Chapter Fourteen: ACIDS AND BASES

Transcription

1 Chapter Fourteen: ACIDS AND BASES 1

2 Contents p622

3 14-1 The Nature of Acids and Bases p623 Arrheniuis postulates that acids produce hydrogen ios in aqueous solution, while bases produce hydroxide ions. Danish chemist Johannes Bronsted and the English chemist Thomas Lowry In terms of the Bronsted-Lowry model, an acids is a proton (H + ) donor, and a base is a proton acceptor. Arrhenius: Acids produce H + in solution, bases produce OH ion. Brønsted-Lowry:Acids are H + donors, bases are proton acceptors. HCl + H 2 O Cl + H 3 O + HCl + H 2 O Cl + H 3 O + acid base 3

4 Models of Acids and Bases Arrhenius: Acids produce H + in solution, bases produce OH - ion. Brønsted-Lowry: Acids are H + donors, bases are proton acceptors. HCl + H 2 O Cl - + H 3 O + acid base 4

5 Conjugate acid-base pair p624 5

6 The equilibrium expression for Ka p625 HA(aq) + H 2 O H 3 O + (aq) + A - (aq) acid base onjugate conjugate cid base 6

7 Brønsted-Lowry Reaction 7

8 Ex 14.1 Acid Dissociation (lonization) Reactions a. Hydrochloric acid (HCl) b. Acetic acid (HC 2 H 3 O 2 ) c. The ammonium ion (NH 4+ ) d. The anilinium ion (C 6 H 5 NH 3 +) e. The hydrated aluminum(iii) ion [Al(H 2 O) 6 ] 3+ P625 Write the simple dissociation (ionization) reaction (omitting water) for each of the following acids.

9

10 The Bronsted- Lowry model is not limited to aqeous solutions p625 10

11 In this reaction is donated by the hydrogen chloride to the ammonia, as shown these Lewis structure. p625 11

12 14-2 Acid Strength The strength of an acid is defined by the equilibrium p626 position of its dissociation (ionization) reaction. Before dissociation After dissociation at equilibrium

13 p626 Figure 14.5 The relationship of acid strength and conjugate base strength for the ionization reaction. 13

14 A strong acid is virtually 100 % p627 dissociated in water. 14

15 Acid Strength p627

16 p627 Figure 14.6 (a) A strong acid HA is completely ionized in water. (b) A weak acid HB exists mostly as undissociated HB molecules in water. Note that the water molecules are not sown in this figure. 16

17 Acid Ionization Equilibrium 17

18 Self-Ionization of Water 18

19 React HA(aq) + H 2 O H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base What is the equilibrium constant expression for an acid acting in water? 19

20 Think! How good is Cl - (aq) as a base? Is A - (aq) a good base? 20

21 Think! H 2 O(aq) + H 2 O H 3 O + (aq) + OH - (aq) acid base conjugate conjugate acid base At 25 C, K w = 1.0 x

22 Values of Ka p628

23 Appendix 5.1 contains a table of Ka values. p628 Organic acids: carboxyl group

24 P628 Ex 14.2 Relative Base Strength Using Table 14.2, arrange the following species according to their strengths as bases: H 2 O, F -, Cl -, NO 2-, and CN -. Solution 24

25 Water as an Acid and a Base p629 25

26 Kw p629 26

27 p630 There are three possible situations: 27

28 P630 Ex 14.3 Calculating [H + ] and [OH - ] Calculate [H + ] or [OH - ] as required for each of the following solutions at 25, and state whether the solution is neutral, acid, or basic. a M OH - b M OH - c M H + 28

29 Solution: p630

30 Ex 14.4 Autoionization of Water P630 At 60, the value of K w is a. Using Le Chatelier s principle, predict whether the reaction is exothermic or endothermic. 2 3 H 2O( l) H O ( aq) OH ( aq) b. Calculate [H + ] and [OH - ] in a neutral solution at

