# Bronsted Acids & Bases

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1 Acids and Bases 1

2 Bronsted Acids & Bases Acid: substance capable of donating a proton (H + ). Base: substance capable of accepting a proton. (definition not dependent on OH - ) 2

3 Conjugate Pairs CH 3 COOH + H 2 O acetic acid water CH 3 COO - + H 3 O + acetate ion hydronium ion Acid: H + donor Base: H + acceptor Acid & conjugate base pair Base and conjugate acid pair How do you recognize a conjugate pair? 3

4 Conjugate Pairs NH 3 + H 2 O NH OH - NH 3 NH 4 + base conjugate acid H 2 O OH - acid conjugate base 4

5 Bronsted Base NH 3 H + H 2 O Bronsted bases must have an atom with a lone pair of electrons to accept a proton ( coordinate covalent bond ) 5

6 Try It!!! Identify the conjugate pairs in this reaction. CN - + H 2 O HCN + OH - 6

7 Autoionization of Water H 2 O H + + OH - or shorthand H 2 O + H 2 O H 3 O + + OH - What are the conjugate pairs? 7

8 Ion Product of Water K c = [H+ ][OH - ] [H 2 O] or since [H 2 O] is ~constant: K c [H 2 O] = K w = [H + ][OH - ] Remember: H + is shorthand for H 3 O + K w = [H 3 O + ][OH - ] 8

9 Ion Product of Water In pure water at 25 o C: [H 3 O + ] = [OH - ] = 1.0 x 10-7 M Thus K w = [H 3 O + ][OH - ] = 1.0 x K w is temperature dependent, but we will use this value for most calculations. 9

10 Acid vs. Base Acid: [H + ] > [OH - ] Base: [H + ] < [OH - ] Neutral: [H + ] = [OH - ] 10

11 [H + ] and [OH - ] Calculations 1.In an ammonia solution, what is [H + ] if [OH - ] is M? 2.What is [OH - ] in an HCl solution with [H + ] of 1.3 M? 11

12 ph: Definition ph = -log[h + ] (no units) ph increases as [H + ] decreases Acidic: [H + ] > 10-7 M & ph < 7 Basic: [H + ] < 10-7 M & ph > 7 Neutral: [H + ] = 10-7 M & ph = 7* *Neutral [H + ] = [OH - ] Neutral is ph = 7 only at 25 o C. 12

13 ph of Common Fluids Stomach fluid 1.5 Vinegar 3.0 Orange juice 3.5 Water + air 5.5 Blood 7.4 Milk of magnesia 11.5 ph is a log function. A ph increase of 1.0 means [H + ] decrease by factor of

14 ph is a log Function Compare the ph of a solution with a ph of 3 to a solution with a ph of 7. 14

15 poh px = log(x) poh = -log[oh - ] K w = [H + ][OH - ] = 1.0 x Taking negative logs: -log[h + ] - log[oh - ] = ph + poh =

16 ph phun If [H + ] = 3.2 x 10-4 M, what is the ph? If the ph of acid rain is 4.82, what is [H + ]? If a basic solution has [OH - ] of 2.9 x 10-4 M, what is ph? 16

17 pkw Temp. Dependence of K w H 2 O H + + OH - Is this reaction endoor exothermic? temp. ( o C) 17

18 Acid Strength Strong acids: completely ionized in water. Know! HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3 Strong oxyacid if #O - #H > 2 This applies to neutral acids, not ions. 100% H SO + H O H O + + HSO

19 Strong Acids HCl + H 2 O H 3 O + + Cl - The anion of a strong acid (Cl - ) doesn t hydrolyze. Cl - does not react with water to bond with an H +. 19

20 Weak Acids Weak acids incompletely ionize in water. HF, NH + 4, organic acids (CH 3 COOH) HF + H 2 O H 3 O + + F - Equilibrium lies far to the left. [HF] >> [F - ] since F - holds onto the H + 20

21 Strong Bases Completely ionized in water. NaOH, KOH, Ba(OH) 2 Group 1 & 2 hydroxides. All ionize completely, even though some are only slightly soluble (e.g. Mg(OH) 2.) KOH H 2 O K + (aq) + OH - (aq) Bronsted base 21

