Chapter 14. Acids and Bases

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1 Chapter 14 Acids and Bases

2 Section 14.1 The Nature of Acids and Bases 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 Copyright Cengage Learning. All rights reserved 2

3 Section 14.1 The Nature of Acids and Bases Brønsted Lowry Reaction 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 3

4 Section 14.1 The Nature of Acids and Bases Acid in Water HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) 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. Copyright Cengage Learning. All rights reserved 4

5 Section 14.1 The Nature of Acids and Bases Acid Ionization Equilibrium 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 14.2 Acid Strength Strong acid: Ionization equilibrium lies far to the right. Yields a weak conjugate base. Weak acid: Ionization equilibrium lies far to the left. Weaker the acid, stronger its conjugate base. Copyright Cengage Learning. All rights reserved 6

9 Section 14.2 Acid Strength 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 ] = No matter what the solution contains, the product of [H + ] and [OH ] must always equal at 25 C. Copyright Cengage Learning. All rights reserved 9

10 Section 14.2 Acid Strength Three Possible Situations [H + ] = [OH ]; neutral solution [H + ] > [OH ]; acidic solution [OH ] > [H + ]; basic solution Copyright Cengage Learning. All rights reserved 10

11 Section 14.2 Acid Strength Self-Ionization of Water 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 11

12 Section 14.2 Acid Strength CONCEPT CHECK! HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base What is the equilibrium constant expression for an acid acting in water? K = H O A 3 HA Copyright Cengage Learning. All rights reserved 12

13 Section 14.2 Acid Strength CONCEPT CHECK! If the equilibrium lies to the right, the value for K a is. large (or >1) If the equilibrium lies to the left, the value for K a is. small (or <1) Copyright Cengage Learning. All rights reserved 13

14 Section 14.2 Acid Strength CONCEPT CHECK! HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) If water is a better base than A, do products or reactants dominate at equilibrium? Does this mean HA is a strong or weak acid? Is the value for K a greater or less than 1? Copyright Cengage Learning. All rights reserved 14

15 Section 14.2 Acid Strength 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) Copyright Cengage Learning. All rights reserved 15

16 Section 14.2 Acid Strength 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 Copyright Cengage Learning. All rights reserved 16

17 Section 14.2 Acid Strength CONCEPT CHECK! Consider a solution of NaA where A is the anion from weak acid HA: A (aq) + H 2 O(l) HA(aq) + OH (aq) base acid conjugate conjugate acid base a) Which way will equilibrium lie? left Copyright Cengage Learning. All rights reserved 17

18 Section 14.2 Acid Strength CONCEPT CHECK! Consider a solution of NaA where A is the anion from weak acid HA: A (aq) + H 2 O(l) HA(aq) + OH (aq) base acid conjugate conjugate acid base b) Is the value for K b greater than or less than 1? less than 1 Copyright Cengage Learning. All rights reserved 18

19 Section 14.2 Acid Strength CONCEPT CHECK! Consider a solution of NaA where A is the anion from weak acid HA: A (aq) + H 2 O(l) HA(aq) + OH (aq) base acid conjugate conjugate acid base c) Does this mean A is a strong or weak base? weak base Copyright Cengage Learning. All rights reserved 19

20 Section 14.2 Acid Strength 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 Copyright Cengage Learning. All rights reserved 20

21 Section 14.2 Acid Strength 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 = The bases from weakest to strongest are: Cl, H 2 O, C 2 H 3 O 2, CN Copyright Cengage Learning. All rights reserved 21

22 Section 14.2 Acid Strength CONCEPT CHECK! Discuss whether the value of K for the reaction: HCN(aq) + F (aq) CN (aq) + HF(aq) is >1 <1 =1 (K a for HCN is ; K a for HF is ) Explain your answer. Copyright Cengage Learning. All rights reserved 22

23 Section 14.2 Acid Strength CONCEPT CHECK! Calculate the value for K for the reaction: HCN(aq) + F (aq) CN (aq) + HF(aq) (K a for HCN is ; K a for HF is ) K = Copyright Cengage Learning. All rights reserved 23

24 Section 14.3 The ph Scale ph = log[h + ] ph changes by 1 for every power of 10 change in [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. Copyright Cengage Learning. All rights reserved 24

32 Section 14.4 Calculating the ph of Strong Acid Solutions Thinking About Acid Base Problems What are the major species in solution? What is the dominant reaction that will take place? Is it an equilibrium reaction or a reaction that will go essentially to completion? React all major species until you are left with an equilibrium reaction. Solve for the ph if needed. Copyright Cengage Learning. All rights reserved 32

