1. 16-1 CHEM 102, Fall 2012, LA TECH CTH 328 10:00-11:15 am Instructor: Dr. Upali Siriwardane e-mail: upali@latech.edu Office: CTH 311 Phone 257-4941 Office Hours: M,W 8:00-9:00 & 11:00-12:00 am; Tu, Th, F 8:00 - 10:00am.. Exams: 10:00-11:15 am, CTH 328. September 27, 2012 (Test 1): Chapter 13 October 18, 2012 (Test 2): Chapter 14 &15 November 13, 2012 (Test 3): Chapter 16 &18 Optional Comprehensive Final Exam: November 15, 2012 : Chapters 13, 14, 15, 16, 17, and 18 Chemistry 102(001) Fall 2012

2. 16-2 CHEM 102, Fall 2012, LA TECH Chapter 16. Acids and Bases 16.1 The Brønsted-Lowry Concept of Acids and Bases 16.2 Types of acids/bases:Organic Acids and Amines 16.3 The Autoionization of Water 16.4 The pH Scale 16.5 Ionization Constants of Acids and Bases 16.6 Problem Solving Using K a and K b 16.7 Molecular Structure and Acid Strength 16.8 Acid-Base Reactions of Salts 16.9 Practical Acid-Base Chemistry 16.10 Lewis Acid and Bases

3. 16-3 CHEM 102, Fall 2012, LA TECH Types of Reactions a) Precipitation Reactions. Reactions of ionic compounds or salts Reactions of ionic compounds or salts b) Acid/base Reactions. Reactions of acids and bases Reactions of acids and bases c) Redox Reactions. reactions of oxidizing & reducing agents reactions of oxidizing & reducing agents

4. 16-4 CHEM 102, Fall 2012, LA TECH What are Acids &Bases? Definition? a) Arrhenius b) Bronsted-Lowry c) Lewis

5. 16-5 CHEM 102, Fall 2012, LA TECH Arrhenius, Svante August (1859-1927), Swedish chemist, 1903 Nobel Prize in chemistry AcidAcid Anything that produces hydrogen ions in a water solution. HCl (aq ) H + ( aq) + Cl - ( aq) Base BaseAnything that producs hydroxide ions in a water solution. NaOH (aq) Na + ( aq) + OH - ( aq) Arrhenius definitions are limited proton acids and hydroxide bases to aqueous solutions. Arrhenius Definitions

6. 16-6 CHEM 102, Fall 2012, LA TECH Expands the Arrhenius definitions to include many bases other than hydroxides and gas phase reactions Acid AcidProton donor Base BaseProton acceptor This definition explains how substances like ammonia can act as bases. Eg. HCl(g) + NH 3 (g) ------> NH 4 Cl(s) HCl (acid), NH 3 (base). NH 3 (g) + H 2 O(l) NH 4 + + OH - Brønsted-Lowry definitions

7. 16-7 CHEM 102, Fall 2012, LA TECH Lewis Definition G.N. Lewis was successful in including acid and bases without proton or hydroxyl ions. Lewis Acid: A substance that accepts an electron pair. Lewis base: A substance that donates an electron pair. E.g. BF 3 (g) + :NH 3 (g) F 3 B:NH 3 (s) the base donates a pair of electrons to the acid forming a coordinate covalent bond common to coordination compounds. Lewis acids/bases will be discussed later in detail

8. 16-8 CHEM 102, Fall 2012, LA TECH 1) Acids and bases can be defined in several ways. Which definitions of the bases that fits the description below? a) a compound that produces more OH- ions in water: b) a proton acceptor: c) an electron pair donor:

9. 16-9 CHEM 102, Fall 2012, LA TECH Types of Acids and Bases Binary acids Oxyacid Organic acids Acidic oxides Basic oxides Amine Polyprotic acids

10. 16-10 CHEM 102, Fall 2012, LA TECH Types of Acids and Bases Binary acids: HCl, HBr, HI, H 2 S More than two elements: HCN Oxyacid: HNO 3, H 2 SO 4, H 3 PO 4 Polyprotic acids: H 2 SO 4, H 3 PO 4 Organic acids: R-COOH, R= CH 3 -, CH 3 CH 2 - Acidic oxides: SO 3, NO 2, CO 2, Basic oxides: Na 2 O, CaO Amine: NH 3. R-NH 2, R= CH 3 -, CH 3 CH 2 - : primary R 2 -NH : secondary, R 3 -N: tertiary R 2 -NH : secondary, R 3 -N: tertiary Lewis acids & bases: BF 3 and NH 3

