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 and is found by removing an H +. Conjugate acid is formed when the proton is transferred to the base and is found by adding an H +.

5. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 5 Practice – Conjugate Acids and Bases Identify the conjugate acid for each base. a)F - b)HCO 3 - c)H 2 O 13- HF H 2 CO 3 H3O+H3O+ Identify the conjugate base for each acid. a)HBr b)HSO 4 - c)H 2 O Br - SO 4 -2 OH -

6. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 6 Water acts as a base accepting a proton from the acid. Forms hydronium ion (H 3 O + ).

7. Section 16.1 Acids and Bases Return to TOC Copyright © Cengage Learning. All rights reserved 7 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 –

8. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 8 Strong Acid Completely ionized or completely dissociated

9. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 9 Weak Acid Most of the acid molecules remain intact.

10. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 10 Behavior of Acids of Different Strengths in Aqueous Solution

11. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 11 A strong acid contains a relatively weak conjugate base.

12. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 12 Ways to Describe Acid Strength

13. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 13 Common Strong Acids Sulfuric acid, H 2 SO 4 Hydrochloric acid, HCl Nitric acid, HNO 3 Perchloric acid, HClO 4

14. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 14 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

15. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 15 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)

16. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 16 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 –

17. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 17 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 –

18. Acid Strength Section 16.2 Return to TOC Copyright © Cengage Learning. All rights reserved 18 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 –

19. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 19 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.

20. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 20 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.

21. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 21 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 – ]

22. Water as an Acid and a Base Section 16.3 Return to TOC Copyright © Cengage Learning. All rights reserved 22 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.

23. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 23 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.

24. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 24 pH Range pH = 7; neutral pH > 7; basic  Higher the pH, more basic. pH < 7; acidic  Lower the pH, more acidic.

25. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 25 The pH Scale and pH Values of Some Common Substances Blood cells can exist only over a narrow pH range.

26. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 26 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) = 4.0 b) 0.040 M OH – K w = [H + ][OH – ] = 1.00 × 10 –14 = [H + ](0.040 M) [H + ] = 2.5 × 10 –13 M H + pH = –log[H + ] = –log(2.5 × 10 –13 M) = 13

27. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 27 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

28. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 28 pH and pOH Recall: K w = [H + ][OH – ] –log K w = –log[H + ] – log[OH – ] pK w = pH + pOH 14.00 = pH + pOH

29. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 29 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) = 4.0; So, 14.00 = 4.00 + pOH; pOH = 10 b) 0.040 M OH – pOH = –log[OH – ] = –log(0.040 M) = 1.4

30. Section 16.4 The pH Scale Return to TOC Copyright © Cengage Learning. All rights reserved 30 Exercise The pH of a solution is 5.85. What is the [OH – ] for this solution? [OH – ] = 7.08 × 10 –9 M 14.00 = 5.85 + pOH; pOH = 8.15 [OH – ] = 10 –8.15 = 7.08 × 10 –9 M

31. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 31 Determine the [H + ]. 2.0 M HCl → 2.0 M H + and 2.0 M Cl – pH =  log[H + ]

32. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 32 Concept Check Consider an aqueous solution of 2.0 × 10 –3 M HCl. What is the pH? pH = 2.7 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 = 2.7

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 –11 M solution of HCl. pH = 7.0

34. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 34 Concept Check Calculate the pH of a 1.5 × 10 –2 M solution of HNO 3. pH = 1.8 pH = –log[H + ] pH = –log(1.5 × 10 –2 M) pH = 1.8

35. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 35 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.

36. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 36 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)

37. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 37 How Buffers Really Work HA  H + + A - A - + H 2 O  HA + OH - Consider the Equilibrium and La Chatelier’s Principle. As you add H + to a buffered system containing equal amounts of weak acid and its conjugate base, it will go through this equilibrium treadmill and keep the H + concentration in check.

38. Section 16.6 Buffered Solutions Return to TOC Copyright © Cengage Learning. All rights reserved 38 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