1. Acids and Bases Topics to be covered: Definitions of acids and bases; Bronsted’s conjugate acid-base pairs concept; Determination of [H 3 O + ], [OH - ], and pH for strong acids and strong bases; Determination of [H 3 O + ], [OH - ], pH, K a, or K b, and percent ionization for weak acids and weak bases; Predicting acid/base properties of salts (ionic compounds) and oxides of metals and nonmetals; The effect of molecular structures and bond energy on the strength of acids and bases.

2. Acids and Bases Arrhenius definition: Acid: substance that increases [H 3 O + ] in solution; Base: substance that increases [OH - ] in solution; Br  nsted-Lowry definition: Acid: reactant that donates proton in a reaction; Base: reactant that accepts proton in a reaction; Lewis definition: - in the formation of covalent bonds, Acid: one that accept electron-pairs Base: one that donate electron-pairs

3. Strong and Weak Acids Strong acids ionize completely in aqueous solution: HCl (aq) + H 2 O  H 3 O + (aq) + Cl - (aq) ; H 2 SO 4 (aq) + H 2 O  H 3 O + (aq) + HSO 4 - (aq) ; Weak acids ionize only partially in aqueous solution: HF (aq) + H 2 O ⇌ H 3 O + (aq) + F - (aq) ; HOCl (aq) + H 2 O ⇌ H 3 O + (aq) + ClO - (aq).

4. Complete Ionization of Hydrochloric Acid

5. Dissociations of Strong and Weak Acids

6. Strong and Weak Bases Strong bases ionize completely in aqueous solution: NaOH (aq)  Na + (aq) + OH - (aq) ; Ba(OH) 2 (aq)  Ba 2+ (aq) + 2 OH - (aq) ; Weak bases ionize only partially in aqueous solution: NH 3 (aq) + H 2 O  NH 4 + (aq) + OH - (aq) ; PO 4 3- (aq) + H 2 O  HPO 4 2- (aq) + OH - (aq)

7. Brønsted-Lowry: Conjugate Acids & Bases Consider the following equilibrium: HA + B ⇌ BH + + A - ; Acid 1 Base 2 Conjugate Conjugate acid 2 base 1 A - is the conjugate base to acid HA; HA & A - are conjugate acid-base pair; BH + is the conjugate acid to base B; BH + & B are also conjugate acid-base pair.

8. Brønsted-Lowry’s Acid-Base Reaction

9. Brønsted’s Conjugate Acid-Base Pairs

10. Examples of Conjugate Acid-Base Pairs HNO 3 – NO 3 - H 2 SO 4 – HSO 4 - H 3 O + – H 2 O HF – F - H 3 PO 4 – H 2 PO 4 - CH 3 COOH – CH 3 COO - H 2 PO 4 - – HPO 4 2- NH 4 + – NH 3 H 2 O – OH -

11. Exercise #1: Conjugate Acids & Bases Write the formulas of the conjugate bases for the following acids: (a) H 2 CO 3 (b) HSO 4 - (c) Al(H 2 O) 6 3+ (d) Cr(OH) 3 (H 2 O) 3 (Answer: (a) HCO 3 -; (b) SO 4 2- ; (c) Al(H 2 O) 5 (OH) 2+ ; (d) Cr(OH) 4 (H 2 O) 2 - )

12. Exercise #2: Conjugate Acids and Bases Write the formulas of the conjugate acids for the following bases: (a) NH 3 (b) CO 3 2- (c) C 5 H 5 N (d) Al(OH) 3 (H 2 O) 3 (Answer: (a) NH 4 + ; (b) HCO 3 - ; (c) C 5 H 5 NH + ; (d) Al(OH) 2 (H 2 O) 4 + )

13. Acid Strength and Ionization Constants For the ionization or dissociation equilibrium of an acid in aqueous solution, such as: HA (aq) + H 2 O  H 3 O + (aq) + A - (aq) ; The ionization or dissociation constant is expressed as follows: The values of K a indicate the relative strength of the acids. Strong acids have very large K a, while weak acids have small K a ’s (K a << 1)

15. Relative Strength of Acids and Their Conjugate Bases AcidsConjugate Bases Very Strong Very Weak Strong Weak Weak Strong Very Weak Very Strong ______________________________ Strong acids lose protons very readily  weak conjugate bases; Weak acids do not lose protons very readily  strong conjugate bases.