22 Strong Bases KOH H 2 O K + (aq) + OH - (aq) The cation of a strong base (K + ) doesn t hydrolyze. K + does not react with water to bond with an OH - Ġroup 1 & 2 ions do not hydrolyze. 22

23 Weak Bases Weak electrolytes. NH 3 + H 2 O NH OH - Unlike an acid, a weak base does not ionize. Rather is causes the ionization of water. 23

24 Common Weak Bases Ammonia, NH 3 Organic amines, e.g. CH 3 NH 2 Pyridines, e.g. What do these have in common? 24

25 General Properties (Table 15.2) 1. The stronger the acid the weaker the conjugate base. HA + H 2 O H 3 O + + A - 2. H 3 O + is strongest acid in water. Stronger acids yield H 3 O +. HCl + H 2 O H 3 O + + Cl - 25

26 General Properties (Table 15.2) 3.All weak acids have the equilibrium far to the left. HF + H 2 O H 3 O + + F - 4.OH - is strongest base in water. Stronger bases produce OH -. O -2 + H 2 O 2OH - 26

27 Prediction Use Table 15.2 to predict whether primarily reactants or products will remain when HNO 2 is mixed with KCN. HNO 2 + CN - NO HCN stronger weaker acid acid 27

28 Prediction Use Table 15.2 to predict whether K c is >1 or <1 for the following reaction: F - + HNO 2 HF + NO - 2 weaker acid stronger acid 28

29 One way arrow Strong Acid ICE Calculate ph of 1.0 x 10-3 M HCl. HCl H + + Cl - Initial: 1.x Change: 1.x x x10-3 Equilib: 0 1.x x10-3 ph = -log(1.0x10-3 ) = 3.00 For strong acids, [H 3 O + ] is same as starting acid concentration. 29

30 Strong Base: Try It What is the ph of a M barium hydroxide solution? 30

31 Weak Acids HA + H 2 O H 3 O + + A - or simply: HA H + + A - K a = [H+ ][A - ] [HA] Two way arrow Acid ionization constant, K a 31

32 Acid Ionization Constant The strength of a weak acid is measured by K a. See Table You will need to look up K a s! 32

33 Problems Involving K a What is the ph of M HF, given K a = 7.1 x 10-4? HF + H 2 O H 3 O + + F - or simply: HF H + + F - (ignore autoionization of water) 33

34 ICE Problems Involving K a Initial: Change: -x +x +x Equlib.: x x x K a = HF H + + F - x 2 (0.050 x) = 7.1 x

35 K a Problem continued x 2 (0.050 x) = 7.1 x 10-4 There are 2 ways to solve!!! 1. quadratic (yuck!) 2. successive approximation 35

36 1. Quadratic x 2 (0.050 x) = 7.1 x x 2 + (7.1x10-4 )x -3.6x10-5 = 0 a b c x = -b ± b 2 4ac 2a 36

37 1. Quadratic x 2 + (7.1x10-4 )x -3.6x10-5 = 0 -b ± b 2 4ac x = 2a You get 2 answers; only one is reasonable. x = 5.6 x 10-3 M = [H + ] ph = -log(5.6 x 10-3 ) =

38 2. Successive Approximation K a = HF H + + F - x 2 (0.050 x) = 7.1 x 10-4 Assume x is very small compared to so that the degree of ionization is small. This works for K a ~ 10-4 or less. 38

39 2. Successive Approximation K a = x 2 (0.050 x) = 7.1 x 10-4 (ignore x here) x = 7.1 x 10-4 x = M 39

40 2. Successive Approximation Plug this value of x into the denominator of the K a equation, and recalculate another x. Repeat until x doesn t change within 2 sig figs. 40

41 2. Successive Approximation Put x = M here & calculate another x. K a = x 2 ( x) = 7.1 x 10-4 x = M (repeat again and get same value) 41

42 3. Successive Approximation x = [H + ] = M ph = -log(0.0056) = 2.25 same as quadratic! Usually 2 cycles will do it! 42

43 Summary (so far) 1. Express K a in terms of x, the change in concentration (ICE). 2. Use successive approximation first, then quadratic if needed. 3. Having x, calculated all else. 4. ph depends on both acid strength (K a ) and acid concentration. 43