31 Solution: p631

32 14.3 The ph Scale p631 32

33 The ph Scale 33

34 P631 Ex 14.5 Calculating ph and poh Calculate ph and poh for each of the following solutions at 25 a M OH - b. 1.0 M OH - 34

35 p632 Solution: (a) (b)

36 p632 The ph scale and ph values of some common substances Figure

37 P633 Ex 14.6 Calculating ph The ph of a sample of human blood was measured to be 7.41 at 25. Calculate poh, [H + ], and [OH - ] for the sample. 37

38 p634

39 14-4 Calculating the ph of Strong Acid Solutions p634 Ex 14.7 ph of Strong Acids b. Calculate the ph of M HCl. a. Calculate the ph of 0.10 M HNO 3. Solution: (a) (b)

40 14-5 Calculating the ph of Weak Acid Solutions p635

41 Ex Calculating Ka from Percent Dissociation p643 Lactic acid is a waste product that accumulates in muscle tissue during exertion, leading to pain and a feeling of fatigue. In a M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for this acid. Solution:

42 14.6 Bases p644 42

43 Ex The ph of Strong Bases p645 Calculate the ph of a 5.0 x M NaOH. Solution: 43

44 The lone pair is located on a nitrogen atom p646 44

45 Kb p647

46 Ex The ph of Weak Bases I Calculates the ph for a 15.0 M NH 3 (K b = 1.8 x 10-5 ). Solution: p647

47 14.7 Polyprotic Acids p650

48 Polyprotic acids p651

49 Ex14.15 The ph of a Polyprotic Acid p651 Calculate the ph of a 5.0 M H 3 PO 4 solution and the equilibrium concentrations of the species H 3 PO 4, H 2 PO 4-, HPO 2-4, and PO 3-4. Solution: a. b. e. c. f. d.

50 Acid-Base Properties of Salts Acidic Cation Anion or Basic Example neutral neutral neutral NaCl neutral conjugate acid of weak base conjugate acid of weak base conjugate base of weak acid basic NaF neutral acidic NH 4 Cl conjugate base of weak acid depends on K a & K b values Al 2 (SO 4 ) 3 50

51 14-8 Acid-Base Properties of Salts p655 Salts That Produce Neutral Solutions KCl, NaCl, NaNO 3, and KNO 3 are neutral. Salts That produce Basic Solutions The ph of the solution will be determined by the conjugate anions of weak acids, such as CN -, acetate ion C 2 H 3 O 2-.

52 Ex Salts as Weak Bases p656 Calculating the ph of a 0.30 M NaF solution. The Ka value for HF is 7.2 x Solution:

53 Base Strength in Aqueous Solutions p657 Relative base strength: OH - > CN - > H 2 O Salts That Produce Acidic Solutions Table 14.5 Qualitative Prediction of ph of Salt Solutions of Salts for Which Both Cation and Anion Have Acidic or basic properties

54 Various Types of Salts 54

55 14-9 The Effect of Structure on Acid-Base Properties p661 Table 14.7 Bond Strengths and Acid Strengths for Hydrogen Halides

56 The structure effect p661 Two factors for acidity in binary compounds: Bond Polarity (high is good) Bond Strength (low is good)

57 The Effect of Structure on Acid-Base Properties p662 The effect of number of attracted oxygens on the O-H bond in a series of chlorine oxyacids. Figure 14.11

58 p Acid-Base Properties of Oxides

59 Oxides Acidic Oxides (Acid Anhydrides): O X bond is strong and covalent. SO 2, NO 2, CrO 3 Basic Oxides (Basic Anhydrides): O X bond is ionic. K 2 O, CaO 59

60 14-11 The Lewis Acid-Base Model p663 Three Models for Acids and Bases 60

61 Lewis model p664 61

62 Lewis acid-lewis base reaction p664 Hydration of a metal ion. Al O H H Al O H H 6 3+ Figure Reaction of BF 3 with NH 3

63 Al 3+ ion accepts one electron pair from each of six water molecules to form Al(H 2 O) p665

64 14-12 Strategy for Solving Acid-Base Problems: A Summary p666