16. Relative Strength of Acids and Their Conjugate Bases

17. Acid-Base Properties of Water Auto-ionization of water: 2H 2 O  H 3 O + (aq) + OH - (aq) K w = [H 3 O + ][OH - ] = 1.0 x 10 -14 at 25 o C Water ionizes to produce both H 3 O + and OH -, thus it has both acid and base properties. K w is called water ionization constant. Pure water at 25 o C: [H 3 O + ] = [OH - ] = 1.0 x 10 -7 M

18. Expressing Acidity in pH Scale pH = -log[H + ] (note: [H + ] = [H 3 O + ]) pOH = -log[OH - ] pK w = -log(K w ); pK a = -log(K a ); pK b = -log(K b ) For water, K w = [H 3 O + ][OH - ] = 1.0 x 10 -14 -log(K w ) = -log [H 3 O + ] + (-log[OH - ]) pK w = pH + pOH = 14.00 At 25 o C, pOH = 14 – pH

19. Acidity and pH Range Acidic solutions: [H 3 O + ] > 1.0 x 10 -7 M; pH < 7; Basic solutions: [OH - ] > 1.0 x 10 -7 M or [H 3 O + ] < 1.0 x 10 -7 M pH > 7; Neutral solutions: [H 3 O + ] = [OH - ] = 1.0 x 10 -7 M; pH = 7.00

20. Relationship between acidity of solution and pH

21. pH of Common Household Items

22. Household Substances

23. [H 3 O + ] and pH of Strong Acids Strong acids like HCl and HClO 4 ionize completely in aqueous solution: HCl (aq) + H 2 O  H 3 O + (aq) + Cl - (aq) ; HClO 4 (aq) + H 2 O  H 3 O + (aq) + ClO 4 - (aq) ; In solutions of strong monoprotic acids HA, such as HCl and HClO 4, [H 3 O + ] = [HA] 0 For example, in 0.10 M HCl, [H 3 O + ] = 0.10 M, and pH = -log(0.10) = 1.00

24. [OH - ] and pH of Strong Bases Like strong acids, strong bases also ionize completely in aqueous solution. Examples: NaOH (aq)  Na + (aq) + OH - (aq) ; Ba(OH) 2 (aq)  Ba 2+ (aq) + 2 OH - (aq) ; In a base solution such as 0.050 M NaOH, [OH - ] = [NaOH] 0 = 0.050 M; pOH = -log(0.050 M) = 1.30; pH = 14.00 - 1.30 = 12.70 In a base solution such as 0.050 M Ba(OH) 2, [OH - ] = 2 x [Ba(OH) 2 ] 0 = 0.10 M; pOH = -log(0.10) = 1.00; pH = 14.00 - 1.00 = 13.00

25. [H 3 O + ] and pH of Weak Acids In weak acid solutions, [H 3 O + ] < [HA ] 0 ; [H 3 O + ] and pH can be calculated from the initial concentration of the acid and its K a value. For example, in 0.100 M acetic acid, CH 3 COOH, with K a = 1.8 x 10 -5, [H 3 O + ] and pH can be calculated using the “ICE” table.