44 Try It!!! 1.Calculate ph of M nitrous acid (K a = 4.5 x 10-4 ) 2. The ph of 0.10 M methanoic acid is What is K a? 44

45 Percent Ionization (PI) Another measure of the strength of an acid. HA H + + A - PI = Ionized acid conc. at eq. Initial acid conc. X 100 = [H+ ] [HA] o x

46 PI = [H+ ] [HF] o x 100 For M HF PI = (See slide 39) x 100 = 11% For 0.50 M HF PI = x 100 = 3.8% More dilute greater PI 46

47 Percent Ionization (PI) LeChatelier s Principle HA + H 2 O H 3 O + + A strong acid PI weak acid 0 [HA] o 47

48 Weak Bases (K b ) Treated in the same way. NH 3 + H 2 O NH OH - Name the conjugate pairs. K = [NH 4 + ][OH - ] [NH 3 ][H 2 O] K[H 2 O] = K b = [NH 4 + ][OH - ] [NH 3 ] 48

49 Base Ionization Constant, K b See Table 15.4 Problems are solved the same as with acids except calculate [OH - ] first. Try it. What is the ph of 0.40 M NH 3 solution? K b = 1.8 x

50 Acid Conjugate Base e.g. acetic acid (HAc) and sodium acetate (NaAc) For acetic acid: HAc + H 2 O H 3 O + + Ac - K a = [H 3 O+ ][Ac - ] [HAc] 50

51 Acid Conjugate Base For sodium acetate: H First: NaAc 2 O Na + + Ac - Nothing happens to Na + WHY? Then: Ac - + H 2 O HAc + OH - K b = [HAc][OH- ] [Ac - ] Group 1 & 2 ions do not hydrolyze. 51

52 Acid Conjugate Base For a conjugate pair (HA and A - ), show that : K a x K b = K w and that the two reactions add to the autoionization of water. (recall the addition rule for chemical equilibria) (Also note: pk a + pk b = pk w = 14) 52

53 Conjugate Pairs 1. Conjugate ion of a weak acid is a weak base. HF H + + F - 2. Conjugate ion of a weak base is a weak acid. NH 3 + H 2 O NH OH - 3. The stronger the acid/base, the weaker its conjugate. 53

54 Acid Conjugate Base K a K b = K w Try it. If K a for acetic acid is 1.8 x 10-5, what is K b for sodium acetate? 54

55 Polyprotic Acids e.g. H 2 CO 3 stepwise ionization Step 1: H 2 CO 3 H + + HCO 3 - K a1 = [H+ ][HCO 3 - ] [H 2 CO 3 ] 55

56 Polyprotic Acids Step 2: conjugate base of step 1 is the acid of step 2. HCO 3 - H + + CO 3-2 K a2 = [H+ ][CO 3-2 ] [HCO 3 - ] 56

57 Polyprotic Acids K a1 is always much greater than K a2. Why? K a1 = 4.2E-7 K a2 = 4.8E-11 See Table

58 Overall K What is the product of K a1 x K a2? H CO 2 3 H + + HCO - 3 HCO - 3 H + + CO -2 3 H CO 2 3 2H + + CO -2 3 K a1 x K a2 = 2.0E-17 58

59 Fun with Oxalic Acid O O H-O-C-C-O-H C 2 H 2 O 4 is diprotic with K a1 = 6.5 x 10-2 K a2 = 6.1 x 10-5 Calculate conc. of all species in 0.10 M solution of C 2 H 2 O 4 59

60 Oxalic Acid Step 1: 1 st ionization. C 2 H 2 O 4 H + + C 2 HO - 4 I C -x +x +x E 0.10-x x x K a1 = x x = 6.5 x

61 Oxalic Acid: Step 1 Solve for x. Approximation method is so far off that you must use quadratic. X x = 0 x = M 61

62 Oxalic Acid: Step 1 Results [H + ] = M [C 2 HO - 4 ] = M [C 2 H 2 O 4 ] = ( ) M = M Now use these as starting values for 2 nd ionization. 62