33 Section 14.4 Calculating the ph of Strong Acid Solutions CONCEPT CHECK! Consider an aqueous solution of M HCl. What are the major species in solution? H +, Cl, H 2 O What is the ph? ph = 2.70 Copyright Cengage Learning. All rights reserved 33

36 Section 14.4 Calculating the ph of Strong Acid Solutions Let s Think About It When HNO 3 is added to water, a reaction takes place immediately: HNO 3 + H 2 O H 3 O + + NO 3 Copyright Cengage Learning. All rights reserved 36

37 Section 14.4 Calculating the ph of Strong Acid Solutions Let s Think About It Why is this reaction not likely? NO 3 (aq) + H 2 O(l) HNO 3 (aq) + OH (aq) Copyright Cengage Learning. All rights reserved 37

38 Section 14.4 Calculating the ph of Strong Acid Solutions Let s Think About It What reaction controls the ph? H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) In aqueous solutions, this reaction is always taking place. But is water the major contributor of H + (H 3 O + )? ph = 1.82 Copyright Cengage Learning. All rights reserved 38

39 Section 14.5 Calculating the ph of Weak Acid Solutions Solving Weak Acid Equilibrium Problems 1. List the major species in the solution. 2. Choose the species that can produce H +, and write balanced equations for the reactions producing H Using the values of the equilibrium constants for the reactions you have written, decide which equilibrium will dominate in producing H Write the equilibrium expression for the dominant equilibrium. Copyright Cengage Learning. All rights reserved 39

40 Section 14.5 Calculating the ph of Weak Acid Solutions Solving Weak Acid Equilibrium Problems 5. List the initial concentrations of the species participating in the dominant equilibrium. 6. Define the change needed to achieve equilibrium; that is, define x. 7. Write the equilibrium concentrations in terms of x. 8. Substitute the equilibrium concentrations into the equilibrium expression. Copyright Cengage Learning. All rights reserved 40

41 Section 14.5 Calculating the ph of Weak Acid Solutions Solving Weak Acid Equilibrium Problems 9. Solve for x the easy way, that is, by assuming that [HA] 0 x about equals [HA] Use the 5% rule to verify whether the approximation is valid. 11.Calculate [H + ] and ph. Copyright Cengage Learning. All rights reserved 41

42 Section 14.5 Calculating the ph of Weak Acid Solutions CONCEPT CHECK! Consider a 0.80 M aqueous solution of the weak acid HCN (K a = ). What are the major species in solution? HCN, H 2 O Copyright Cengage Learning. All rights reserved 42

44 Section 14.5 Calculating the ph of Weak Acid Solutions Consider This HCN(aq) + H 2 O(l) K a = H 3 O + (aq) + CN (aq) H 2 O(l) + H 2 O(l) K w = H 3 O + (aq) + OH (aq) Which reaction controls the ph? Explain. Copyright Cengage Learning. All rights reserved 44

46 Section 14.5 Calculating the ph of Weak Acid Solutions Let s Think About It What are the major species in solution? HF, H 2 O Why aren t H + and F major species? Copyright Cengage Learning. All rights reserved 46

47 Section 14.5 Calculating the ph of Weak Acid Solutions Let s Think About It What are the possibilities for the dominant reaction? HF(aq) + H 2 O(l) H 3 O + (aq) + F (aq) K a = H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) K w = Which reaction controls the ph? Why? Copyright Cengage Learning. All rights reserved 47

48 Section 14.5 Calculating the ph of Weak Acid Solutions Steps Toward Solving for ph HF(aq) + H 2 O H 3 O + (aq) + F (aq) Initial 0.50 M ~ 0 ~ 0 Change x +x +x Equilibrium 0.50 x x x K a = ph = 1.72 Copyright Cengage Learning. All rights reserved 48

49 Section 14.5 Calculating the ph of Weak Acid Solutions Percent Dissociation (Ionization) amount dissociated (mol/l) Percent dissociation = 100% initial concentration (mol/l) For a given weak acid, the percent dissociation increases as the acid becomes more dilute. Copyright Cengage Learning. All rights reserved 49

50 Section 14.5 Calculating the ph of Weak Acid Solutions EXERCISE! A solution of 8.00 M formic acid (HCHO 2 ) is 0.47% ionized in water. Calculate the K a value for formic acid. K a = Copyright Cengage Learning. All rights reserved 50