26. ICE Table for Acetic Acid Ionization: CH 3 CO 2 H + H 2 O  H 3 O + + CH 3 CO 2 -  I nitial [M] 0.100 0.00 0.00 Change [M] - x + x + x Equilm. [M] (0.100 – x ) x x 

27. Calculating [H 3 O + ] from initial concentration and K a by approximation method. Solving for x in the expression for K a :

28. Calculation Percent Ionization We calculated that, in 0.10 M CH 3 COOH, the concentration of acid that ionizes is 1.34 x 10 -3 M. The percent ionization of 0.10 M acetic acid:

29. Increase in Percent Ionization with Dilution Percent ionization of weak acid increases as the dilution increases. For example, if [CH 3 COOH] initial = 0.0100 M [CH 3 COOH] ionized ~ 4.24 x 10 -4 M (calculated using approximation method);

30. Percent Ionization Increases with Dilution At [CH 3 COOH] initial = 0.00100 M, [CH 3 COOH] ionized = 1.34 x 10 -4 M (by approximation) The more we dilute the acid solution, the higher the fraction of the acid that will ionize, which increases the degree of ionization. Is this related to the Le Chatelier’s principle? Explain.

31. Exercise #3: pH and Percent Ionization 1)Nitrous acid, HNO 2, has K a = 4.0 x 10 -4 at 25 o C. Calculate the pH and percent ionization of HNO 2 in 0.10 M solution of the acid. (Answer: pH = 2.20; % ionization = 6.3%) 2)Chlorous acid, HOCl, has K a = 3.5 x 10 -8 at 25 o C. Calculate the pH and percent ionization of HOCl in 0.10 M solution of the acid. (Answer: pH = 4.23; % ionization = 0.059%)

32. [OH - ] and pH of a Weak Base In a weak base, [OH - ] < [Base] 0 ; [OH - ] and pH can be calculated from the initial concentration of the base and its K b value. For example, in 0.100 M ammonia, NH 3, with K b = 1.8 x 10 -5, [OH - ] and pH can be calculated using the following “ICE” table.

33. ICE Table for the Ionization of Ammonia Ionization: NH 3 + H 2 O  NH 4 + + OH -  Initial [M] 0.100 0.000.00 Change [M] -x +x +x Equilm. [M] (0.100 – x) x x 

34. Calculate [OH - ] from initial concentration and K b by approximation method. Solving for x in the expression for K b :

35. Exercise #4: pH of Strong and Weak Bases 1)Ethylamine, C 2 H 5 NH, has K b = 5.6 x 10 -4. Calculate the concentration of H 3 O + in 0.10 M ethylamine solution and the pH of the solution? (Answer: [H 3 O + ] = 1.3 x 10 -12 M; pH = 11.87) 2)Pyridine, C 5 H 5 N, has K b = 1.7 x 10 -9. Calculate the concentration of H 3 O + in 0.020 M pyridine solution and the pH of the solution? (Answer: [H 3 O + ] = 1.7 x 10 -9 M; pH = 8.77)

36. Polyprotic Acids Acids with more than one ionizable hydrogen ion, such as H 2 SO 3, H 2 SO 4, H 3 PO 4, H 3 C 6 H 5 O 7, etc. Their hydrogen ionizes in stages, for example, H 3 PO 4 (aq) + H 2 O ⇄ H 3 O + (aq) + H 2 PO 4 - (aq) ; K a1 = 7.5 x 10 -3 H 2 PO 4 - (aq) + H 2 O ⇄ H 3 O + (aq) + HPO 4 2- (aq) ; K a2 = 6.2 x 10 -8 HPO 4 2- (aq) + H 2 O ⇄ H 3 O + (aq) + PO 4 3- (aq) ; K a3 = 4.8 x 10 -13 Acid strength decreases in the order: H 3 PO 4 >> H 2 PO 4 - >> HPO 4 2- ; pH of solution is determined mainly by ionization of H 3 PO 4

37. Exercise #5: pH of H 2 SO 4 1)Calculate the concentrations of H 2 SO 4, H 3 O +, HSO 4 -, and SO 4 2-, in 0.10 M H 2 SO 4 solution. What is the pH of the solution? (H 2 SO 4 is a strong acid and HSO 4 - has K a = 1.2 x 10 -2 ) (Answer: [H 2 SO 4 ] = 0.0 M; [H 3 O + ] = 0.11 M; [HSO 4 - ] = 0.090 M; [SO 4 2- ] = 0.0098 M; pH = 0.96]

38. Salts and Their Corresponding Acids & Bases Acids and bases from which each of the following salts are produced. AcidsBases NaClO 4 KNO 3 (NH 4 ) 2 SO 4 NaC 2 H 3 O 2 BaCl 2 Na 3 PO 4

39. Acid-Base Properties of Salt Solutions Soluble salts dissociate completely when dissolved in water; Ions produced may react with water that release H 3 O + and makes the solution acidic, or produces OH - and makes the solution basic.