63 Oxalic Acid: Step 2 2 nd ionization C 2 HO - 4 H + + C 2 O -2 4 I C -y +y +y E y y y K a2 = (0.054+y) y (0.054-y) = 6.1 x

64 Oxalic Acid: Step 2 y = [C 2 O 4-2 ] = 6.1 x 10-5 Same value as K a2! 64

65 Oxalic Acid: Final Results [C 2 H 2 O 4 ] = M [C 2 HO 4 - ] = ( x 10-5 )M = M [H + ] = ( x 10-5 )M = M [C 2 O 4-2 ] = 6.1 x 10-5 M 1 st only 1 st only 1 st only Ka2 65

66 Oxalic Acid Note these general results: 1.If K a1 >> K a2, [H + ] & [first conjugate base] are result of 1 st stage ionization only. 2.Second stage conjugate base concentration is equal to K a2. 66

67 Molecular Structure vs. Acid Strength HX H + + X - Acid strength is measured by degree of ionization (K a ). How is molecular structure of the acid related to acid strength? 67

68 Acid Strength d+ d- H A Determine by two factors: 1. Bond strength: the higher the bond dissociation energy, the weaker the acid. 2. Bond polarity: the more polar the bond, the stronger the acid. Why? 68

69 Hydrohalic Acids d+ d- H A acid strength HF<<HCl<HBr<HI weak strong strong strong HF HCl HBr HI Bond Energy Wins! Bond Polarity 69

70 Strength of Oxyacids Draw Lewis dot structures of: Carbonic acid Nitric acid Sulfuric acid 70

71 Oxyacids d- d+ General structure Z O H If Z is highly electronegative or in high oxidation state, it will attract bonding electrons and make the O H bond more polar, resulting in a stronger acid. 71

72 Oxyacids with Z Atom from same Group HClO 3 vs. HBrO 3 O H O Z O What is oxidation number of Z? Cl more electronegative, therefore HClO 3 is stronger. 72

73 Oxyacids with Same Z Ox.# HClO 4 >HClO 3 >HClO 2 >HClO acid strength As oxidation state of Cl increases, acid strength increases. Remember the rule for strong acids. 73

74 Oxyacids: Summary General structure Z O H For element Z: High electronegativity High oxidation number stronger acid 74

75 Predict!!! Place in order of decreasing strength and state which are strong/weak: 1.HIO, HBrO, HClO 2.HNO 3, HNO 2 3.H 3 PO 3, H 3 PO 4 75

76 Salt Hydrolysis Reaction of a cation and/or anion of a salt with water, changing ph. 76

77 Which Ions React with H O 2 to Change ph? Anions from strong acids do not hydrolyze. E.g. Cl - comes from the strong acid HCl. Cations from strong bases do not hydrolyze. E.g. Na + comes from the strong base NaOH. All other ions hydrolyze. Know strong acids and bases. 77

78 Neutral Salts Group 1 and 2 cations (except Be +2 ) combined with anions of a strong acid do not hydrolyze and are neutral. NaNO 3 Na + (aq) + NO 3 - (aq) ph ~ 7 NaOH HNO 3 (strong base, (strong acid no hydrolysis) no hydrolysis) 78

79 Basic Salts Salt of weak acid and strong base is basic. H 2 O CH 3 COONa Na + + CH 3 COO - Na + doesn t hydrolyze, but: CH 3 COO - + H 2 O CH 3 COOH + OH - basic 79

80 Basic Salts CH 3 COO - + H 2 O CH 3 COOH + OH - K b = [CH 3COOH ][OH - ] [CH 3 COO - ] CH 3 COONa acts like a weak base when added to water. 80

81 Try It! What is the ph of a 0.15 M solution of CH 3 COONa? (K a of acetic acid is = 1.8E-5) 81

82 Acidic Salts Salt of strong acid and weak base is acidic. NH 4 Cl H 2 O NH 4 + (aq) + Cl - (aq) Cl - doesn t hydrolyze, but: NH H 2 O NH 3 + H 3 O + acidic 82

83 Acidic Salts NH H 2 O NH 3 + H 3 O + or simply: NH 4 + NH 3 + H + K a = [NH 3 ][H+ ] [NH 4 + ] Can look up K a of NH 4 + (or K b of NH 3 ) to solve problems. 83