11. 16-11 CHEM 102, Fall 2012, LA TECH Acidic Oxides Acidic Oxides These are usually oxides of non-metallic elements such as P, S and N. E.g. NO 2, SO 2, SO 3, CO 2 E.g. NO 2, SO 2, SO 3, CO 2 They produce oxyacids when dissolved in water SO 3 + H 2 O ---> H 2 SO 4 CO 2 + H 2 O ---> H 2 CO 3 NO2 + H 2 O ---> HNO 3

12. 16-12 CHEM 102, Fall 2012, LA TECH Basic Oxides Basic Oxides Oxides oxides of metallic elements such as Na, K, Ca. They produce hydroxyl bases when dissolved in water. e.g. CaO + H 2 O ---> Ca(OH) 2 Na 2 O + H 2 O ---> 2 NaOH Na 2 O + H 2 O ---> 2 NaOH

13. 16-13 CHEM 102, Fall 2012, LA TECH Protic Acids Monoprotic Acids: The form protic refers to acidity due to protons. Monoprotic acids have only one acidic proton. e.g. HCl. Polyprotic Acids: They have more than one acidic proton. e.g. H 2 SO 4 - diprotic acid H 3 PO 4 - triprotic acid.

14. 16-14 CHEM 102, Fall 2012, LA TECH Polyprotic Acids acids where more than one hydrogen per molecule is released

15. 16-15 CHEM 102, Fall 2012, LA TECH Amines Amines Class of organic bases derived from ammonia NH 3 by replacing hydrogen by organic groups. They are defined as bases similar to NH 3 by Bronsted- Lowery or Lewis acid/base definitions.

16. 16-16 CHEM 102, Fall 2012, LA TECH Amines

17. 16-17 CHEM 102, Fall 2012, LA TECH 2) Identify types of acids/bases as: binary acids, oxy acids, organic acids, acidic oxides, basic oxides, amine and polyprotic acids. a) HF b) HBrc) H 3 PO 4 d) H 2 SO 4 e) HNO 3 f) R-COOH g) NO 2 h) SO 3 j)CaO k) R-NH 2

18. 16-18 CHEM 102, Fall 2012, LA TECH Influence of Molecular Structure on Acid Strength Binary Hydrides hydrogen & one other element Bond Strengths weaker the bond, the stronger the acid Stability of Anion higher the electronegativity, stronger the acid

19. 16-19 CHEM 102, Fall 2012, LA TECH Binary Acids Binary Acids Compounds containing acidic protons bonded to a more electronegative atom. e.g. HF, HCl, HBr, HI, H 2 S The acidity of the haloacid (HX; X = Cl, Br, I, F) Series increase in the following order: HF < HCl < HBr < HI HF < HCl < HBr < HI

20. 16-20 CHEM 102, Fall 2012, LA TECH Oxyacids Oxyacids Compounds containing acidic - OH groups in the molecule. Acidity of H 2 SO 4 is greater than H 2 SO 3 because of the extra O (oxygens) The order of acidity of oxyacids from the a halogen (Cl, Br, or I) shows a similar trend. HClO 4 > HClO 3 > HClO 2 > HClO HClO 4 > HClO 3 > HClO 2 > HClO perchloric chloric chlorus hyphochlorus perchloric chloric chlorus hyphochlorus

21. 16-21 CHEM 102, Fall 2012, LA TECH Influence of Molecular Structure on Acid Strength Oxyacids hydrogen, oxygen, & one other elementH-O-E higher the electronegativity on E, stronger the acid as this weakens the bond between the O and H

22. 16-22 CHEM 102, Fall 2012, LA TECH <<< < Oxo Acid

23. 16-23 CHEM 102, Fall 2012, LA TECH 3) Which of the following is stronger acid or base: a) a) H 2 SO 4 or H 2 SO 3 : b) b) HCl or HI: c) c) HClO or HClO 3 : d) d) H 2 S or HF: e) e) CF 3 COOH or CH 3 COOH: f) f) CH 3 COOH or CH 3 CH 2 COOH