40. Acid-Base Properties of Salt Solutions The acid-base property of a salt solution depends on whether the compound is a product of: 1.Strong acid-strong base reaction: produces neutral salt 2.Strong acid-weak base reaction: produces acidic salt 3.Strong base-weak acid reaction: produces basic salt 4.Weak acid-weak base reaction: produces salt that is either acidic or basic, depending on the relative strength of the acid and the base.

41. Types of Salts and Their Solutions Salts of Strong Acid-Strong Base Reactions: NaCl, NaNO 3, KBr, etc.; solutions are neutral Salts of Weak Acid-Strong Base Reactions: NaF, NaNO 2, NaC 2 H 3 O 2, etc.; solutions are basic Salts of Strong Acid-Weak Base Reactions: NH 4 Cl, NH 4 NO 3, (CH 3 ) 2 NH 2 Cl, C 5 H 5 NHCl; Solutions of these salts are acidic Salts of Weak Acid-Weak Base Reactions: NH 4 C 2 H 3 O 2, NH 4 CN, NH 4 NO 2, etc.. These compounds can be acidic, basic, or neutral, which depends on the relative strength of the acid and the base.

42. Salts of Strong Acid-Strong Base Reactions Dissociation and reaction of a neutral salt: NaCl (aq)  Na + (aq) + Cl - (aq) ; Na + (aq) + H 2 O  NR Cl - (aq) + H 2 O  NR (The concentrations of H 3 O + and OH - in NaCl solution are the same as in pure water  solution is neutral.)

43. Salts of Weak Acid-Strong Base Reactions: Dissociation and reaction of a basic salt: NaNO 2 (aq)  Na + (aq) + NO 2 - (aq) ; Na + (aq) + H 2 O  NR NO 2 - (aq) + H 2 O  HNO 2 (aq) + OH - (aq) (The reaction of NO 2 - with water causes [ OH - ] > [ H 3 O + ] and the solution becomes basic.)

44. Salts of Strong Acid-Weak Base Reactions Dissociation and reaction of an acidic salt: NH 4 NO 3 (aq)  NH 4 + (aq) + NO 3 - (aq) ; NH 4 + (aq) + H 2 O  NH 3 (aq) + H 3 O + (aq) ; NO 3 - (aq) + H 2 O  NR; (The reaction of NH 4 + with water causes [H 3 O + ] > [OH - ], and the solution becomes acidic.)

45. Salts of Weak Acid-Weak Base Reactions Salts produced by reactions of weak acids and weak bases can be neutral, acidic, or basic, depending on the relative magnitude of the K a of the weak acid and the K b of the weak base. If K a ~ K b, salts are neutral; example: NH 4 C 2 H 3 O 2 If K a > K b, salts are acidic; example: NH 4 NO 2 If K a < K b, salts are basic; example: NaC 2 H 3 O 2

46. Predicting Acid-Base Property of Salts Consider a solution containing NH 4 C 2 H 3 O 2 NH 4 C 2 H 3 O 2 (aq)  NH 4 + (aq) + C 2 H 3 O 2 - (aq) ; NH 4 + (aq) + H 2 O  H 3 O + (aq) + NH 3 (aq) ; K a = 5.6 x 10 -10 C 2 H 3 O 2 - (aq) + H 2 O  HC 2 H 3 O 2 (aq) + OH - (aq) ; K b = 5.6 x 10 -10 K a = K b = 5.6 x 10 -10,  NH 4 C 2 H 3 O 2 is neutral

