1. Chapter 16 Acids and Bases

2. Chapter 16 Table of Contents Copyright © Cengage Learning. All rights reserved 2 16.1Acids and Bases 16.2Acid Strength 16.3Water as an Acid and a Base 16.4The pH Scale 16.5 Calculating the pH of Strong Acid Solutions 16.6Buffered Solutions

3. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 3 Models of Acids and Bases Arrhenius: Acids produce H + ions in solution, bases produce OH – ions. Brønsted – Lowry: Acids are proton (H + ) donors, bases are proton acceptors. HCl + H 2 O → Cl – + H 3 O + acid base

4. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 4 Acid in Water HA(aq) + H 2 O(l) H 3 O + (aq) + A – (aq) acid base conjugate conjugate acid base Conjugate base is everything that remains of the acid molecule after a proton is lost. Conjugate acid is formed when the proton is transferred to the base.

5. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 5 Water acts as a base accepting a proton from the acid. Forms hydronium ion (H 3 O + ).

6. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 6 Concept Check Which of the following represent conjugate acid–base pairs? a)HCl, HNO 3 b)H 3 O +, OH – c)H 2 SO 4, SO 4 2– d)HCN, CN –

7. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 7 Strong Acid Completely ionized or completely dissociated

8. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 8 Weak Acid Most of the acid molecules remain intact.

9. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 9 Behavior of Acids of Different Strengths in Aqueous Solution

10. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 10 A strong acid contains a relatively weak conjugate base.

11. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 11 Ways to Describe Acid Strength

12. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 12 Common Strong Acids Sulfuric acid, H 2 SO 4 Hydrochloric acid, HCl Nitric acid, HNO 3 Perchloric acid, HClO 4

13. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 13 Oxyacid – acidic proton is attached to an oxygen atom Organic acid – have a carbon atom backbone and commonly contain the carboxyl group:  Typically a weak acid

14. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 14 Concept Check Consider a 1.0 M solution of HCl. Order the following from strongest to weakest base and explain: H 2 O(l) A – (aq) (from weak acid HA) Cl – (aq)

15. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 15 Let’s Think About It… How good is Cl – (aq) as a base? Is A – (aq) a good base? The bases from strongest to weakest are: A –, H 2 O, Cl –

16. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 16 Concept Check Acetic acid (HC 2 H 3 O 2 ) and HCN are both weak acids. Acetic acid is a stronger acid than HCN. Arrange these bases from weakest to strongest and explain your answer: H 2 O Cl – CN – C 2 H 3 O 2 –

17. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 17 Let’s Think About It… H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq) acid base conjugate conjugate acid base At 25  C, K w = 1.0 x 10 –14 The bases from weakest to strongest are: Cl –, H 2 O, C 2 H 3 O 2 –, CN –

18. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 18 Water as an Acid and a Base Water is amphoteric:  Behaves either as an acid or as a base. At 25°C: K w = [H + ][OH – ] = 1.0 × 10 –14 No matter what the solution contains, the product of [H + ] and [OH – ] must always equal 1.0 × 10 –14 at 25°C.

19. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 19 Three Possible Situations [H + ] = [OH – ]; neutral solution [H + ] > [OH – ]; acidic solution [H + ] < [OH – ]; basic solution In each case, however, K w = [H + ][OH – ] = 1.0 × 10 –14.

20. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 20 Concept Check In an acidic aqueous solution, which statement below is correct? a)[H + ] < 1.0 × 10 –7 M b)[H + ] > 1.0 × 10 –7 M c)[OH – ] > 1.0 × 10 –7 M d)[H + ] < [OH – ]

21. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 21 Exercise In an aqueous solution in which [OH – ] = 2.0 x 10 –10 M, the [H + ] = ________ M, and the solution is ________. a)2.0 × 10 –10 M ; basic b)1.0 × 10 –14 M ; acidic c)5.0 × 10 –5 M ; acidic d)5.0 × 10 –5 M ; basic [H + ] = K w /[OH – ] = 1.0 × 10 –14 /2.0 × 10 –10 = 5.0 × 10 –5 M. Since [H + ] is greater than [OH – ], the solution is acidic.

22. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 22 pH = –log[H + ] A compact way to represent solution acidity. pH decreases as [H + ] increases. Significant figures:  The number of decimal places in the log is equal to the number of significant figures in the original number.

23. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 23 pH Range pH = 7; neutral pH > 7; basic  Higher the pH, more basic. pH < 7; acidic  Lower the pH, more acidic.

24. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 24 The pH Scale and pH Values of Some Common Substances

25. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 25 Exercise Calculate the pH for each of the following solutions. a) 1.0 × 10 –4 M H + pH = –log[H + ] = –log(1.0 × 10 –4 M) = 4.00 b) 0.040 M OH – K w = [H + ][OH – ] = 1.00 × 10 –14 = [H + ](0.040 M) = 2.5 × 10 –13 M H + pH = –log[H + ] = –log(2.5 × 10 –13 M) = 12.60

26. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 26 Exercise The pH of a solution is 5.85. What is the [H + ] for this solution? [H + ] = 1.4 × 10 –6 M [H + ] = 10^–5.85 = 1.4 × 10 –6 M

27. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 27 pH and pOH Recall: K w = [H + ][OH – ] –log K w = –log[H + ] – log[OH – ] pK w = pH + pOH 14.00 = pH + pOH

28. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 28 Exercise Calculate the pOH for each of the following solutions. a) 1.0 × 10 –4 M H + pH = –log[H + ] = –log(1.0 × 10 –4 M) = 4.00; So, 14.00 = 4.00 + pOH; pOH = 10.00 b) 0.040 M OH – pOH = –log[OH – ] = –log(0.040 M) = 1.40

29. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 29 Exercise The pH of a solution is 5.85. What is the [OH – ] for this solution? [OH – ] = 7.1 × 10 –9 M 14.00 = 5.85 + pOH; pOH = 8.15 [OH – ] = 10^–8.15 = 7.1 × 10 –9 M

30. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 30 Determine the [H + ]. 2.0 M HCl → 2.0 M H + and 2.0 M Cl – pH =  log[H + ]

31. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 31 Concept Check Consider an aqueous solution of 2.0 × 10 –3 M HCl. What is the pH? pH = 2.70 2.0 × 10 –3 M HCl → 2.0 × 10 –3 M H + and 2.0 × 10 –3 M Cl – pH = –log[H + ] pH = –log(2.0 × 10 –3 M) pH = 2.70

32. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 32 Concept Check Calculate the pH of a 1.5 × 10 –11 M solution of HCl. pH = 7.00

33. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 33 Concept Check Calculate the pH of a 1.5 × 10 –2 M solution of HNO 3. pH = 1.82 pH = –log[H + ] pH = –log(1.5 × 10 –2 M) pH = 1.82

34. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 34 Buffered solution – resists a change in its pH when either an acid or a base has been added.  Presence of a weak acid and its conjugate base buffers the solution.

35. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 35 The Characteristics of a Buffer 1.The solution contains a weak acid HA and its conjugate base A –. 2.The buffer resists changes in pH by reacting with any added H + or OH – so that these ions do not accumulate. 3.Any added H + reacts with the base A –. H + (aq) + A – (aq) → HA(aq) 4.Any added OH – reacts with the weak acid HA. OH – (aq) + HA(aq) → H 2 O(l) + A – (aq)

36. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 36 Concept Check If a solution is buffered with NH 3 to which has been added NH 4 Cl, what reaction will occur if a strong base such as NaOH is added? a)NaOH + NH 3 → NaNH 4 O b)NaOH + NH 4 + → Na + + NH 3 + H 2 O c)NaOH + Cl – → NaCl + OH – d)NaOH + H 2 O → NaH 3 O 2

37. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 37 Copyright © Cengage Learning. All rights reserved 37 Copyright © Cengage Learning. All rights reserved 37 Copyright © Cengage Learning. All rights reserved 37 Homework Reading assignment –Pages 515 through 536 Homework Problems –Questions and problems 3, 5, 9, 11, 13, 15, 19, 21, 23, 25, 29, 33, 35, 39, 43, 45, 47, 49, 51, 53, 61. Due on Chapter 16 Homework