24. 16-24 CHEM 102, Fall 2012, LA TECH Dissociation Strong Acids: HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl - (aq) HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl - (aq) H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq) H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq) Dissociation Equilibrium Weak Acid/base: Dissociation Equilibrium Weak Acid/base: H 2 O(l) + H 2 O(l) H 3 + O(aq) + OH - (aq) This dissociation is called autoionization of water. HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 - (aq) NH 3 (aq) + H 2 O(l) NH 4 + + OH - (aq) Equilibrium constants: K a, K b and K w

25. 16-25 CHEM 102, Fall 2012, LA TECH 4) Write equations for the dissociation equilibrium reactions for the following acids and bases in water. Which of these are acid or dissociations? a) HCl: b) H 2 SO 4 :

26. 16-26 CHEM 102, Fall 2012, LA TECH 4) Write equations for the dissociation equilibrium reactions for the following acids and bases in water. Which of these are acid or dissociations? c) H 2 O (autoionization): What is auto ionization? d) HC 2 H 3 O(acetic acid): e) NH 3 :

27. 16-27 CHEM 102, Fall 2012, LA TECH Bronsted acid/conjugate base and base/conjugate acid pairs in acid/base equilibria HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl - (aq) HCl(aq): acid H 2 O(l):base H 3 + O(aq):conjugate acid Cl - (aq):conjugate base H 2 O/ H 3 + O: base/conjugate acid pair HCl/Cl - :acid/conjugate base pair

28. 16-28 CHEM 102, Fall 2012, LA TECH Conjugate acid-base pairs. Acids and bases that are related by loss or gain of H+ as H 3 O + and H 2 O. Examples. Examples.AcidBase H 3 O + H 2 O HC 2 H 3 O 2 C 2 H 3 O 2 - NH 4 + NH 3 H 2 SO 4 HSO 4 - HSO 4 - SO 4 2- Brønsted-Lowry Definitions

29. 16-29 CHEM 102, Fall 2012, LA TECH Select acid, base, acid/conjugate base pair, base/conjugate acid pair H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq) acid base conjugate acid conjugate base base/conjugate acid pair acid/conjugate base pair

30. 16-30 CHEM 102, Fall 2012, LA TECH 5) For HOCl write: a) Dissociation equilibrium reaction for the HOCl: b) Identify the acid/conjugate base pair: c) Identify the base/conjugate acid pair: d) The equilibrium constant expression:

31. 16-31 CHEM 102, Fall 2012, LA TECH Strong Acid vs. Weak Acids Strong acid completely ionized Hydrioidic HI K a ~ 10 11 pK a = -11 Hydrobromic HBr K a ~ 10 9 pK a = -9 Perchloric HClO 4 K a ~ 10 7 pK a = -7 Hyrdrochloric HCl K a ~ 10 7 pK a = -7 Chloric HClO 3 K a ~ 10 3 pK a = -3 Sulfuric H 2 SO 4 K a ~ 10 2 pK a = -2 Nitric HNO 3 K a ~ 20 pK a = -1.3 Weak acid partially ionized Hydrofluoric acid HF K a = 6.6x10 -4 pK a = 3.18 Formic acid HCOOH K a = 1.77x10 -4 pK a = 3.75 Acetic acid CH 3 COOH K a = 1.76x10 -5 pK a = 4.75 Nitrous acid HNO 2 K a = 4.6x10 -4 pK a = 3.34 Acetyl Salicylic acid C 9 H 8 O 4 K a = 3x10 -4 pK a = 3.52 Hydrocyanic acid HCN K a = 6.17x10 -10 pK a = 9.21

