1. Chapter 16 Acids and Bases

2. Chapter 16 Table of Contents 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 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 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 8 Strong Acid Completely ionized or completely dissociated

9. Acid Strength Section 16.2 Return to TOC 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 11 A strong acid contains a relatively weak conjugate base.

12. Acid Strength Section 16.2 Return to TOC 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 Common Weak Acids Hydroflouric Acid HF Phosphoric Acid H 3 PO 4 Organic Acids

14. Acid Strength Section 16.2 Return to TOC 14 Common Strong Bases Ammonia NH 3 Organic Ammines R-NH 2 Insoluble Hydroxides Common Weak Bases Group I Hydroxides NaOH

15. Acid Strength Section 16.2 Return to TOC 15 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

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

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

18. Acid Strength Section 16.2 Return to TOC 18 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 – 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 19 Water as an Acid and a Base Water is amphoteric: H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq)  Behaves either as an acid or as a base. At 25°C: K w = [H + ][OH – ] = 1.0 × 10 –14 [X] = molarity of X or moles X per Liter 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 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 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 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 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) = 4.00 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) = 12.60

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 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) = 4.00; So, 14.00 = 4.00 + pOH; pOH = 10.00 b) 0.040 M OH – pOH = –log[OH – ] = –log(0.040 M) = 1.40

30. Section 16.4 The pH Scale Return to TOC 30 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

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 = 10.82

33. Section 16.5 Calculating the pH of Strong Acid Solutions Return to TOC 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 16.4 The pH Scale Return to TOC Practice Calculating ph, pOH, [H + ] and [OH - ] 34 pH [H + ] [OH - ]pOH _____ ______ 6.8x10 -7 ______ 9.2 ______ ______ ______ 6.17 7.83 4.8 1.5x10 -8 6 x10 -10 2x10 -5

35. Section 16.6 Buffered Solutions Return to TOC 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 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