51 Section 14.5 Calculating the ph of Weak Acid Solutions EXERCISE! Calculate the ph of an 8.00 M solution of formic acid. Use the data from the previous slide to help you solve this problem. ph = 1.42 Copyright Cengage Learning. All rights reserved 51

52 Section 14.5 Calculating the ph of Weak Acid Solutions EXERCISE! The value of K a for a 4.00 M formic acid solution should be: higher than lower than the same as the value of K a of an 8.00 M formic acid solution. Explain. Copyright Cengage Learning. All rights reserved 52

53 Section 14.5 Calculating the ph of Weak Acid Solutions CONCEPT CHECK! The percent ionization of a 4.00 M formic acid solution should be: higher than lower than the same as the percent ionization of an 8.00 M formic acid solution. Explain. Copyright Cengage Learning. All rights reserved 53

54 Section 14.5 Calculating the ph of Weak Acid Solutions CONCEPT CHECK! The ph of a 4.00 M formic acid solution should be: higher than lower than the same as the ph of an 8.00 M formic acid solution. Explain. Copyright Cengage Learning. All rights reserved 54

55 Section 14.5 Calculating the ph of Weak Acid Solutions EXERCISE! Calculate the percent ionization of a 4.00 M formic acid solution in water. % Ionization = 0.67% Copyright Cengage Learning. All rights reserved 55

57 Section 14.6 Bases Arrhenius: bases produce OH ions. Brønsted Lowry: bases are proton acceptors. In a basic solution at 25 C, ph > 7. Ionic compounds containing OH - are generally considered strong bases. LiOH, NaOH, KOH, Ca(OH) 2 poh = log[oh ] ph = poh Copyright Cengage Learning. All rights reserved 57

61 Section 14.6 Bases ph calculations for solutions of weak bases are very similar to those for weak acids. K w = [H + ][OH ] = poh = log[oh ] ph = poh Copyright Cengage Learning. All rights reserved 61

63 Section 14.7 Polyprotic Acids Acids that can furnish more than one proton. Always dissociates in a stepwise manner, one proton at a time. The conjugate base of the first dissociation equilibrium becomes the acid in the second step. For a typical weak polyprotic acid: K a1 > K a2 > K a3 For a typical polyprotic acid in water, only the first dissociation step is important to ph. Copyright Cengage Learning. All rights reserved 63

65 Section 14.7 Polyprotic Acids CONCEPT CHECK! Calculate the equilibrium concentration of PO 4 3- in a 1.00 M solution of H 3 PO 4. K a1 = K a2 = K a3 = [PO 4 3- ] = M Copyright Cengage Learning. All rights reserved 65

66 Section 14.8 Acid-Base Properties of Salts Salts Ionic compounds. When dissolved in water, break up into its ions (which can behave as acids or bases). Copyright Cengage Learning. All rights reserved 66

68 Section 14.8 Acid-Base Properties of Salts Salts A basic solution is formed if the anion of the salt is the conjugate base of a weak acid. NaF, KC 2 H 3 O 2 K w = K a K b Use K b when starting with base. Copyright Cengage Learning. All rights reserved 68

69 Section 14.8 Acid-Base Properties of Salts Salts An acidic solution is formed if the cation of the salt is the conjugate acid of a weak base. NH 4 Cl K w = K a K b Use K a when starting with acid. Copyright Cengage Learning. All rights reserved 69

70 Section 14.8 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 Copyright Cengage Learning. All rights reserved 70

72 Section 14.8 Acid-Base Properties of Salts EXERCISE! HC 2 H 3 O 2 K a = HCN K a = Calculate the K b values for: C 2 H 3 O 2 and CN K b (C 2 H 3 O 2- ) = K b (CN - ) = Copyright Cengage Learning. All rights reserved

73 Section 14.8 Acid-Base Properties of Salts CONCEPT CHECK! Arrange the following 1.0 M solutions from lowest to highest ph. HBr NaOH NH 4 Cl NaCN NH 3 HCN NaCl HF Justify your answer. HBr, HF, HCN, NH 4 Cl, NaCl, NaCN, NH 3, NaOH Copyright Cengage Learning. All rights reserved 73

74 Section 14.8 Acid-Base Properties of Salts CONCEPT CHECK! Consider a 0.30 M solution of NaF. The K a for HF is What are the major species? Na +, F -, H 2 O Copyright Cengage Learning. All rights reserved 74