32. 16-32 CHEM 102, Fall 2012, LA TECH Strong Base vs. Weak Base Strong Base completely ionized Lithium hydroxide LiOH Sodium hydroxide NaOH Potassium hydroxide KOH K b ~ 10 2 -10 3 Rubidium hydroxide RbOH Cesium hydroxide CsOH Boarder-line Bases Magnesium hydroxide Mg(OH) 2 Calcium hydroxide Ca(OH) 2 Strotium hydroxide Sr(OH) 2 K b ~ 0.01 to0.1 Barium hydroxide Ba(OH) 2 Weak Base partially ionized Ammonia NH 3 K b =1.79x10 -5 pK b = 4.74 Ethyl amine CH 3 CH 2 NH 2 K b =5.6x10 -4 pK b = 3.25

33. 16-33 CHEM 102, Fall 2012, LA TECH Strong acidsStrong acids Ionize completely in water. HCl, HBr, HI, HClO 3, HNO 3, HClO 4, H 2 SO 4. Weak acidsWeak acids Partially ionize in water. Most acids are weak. Strong basesStrong bases Ionize completely in water. Strong bases are metal hydroxides - NaOH, KOH Weak basesWeak bases Partially ionize in water. Acid and Base Strength

34. 16-34 CHEM 102, Fall 2012, LA TECH Common Acids and Bases Acids Formula Molarity* nitricHNO 3 16 hydrochloric HCl 12 sulfuricH 2 SO 4 18 aceticHC 2 H 3 O 2 18 Bases ammoniaNH 3 (aq) 15 sodium hydroxideNaOH solid *undiluted.

35. 16-35 CHEM 102, Fall 2012, LA TECH 6.Identify stronger and weaker acids:

36. 16-36 CHEM 102, Fall 2012, LA TECH Autoionization Autoionization When water molecules react with one another to form ions. Acids and bases alter the dissociation equilibrium of water based on Le Chaterlier’s principle Kw = [ H 3 O + ] [ OH - ] = 1.0 x 10 -14 at 25 o C Note: Note: [H 2 O] is constant and is included in K w. ion product of water ion product of water H 2 O(l) + H 2 O(l) H 3 O+(aq) + OH - (aq) (10 -7 M) (10 - 7M) Autoionization of Water

37. 16-37 CHEM 102, Fall 2012, LA TECH What is pH? K w = [H 3 + O][OH - ] = 1 x 10 -14 [H 3 + O][OH - ] = 10 -7 x 10 -7 Extreme cases: Extreme cases: Basic medium [H 3 + O][OH - ] = 10 -14 x 10 0 [H 3 + O][OH - ] = 10 -14 x 10 0 Acidic medium [H 3 + O][OH - ] = 10 0 x 10 -14 [H 3 + O][OH - ] = 10 0 x 10 -14 pH value is -log[H + ] spans only 0-14 in water.

38. 16-38 CHEM 102, Fall 2012, LA TECH We need to measure and use acids and bases over a very large concentration range. pH and pOH are systems to keep track of these very large ranges. pH = -log[H 3 O + ] pOH = -log[OH - ] pH + pOH = 14 Substance pH 1 M HCl0.0 Gastric juices 1.0 - 3.0 Lemon juice2.2 - 2.4 Classic Coke2.5 Coffee5.0 Pure Water7.0 Blood 7.35 - 7.45 Milk of Magnesia 10.5 Household ammonia 12.0 1M NaOH 14.0 Substance pH 1 M HCl0.0 Gastric juices 1.0 - 3.0 Lemon juice2.2 - 2.4 Classic Coke2.5 Coffee5.0 Pure Water7.0 Blood 7.35 - 7.45 Milk of Magnesia 10.5 Household ammonia 12.0 1M NaOH 14.0 pH and other “p” scales

39. 16-39 CHEM 102, Fall 2012, LA TECH A logarithmic scale used to keep track of the large changes in [H+]. 0 7 14 10 -14 M 10 -7 M 10 -14 M Very Neutral Very acidic Basic When you add an acid to, the pH gets smaller. When you add a base to, the pH gets larger. pH scale

40. 16-40 CHEM 102, Fall 2012, LA TECH pH of Aqueous Solutions

41. 16-41 CHEM 102, Fall 2012, LA TECH pH, pK w and pOH The relation of pH, K w and pOH K w = [H + ][OH - ] K w = [H + ][OH - ] log K w = log [H + ] + log [OH - ] log K w = log [H + ] + log [OH - ] -log K w = -log [H + ] -log [OH - ] ; -log K w = -log [H + ] -log [OH - ] ; previous equation multiplied by -1 pK w = pH + pOH; pK w = 14 pK w = pH + pOH; pK w = 14 since K w =1 x 10 -14 since K w =1 x 10 -14 14 = pH + pOH pH = 14 - pOH pOH = 14 - pH 14 = pH + pOH pH = 14 - pOH pOH = 14 - pH

