Chapter 15. Acid-Base Equilibria

Transcription

1 Chapter 15 Acid-Base Equilibria

2 Section 15.1 Solutions of Acids or Bases Containing a Common Ion 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. Copyright Cengage Learning. All rights reserved 2

3 Section 15.1 Solutions of Acids or Bases Containing a Common Ion 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.

4 Section 15.2 Buffered Solutions 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. Copyright Cengage Learning. All rights reserved 4

5 Section 15.2 Buffered Solutions Adding an Acid to a Buffer To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright Cengage Learning. All rights reserved 5

6 Section 15.2 Buffered Solutions Buffers To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright Cengage Learning. All rights reserved 6

7 Section 15.2 Buffered Solutions Solving Problems with Buffered Solutions Copyright Cengage Learning. All rights reserved 7

8 Section 15.2 Buffered Solutions Buffering: How Does It Work? Copyright Cengage Learning. All rights reserved 8

9 Section 15.2 Buffered Solutions Buffering: How Does It Work? Copyright Cengage Learning. All rights reserved 9

10 Section 15.2 Buffered Solutions Henderson Hasselbalch Equation A ph = p K + log HA a For a particular buffering system (conjugate acid base pair), all solutions that have the same ratio [A ] / [HA] will have the same ph. Copyright Cengage Learning. All rights reserved 10

11 Section 15.2 Buffered Solutions 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 ph = 5.02 Copyright Cengage Learning. All rights reserved 11

12 Section 15.2 Buffered Solutions Copyright Cengage Learning. All rights reserved 12

13 Section 15.2 Buffered Solutions Buffered Solution Characteristics Buffers contain relatively large concentrations of a weak acid and corresponding conjugate base. Added H + reacts to completion with the weak base. Added OH - reacts to completion with the weak acid. Copyright Cengage Learning. All rights reserved 13

14 Section 15.2 Buffered Solutions Buffered Solution Characteristics The ph in the buffered solution is determined by the ratio of the concentrations of the weak acid and weak base. As long as this ratio remains virtually constant, the ph will remain virtually constant. This will be the case as long as the concentrations of the buffering materials (HA and A or B and BH + ) are large compared with amounts of H + or OH added. Copyright Cengage Learning. All rights reserved 14

15 Section 15.3 Buffering Capacity 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. Copyright Cengage Learning. All rights reserved 15

16 Section 15.3 Buffering Capacity 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. Copyright Cengage Learning. All rights reserved 16

17 Section 15.3 Buffering Capacity 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. Copyright Cengage Learning. All rights reserved 17

18 Titration Curve Plotting the ph of the solution being analyzed as a function of the amount of titrant added. Equivalence (Stoichiometric) Point point in the titration when enough titrant has been added to react exactly with the substance in solution being titrated. Copyright Cengage Learning. All rights reserved 18

19 Neutralization of a Strong Acid with a Strong Base To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright Cengage Learning. All rights reserved 19

20 The ph Curve for the Titration of 50.0 ml of M HNO 3 with M NaOH Copyright Cengage Learning. All rights reserved 20

21 The ph Curve for the Titration of ml of 0.50 M NaOH with 1.0 M HCI Copyright Cengage Learning. All rights reserved 21

22 Weak Acid Strong Base Titration Step 1: Step 2: A stoichiometry problem (reaction is assumed to run to completion) then determine concentration of acid remaining and conjugate base formed. An equilibrium problem (determine position of weak acid equilibrium and calculate ph). Copyright Cengage Learning. All rights reserved 22

23 CONCEPT CHECK! Consider a solution made by mixing 0.10 mol of HCN (K a = ) with 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 Copyright Cengage Learning. All rights reserved 23

24 Let s Think About It Why isn t NaOH a major species? Why aren t H + and CN major species? List all possibilities for the dominant reaction. Copyright Cengage Learning. All rights reserved 24

25 Let s Think About It The possibilities for the dominant reaction are: 1. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) 2. HCN(aq) + H 2 O(l) H 3 O + (aq) + CN (aq) 3. HCN(aq) + OH (aq) CN (aq) + H 2 O(l) 4. Na + (aq) + OH (aq) NaOH 5. Na + (aq) + H 2 O(l) NaOH + H + (aq) Copyright Cengage Learning. All rights reserved 25