75 Section 14.8 Acid-Base Properties of Salts Let s Think About It Why isn t NaF considered a major species? What are the possibilities for the dominant reactions? Copyright Cengage Learning. All rights reserved 75

76 Section 14.8 Acid-Base Properties of Salts Let s Think About It The possibilities for the dominant reactions are: 1. F (aq) + H 2 O(l) HF(aq) + OH (aq) 2. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) 3. Na + (aq) + H 2 O(l) NaOH + H + (aq) 4. Na + (aq) + F (aq) NaF Copyright Cengage Learning. All rights reserved 76

77 Section 14.8 Acid-Base Properties of Salts Let s Think About It How do we decide which reaction controls the ph? F (aq) + H 2 O(l) H 2 O(l) + H 2 O(l) HF(aq) + OH (aq) H 3 O + (aq) + OH (aq) Determine the equilibrium constant for each reaction. Copyright Cengage Learning. All rights reserved 77

Section 14.8 Acid-Base Properties of Salts EXERCISE! Calculate the ph of a 0.

79 Section 14.8 Acid-Base Properties of Salts Let s Think About It What are the major species in solution? Na +, CN, H 2 O Why isn t NaCN considered a major species? Copyright Cengage Learning. All rights reserved 79

80 Section 14.8 Acid-Base Properties of Salts Let s Think About It What are all possibilities for the dominant reaction? The possibilities for the dominant reaction are: 1. CN (aq) + H 2 O(l) HCN(aq) + OH (aq) 2. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) 3. Na + (aq) + H 2 O(l) NaOH + H + (aq) 4. Na + (aq) + CN (aq) NaCN Which of these reactions really occur? Copyright Cengage Learning. All rights reserved 80

81 Section 14.8 Acid-Base Properties of Salts Let s Think About It How do we decide which reaction controls the ph? CN (aq) + H 2 O(l) H 2 O(l) + H 2 O(l) HCN(aq) + OH (aq) H 3 O + (aq) + OH (aq) Copyright Cengage Learning. All rights reserved 81

82 Section 14.8 Acid-Base Properties of Salts Steps Toward Solving for ph CN (aq) + H 2 O HCN(aq) + OH (aq) Initial 0.75 M 0 ~ 0 Change x +x +x Equilibrium 0.75 x x x K b = ph = 11.54

83 Section 14.9 The Effect of Structure on Acid-Base Properties Models of Acids and Bases Two factors for acidity in binary compounds: Bond Polarity (high is good) Bond Strength (low is good) Copyright Cengage Learning. All rights reserved 83

85 Section 14.9 The Effect of Structure on Acid-Base Properties Oxyacids Contains the group H O X. For a given series the acid strength increases with an increase in the number of oxygen atoms attached to the central atom. The greater the ability of X to draw electrons toward itself, the greater the acidity of the molecule. Copyright Cengage Learning. All rights reserved 85

88 Section Acid-Base Properties of Oxides Oxides Acidic Oxides (Acid Anhydrides): O X bond is strong and covalent. SO 2, NO 2, CO 2 When H O X grouping is dissolved in water, the O X bond will remain intact. It will be the polar and relatively weak H O bond that will tend to break, releasing a proton. Copyright Cengage Learning. All rights reserved 88

89 Section Acid-Base Properties of Oxides Oxides Basic Oxides (Basic Anhydrides): O X bond is ionic. K 2 O, CaO If X has a very low electronegativity, the O X bond will be ionic and subject to being broken in polar water, producing a basic solution. Copyright Cengage Learning. All rights reserved 89

90 Section The Lewis Acid-Base Model Lewis Acids and Bases Lewis acid: electron pair acceptor Lewis base: electron pair donor Al O H H Al O H H 6 3+ Lewis acid Lewis base Copyright Cengage Learning. All rights reserved 90

92 Section Strategy for Solving Acid-Base Problems: A Summary When analyzing an acid-base equilibrium problem: Ask this question: What are the major species in the solution and what is their chemical behavior? What major species are present? Does a reaction occur that can be assumed to go to completion? What equilibrium dominates the solution? Let the problem guide you. Be patient. Copyright Cengage Learning. All rights reserved 92

Chapter 16. Acids and Bases. Copyright Cengage Learning. All rights reserved 1

Chapter 16. Acids and Bases. Copyright Cengage Learning. All rights reserved 1 Chapter 16 Acids and Bases Copyright Cengage Learning. All rights reserved 1 Section 16.1 Acids and Bases Models of Acids and Bases Arrhenius: Acids produce H + ions in solution, bases produce OH ions.