42. 16-42 CHEM 102, Fall 2012, LA TECH Measuring pH Arnold Beckman inventor of the pH meter father of electronic instrumentation

43. 16-43 CHEM 102, Fall 2012, LA TECH 7.Identify the following as acidic/basic/neutral and estimate/calculate pH. Solution Acidic/basic/neutral pH of the solution a) [ H + ] > [OH - ] and [H + ] > 1.0 x 10 -7 M : : b) [H + ] < [OH - ] and [H + ] < 1.0 x 10 -7 M : : c) [H + ] = [OH - ] = 1.0 x 10 -7 M : : d) [H + ] > [OH - ] = 1.0 × 10- 14 M : : e) [H + ]< [OH - ] = 1.0 x 10 +7 M : :

44. 16-44 CHEM 102, Fall 2012, LA TECH pH and pOH calculations of acid and base solutions a) Strong acids/bases dissociation is complete for strong acid such as HNO 3 or base NaOH [H + ] is calculated from molarity (M) of the solution b) weak acids/bases needs K a, K b or percent(%)dissociation

45. 16-45 CHEM 102, Fall 2012, LA TECH pH of 0.5 M H 2 SO 4 Solution H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq ) H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq ) HSO 4 - (aq ) + H 2 O(l) H 3 + O(aq) + SO 4 2- (aq ) HSO 4 - (aq ) + H 2 O(l) H 3 + O(aq) + SO 4 2- (aq ) [H 3 + O][HSO 4 - ] [H 3 + O][HSO 4 - ] H 2 SO 4 ; K a1 = ------------------- H 2 SO 4 ; K a1 = ------------------- [H 2 SO 4 ] [H 2 SO 4 ] [H 3 + O][ SO 4 2- ] [H 3 + O][ SO 4 2- ] H 2 SO 4 ; K a2 = ------------------- ; K a2 ignored H 2 SO 4 ; K a2 = ------------------- ; K a2 ignored [HSO 4 - ] [HSO 4 - ]

46. 16-46 CHEM 102, Fall 2012, LA TECH H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq ) the moles of H + ions in the solution is equal to moles of H 2 SO 4 at the beginning. [H 2 SO 4 ] = [H + ] = 0.5 mole/L pH = -log [H + ] pH = -log(0.5) pH = 0.30 pH of 0.5 M H 2 SO 4 Solution

47. 16-47 CHEM 102, Fall 2012, LA TECH 1.5 x 10 -2 M NaOH. NaOH is also a strong base dissociates completely in water. NaOH is also a strong base dissociates completely in water. [NaOH] = [HO - ] = 1.5 x 10 -2 mole/L [NaOH] = [HO - ] = 1.5 x 10 -2 mole/L pOH = -log[HO - ]= -log(1.5 x 10 -2 ) pOH = -log[HO - ]= -log(1.5 x 10 -2 ) pOH = 1.82 pOH = 1.82 As defined and derived previously: As defined and derived previously: pK w = pH + pOH; pK w = 14 pH = pK w + pOH pH = 14 - pOH pH = 14 - 1.82 ; pH = 12.18

48. 16-48 CHEM 102, Fall 2012, LA TECH 8) For a 0.10 M solution of Ba(OH) 2.Is it a strong base? a)How many OH- are in the compound? b) Calculate the [OH - ] and [H+]: c) pH of the solution:

49. 16-49 CHEM 102, Fall 2012, LA TECH 9) Calculate the pH of the strong acid 0.2 M H 2 SO 4. a) Is it a strong acid? b) Is it polyprotic acid? c) Dissociation equilibria: d) Why second dissociation equilibria is not considered for [H + ] concentration? e) Calculate the [H + ] f) pH of the solution:

50. 16-50 CHEM 102, Fall 2012, LA TECH pH of Mixtures of Strong and Weak Acids the presence of the strong acid retards the dissociation of the weak acid The pH of the solution is mainly based on the strong acid Eg. 1.0 M HCl and 1.0 HC 2 H 3 O 2 HCl(aq) + H 2 O(l)  H 3 + O(aq) + Cl - (aq) [ HC 2 H 3 O2(aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 -(aq) HC 2 H 3 O2(aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 -(aq)

51. 16-51 CHEM 102, Fall 2012, LA TECH pH of Mixtures of Acids and Bases The pH of the solution is mainly based on the excess acid or base present Eg. 10 mL of 1.0 M HCl and 20 mL 1.0 NaOH Moles of excess NaOH ( M x L) = 1.0 x 0.001= 0.001 Mixed together volume = 30 mL = 0.030 L Molarity of excess NaOH = 0.001/0.030= 0.030 Calculate pOH and then pH [

52. 16-52 CHEM 102, Fall 2012, LA TECH Equilibrium, Constant, K a & K b K a : Acid dissociation constant for a equilibrium reaction. K b : Base dissociation constant for a equilibrium reaction. Acid: HA + H 2 O H 3 + O + A - Base: BOH + H 2 O B + + OH - [H 3 + O][ A - ] [B + ][OH - ] K a = --------------- ; K b = ----------------- [HA] [BOH]

53. 16-53 CHEM 102, Fall 2012, LA TECH HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl - (aq) HCl(aq) + H 2 O(l) H 3 + O(aq) + Cl - (aq) [H 3 + O][Cl-] [H 3 + O][Cl-] K a = ----------------- [HCl] [HCl] [H + ][Cl-] [H + ][Cl-] K a = ----------------- [HCl] [HCl] Acid Dissociation Constant

54. 16-54 CHEM 102, Fall 2012, LA TECH Base Dissociation Constant NH 3 + H 2 O NH 4 + + OH - [NH 4 + ][OH - ] K = [NH 3 ]

55. 16-55 CHEM 102, Fall 2012, LA TECH Comparing K w and K a & K b Any compound with a K a value greater than K w of water will be a an acid in water. Any compound with a K b value greater than K w of water will be a base in water.

56. 16-56 CHEM 102, Fall 2012, LA TECH Ionization Constants for Acids

57. 16-57 CHEM 102, Fall 2012, LA TECH WEAKER/STRONGER Acids and Bases & K a and K b values A larger value of K a or K b indicates an equilibrium favoring product side. Acidity and basicity increase with increasing K a or K b. Acidity and basicity increase with increasing K a or K b. pK a = - log K a and pK b = - log K b Acidity and basicity decrease with increasing pK a or pK b.

58. 16-58 CHEM 102, Fall 2012, LA TECH Which is weaker? a. HNO 2 ; K a = 4.0 x 10 -4. b. HOCl 2 ; K a = 1.2 x 10 -2. c. HOCl ; K a = 3.5 x 10 -8. d. HCN ; K a = 4.9 x 10 -10.

59. 16-59 CHEM 102, Fall 2012, LA TECH What is K a1 and K a2 ? H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq) HSO 4 - (aq) + H 2 O(l) H 3 + O(aq) + SO 4 2- (aq)

60. 16-60 CHEM 102, Fall 2012, LA TECH H 2 SO 4 (aq) + H 2 O(l) H 3 + O(aq) + HSO 4 - (aq ) HSO 4 - (aq ) + H 2 O(l) H 3 + O(aq) + SO 4 2- (aq ) [H 3 + O][HSO 4 - ] [H 3 + O][HSO 4 - ] H 2 SO 4 ; K a1 = ------------------- [H 2 SO 4 ] [H 2 SO 4 ] [H 3 + O][SO 4 2- ] H 2 SO 4 ; K a2 = ------------------- [HSO 4 - ] [HSO 4 - ] K a Examples

61. 16-61 CHEM 102, Fall 2012, LA TECH HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 + O(aq) + C 2 H 3 O 2 - (aq) [H + ][C 2 H 3 O 2 - ] [H + ][C 2 H 3 O 2 - ] H C 2 H 3 O 2 ; K a = ------------------ H C 2 H 3 O 2 ; K a = ------------------ [H C 2 H 3 O 2 ] [H C 2 H 3 O 2 ] NH 3 (aq) + H 2 O(l) NH 4 + + OH - (aq) [NH 4 + ][OH - ] [NH 4 + ][OH - ] NH 3 ; K b = -------------- NH 3 ; K b = -------------- [ NH 3 ] [ NH 3 ] K a Examples