26 Let s Think About It How do we decide which reaction controls the ph? H 2 O(l) + H 2 O(l) HCN(aq) + H 2 O(l) HCN(aq) + OH (aq) H 3 O + (aq) + OH (aq) H 3 O + (aq) + CN (aq) CN (aq) + H 2 O(l)

27 Let s Think About It HCN(aq) + OH (aq) CN (aq) + H 2 O(l) What are the major species after this reaction occurs? HCN, CN, H 2 O, Na + Copyright Cengage Learning. All rights reserved 27

28 Let s Think About It Now you can treat this situation as before. List the possibilities for the dominant reaction. Determine which controls the ph. Copyright Cengage Learning. All rights reserved 28

29 CONCEPT CHECK! Calculate the ph of a solution made by mixing 0.20 mol HC 2 H 3 O 2 (K a = ) with mol NaOH in 1.0 L of aqueous solution. Copyright Cengage Learning. All rights reserved 29

30 Let s Think About It What are the major species in solution? Na +, OH, HC 2 H 3 O 2, H 2 O Why isn t NaOH a major species? Why aren t H + and C 2 H 3 O 2 major species? Copyright Cengage Learning. All rights reserved 30

31 Let s Think About It What are the possibilities for the dominant reaction? 1. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) 2. HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 O + (aq) + C 2 H 3 O 2 (aq) 3. HC 2 H 3 O 2 (aq) + OH (aq) C 2 H 3 O 2 (aq) + H 2 O(l) 4. Na + (aq) + OH (aq) NaOH(aq) 5. Na + (aq) + H 2 O(l) NaOH + H + (aq) Which of these reactions really occur? Copyright Cengage Learning. All rights reserved 31

32 Let s Think About It Which reaction controls the ph? H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 O + (aq) + C 2 H 3 O 2 (aq) HC 2 H 3 O 2 (aq) + OH (aq) C 2 H 3 O 2 (aq) + H 2 O(l) How do you know? Copyright Cengage Learning. All rights reserved 32

33 Let s Think About It HC 2 H 3 O 2 (aq) + OH C 2 H 3 O 2 (aq) + H 2 O Before 0.20 mol mol 0 Change mol mol mol After 0.17 mol mol K = Copyright Cengage Learning. All rights reserved 33

34 Steps Toward Solving for ph HC 2 H 3 O 2 (aq) + H 2 O H 3 O + + C 2 H 3 O 2- (aq) Initial M ~ M Change x +x +x Equilibrium x x x K a = ph = 3.99 Copyright Cengage Learning. All rights reserved 34

35 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 ph = 2.87 Copyright Cengage Learning. All rights reserved 35

36 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 , and 50.0 ml of a 0.10 M NaOH solution. ph = 4.74 Copyright Cengage Learning. All rights reserved 36

37 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 , is titrated with a 0.10 M NaOH solution. ph = 8.72 Copyright Cengage Learning. All rights reserved 37

38 The ph Curve for the Titration of 50.0 ml of M HC 2 H 3 O 2 with M NaOH Copyright Cengage Learning. All rights reserved 38

39 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 Copyright Cengage Learning. All rights reserved 39

40 The ph Curve for the Titration of ml of M NH 3 with 0.10 M HCl Copyright Cengage Learning. All rights reserved 40

41 Section 15.5 Acid-Base Indicators 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). Copyright Cengage Learning. All rights reserved 41

42 Section 15.5 Acid-Base Indicators The Acid and Base Forms of the Indicator Phenolphthalein Copyright Cengage Learning. All rights reserved 42

43 Section 15.5 Acid-Base Indicators The Methyl Orange Indicator is Yellow in Basic Solution and Red in Acidic Solution Copyright Cengage Learning. All rights reserved 43

44 Section 15.5 Acid-Base Indicators Useful ph Ranges for Several Common Indicators Copyright Cengage Learning. All rights reserved 44