47. Predicting Acid-Base Property of Salts Consider a solution containing (NH 4 ) 2 SO 4 (NH 4 ) 2 SO 4 (aq)  2NH 4 + (aq) + SO 4 2 - (aq) ; NH 4 + (aq) + H 2 O  H 3 O + (aq) + NH 3 (aq) ; K a = 5.6 x 10 -10 SO 4 2 - (aq) + H 2 O  HSO 4 - (aq) + OH - (aq) ; K b = 8.3 x 10 -13 K a > K b  (NH 4 ) 2 SO 4 is acidic;

48. Predicting Acid-Base Characteristics of Salts Now consider a solution containing NH 4 CN. NH 4 CN (aq)  NH 4 + (aq) + CN - (aq) ; NH 4 + (aq) + H 2 O  H 3 O + (aq) + NH 3 (aq) ; K a = 5.6 x 10 -10 CN - (aq) + H 2 O  HCN (aq) + OH- (aq) ; K b = 1.6 x 10 -5 K b > K a  NH 4 CN is basic

49. Effect of Structure on Acid-Base Properties Relative bond strength: H─F > H─Cl > H─Br > H─I Relative acid strength: HI > HBr > HCl > HF (HF is the only weak acid in this group) Relative acid strength: H 2 Te > H 2 Se > H 2 S > H 2 O (all very weak acids)

50. Effect of Structure on Acid-Base Properties Electronegativity Effect on Relative Strength of Oxo-Acids: Same central atom, but different numbers of oxygen atoms: HOClO 3 > HOClO 2 > HOClO > HOCl; Same number of oxygen, but different central atoms: 1.Trend of electronegativity: F > Cl > Br > I; 2.Trend of relative acidity: HOF > HClO > HOBr > HOI; 3.Relative acidity: CF 3 COOH > CCl 3 COOH > CH 3 COOH 4.Trend of electronegativity: Cl > S > P > Si 5.Relative acidity: HClO 4 > H 2 SO 4 > H 3 PO 4 > H 4 SiO 4

51. Acid-Base Properties of Oxides Metal oxides are basic or amphoteric Basic oxides: Na 2 O (s) + H 2 O  2NaOH (aq)  2Na + (aq) + 2OH - (aq) ; MgO (aq) + HCl (aq)  MgCl 2 (aq) + H2O Amphoteric oxides: Al 2 O 3 (s) + HCl (aq)  2AlCl 3 (aq) + H 2 O Al 2 O 3 (s) + 2NaOH (aq) + 3H 2 O  2NaAl(OH) 4 (aq)

52. Acid-Base Properties of Oxides Nonmetal oxides are acidic; N 2 O 5 + H 2 O  2HNO 3 (aq) ; SO 3 (g) + H 2 O  H 2 SO 4 (aq) ; Cl 2 O 7 + H 2 O  2HClO 4 (aq) Trends: Acidity increases left to right across a period SiO 2 < P 4 O 10 < SO 3 < Cl 2 O 7 Acidity decreases top-to-bottom in periodic table N 2 O 5 > P 4 O 10 > As 2 O 5 > Sb 2 O 5

53. Acid-Base Properties of Oxides Trend of acid-base properties of oxides in periodic table: from the most basic (on the left) to the most acidic (on the right): Na 2 O, MgO, Al 2 O 3, SiO 2, P 4 O 10, SO 3, Cl 2 O 7 Trend of acid-base properties going down a group: most acidic on the left to the most basic on the right: N 2 O 5, P 4 O 10, As 2 O 5, Sb 2 O 5

54. Lewis Acids and Bases Identify the Lewis acids and Lewis bases in the following reactions: (a) H 2 O + CO 2 (aq)  H 2 CO 3 (aq) ; (b) Cu 2+ (aq) + 4NH 3 (aq)  Cu(NH 3 ) 4 2+ (aq) ; (c) AlCl 3 + Cl -  AlCl 4 - ; (d) BF 3 + NH 3  F 3 B─NH 3