100 M NaOH

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

23 Section 15.4 Titrations and pH Curves Return to TOC Copyright © Cengage Learning. All rights reserved 23 The pH Curve for the Titration of 100.0mL of 0.050 M NH 3 with 0.10 M HCl

24 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.

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

26 Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 26 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).

27 Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 27 The Acid and Base Forms of the Indicator Phenolphthalein

28 Section 15.5 Acid–Base Indicators Return to TOC Copyright © Cengage Learning. All rights reserved 28 The Methyl Orange Indicator is Yellow in Basic Solution and Red in Acidic Solution

29 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

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

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


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