84 Acidic Salts The most acidic cations are small and highly charged. (high charge density) e.g. Al +3, Cr +3, Fe +3, Bi +3, Be +2 Larger, +1 cations do not hydrolyze (K +, Na + ). 84

85 Try It!! Al +3 ion becomes hydrated in water, forming Al(H 2 O) +3 6, which then hydrolyzes as follows: Al(H 2 O) +3 6 Al(OH)(H 2 O) H + K a = 1.3 x 10-5 What is the ph of M AlCl 3? 85

86 Salts in Which Both Cation & Anion Hydrolyze The math is involved, so let s just make some qualitative predictions. 86

87 Salts in Which Both Cation & Anion Hydrolyze K b for anion > K a for cation, solution will be basic. K b for anion < K a for cation, solution will be acidic. K b for anion ~ K a for cation, solution will be ~ neutral. 87

88 Predict Are solutions of the following salts acidic, neutral, or basic? NH 4 I CaCl 2 Fe(NO 3 ) 3 LiClO 4 KCN NH 4 CN K a ~E-10 K b ~E-5 88

89 HCO 3 - Acidic or Basic? HCO H 2 O H 3 O + + CO -2 3 K a = 4.8 x HCO H 2 O H 2 CO 3 + OH - K b = 2.4 x 10-8 Since K b > K a, solution is basic. 89

90 HSO 4 - : An Exception! HSO 4 - is the only common acidic anion. Cl -, NO - 3, ClO - 4 F -, CO 2-3, HCO - 3, H 2 PO - 4 HSO - 4 neutral basic acidic Remember this guy! 90

91 Oxides See Table basic amphoteric acidic 91

92 Basic (Metallic) Oxides Na 2 O + H 2 O 2NaOH BaO + H 2 O Ba(OH) 2 Metal oxides are anhydrous bases (Bases with the water removed). 92

93 Acidic (Nonmetal) Oxides Nonmetal oxides are anhydrous acids. CO 2 + H 2 O H 2 CO 3 (water exposed to air) SO 3 + H 2 O H 2 SO 4 (acid rain) 93

94 Try It!!! Write the reaction for: CO 2 (g) + NaOH(aq)? BaO(s) + HNO 3 (aq)? 94

95 Amphoteric Oxides Al 2 O 3 is both basic & acidic! Al 2 O 3 + 6HCl 2AlCl 3 + 3H 2 O Al 2 O 3 + 2NaOH + 3H 2 O 2NaAl(OH) 4 95

96 Transition Metal Oxides Some transition metal oxides are acidic if oxidation number is high Mn 2 O 7 + H 2 O 2HMnO CrO 3 + H 2 O H 2 CrO 4 (No change in oxidation no.) 96

97 ph and Solubility Solubility of insoluble basic salts are greatly affected by ph due to common ion effect. e.g. Mg(OH) 2 and BaF 2 Since OH - and F - both react with H +, the solubility of Mg(OH) 2 and BaF 2 are affected by ph. 97

98 ph and Solubility Mg(OH) 2 has K sp = 1.2 x Its ph in water is given by: Mg(OH) 2 Mg OH - K sp = 1.2 x = (Mg +2 )(OH - ) 2 = (s)(2s) 2 s = 1.4 x 10-4 M 2s = [OH - ] = 2.8 x 10-4 M ph =

99 ph and Solubility Mg(OH) 2 (s) Mg +2 (aq) + 2OH - (aq) If H + is added to lower ph below 10.45, the added H + reacts with OH -, thus lowering the [OH - ]. Thus more Mg(OH) 2 will dissolve to keep K sp constant (LeChatelier) 99

100 ph and Solubility Similarly for a basic salt like BaF 2 : BaF 2 (s) Ba +2 (s) + 2F - (s) The added H + to lower ph reacts with F -, forming the weak acid HF. H + (aq) + F - (aq) HF(aq) Thus more BaF 2 will dissolve at lower ph (LeChat.) 100

101 ph and Solubility Try it!!! Which are more soluble in acidic media than in water? CuS AgCl PbSO

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