62. 16-62 CHEM 102, Fall 2012, LA TECH % Dissociation gives x (amount dissociated) need for pH calculation Amount dissociated Amount dissociated % Dissoc. = ------------------------- x 100 Initial amount/con. Initial amount/con. x % Dissoc. = --------------------------- x 100 % Dissoc. = --------------------------- x 100 concentration concentration

63. 16-63 CHEM 102, Fall 2012, LA TECH How do you calculate pH of weak acids/bases? a) From % dissociation b) From K a or K b What is % dissociation Amount dissociated Amount dissociated % Dissoc. = ------------------------- x 100 Initial amount Initial amount

64. 16-64 CHEM 102, Fall 2012, LA TECH How do you calculate % dissociation from K a or K b 1.00 M solution of HCN; K a = 4.9 x 10 -10 What is the % dissociation for the acid?

65. 16-65 CHEM 102, Fall 2012, LA TECH 1.00 M solution of HCN; K a = 4.9 x 10 -10 First write the dissociation equilibrium equation: First write the dissociation equilibrium equation: HCN(aq) + H 2 O(l) H 3 + O(aq) + CN - (aq) [HCN] [H + ] [CN - ] [HCN] [H + ] [CN - ] Ini. Con. 1.00 M 0.0 M 0.00 M Cha. Con -x x x Eq. Con. 1.0 - x xx [H 3 + O ][CN - ] x 2 [H 3 + O ][CN - ] x 2 K a = ------------------- =---------------- K a = ------------------- =---------------- [HCN] 1.0 - x [HCN] 1.0 - x 1.00 M solution of HCN; K a = 4.9 x 10 -10

66. 16-66 CHEM 102, Fall 2012, LA TECH 1.0 - x ~ 1.00 since x is small x 2 x 2 K a = -----------; K a =4.9 x 10 -10 = x 2 K a = -----------; K a =4.9 x 10 -10 = x 2 1.0 1.0 x = 4.9 x 10 -10 = 2.21 x 10 -5 x = 4.9 x 10 -10 = 2.21 x 10 -5 Amount disso. 2.21 x 10 -5 ----------------- x 100 =- ------------- x 100 Ini. amount 1.00 Ini. amount 1.00 % Diss.=2.21 x 10 -5 x 100 = 0.00221 % % Diss.=2.21 x 10 -5 x 100 = 0.00221 % 1.00 M solution of HCN; K a = 4.9 x 10 -10

67. 16-67 CHEM 102, Fall 2012, LA TECH 10) Calculate the % dissociation of 2.00 M solutions of HCN (K a = 4.9 x 10 -10 ) a) Dissociation equilibria: b) ICE setup: c) Amount dissociated: d) % dissociation : :

68. 16-68 CHEM 102, Fall 2012, LA TECH 1 M HF, 2.7% dissociated Notice the conversion of % dissociation to a fraction (x): 2.7/100=0.027) x=0.027 Calculate the pH of a weak acid from % dissociation

69. 16-69 CHEM 102, Fall 2012, LA TECH HF(aq) + H 2 O(l) H 3 + O(aq) + F - (aq) [H + ][F - ] [H + ][F - ] K a = ----------- [HF] [HF] [HF] [H + ] [F - ] [HF] [H + ] [F - ] Ini. Con. 1.00 M 0.0 M0.00 M Chg. Con -x x x Eq.Con. 1.0-0.027 0.0270.027 pH = -log [H + ] pH = -log [H + ] pH = -log(0.027) pH = -log(0.027) pH = 1.57 pH = 1.57 Calculate the pH of a weak acid from % dissociation

70. 16-70 CHEM 102, Fall 2012, LA TECH 11) Calculate the K a of if 5.0 M HF, 2.7% dissociated: a) Dissociation equilibria: b) ICE setup: c) Amount dissociated: d) K a :

71. 16-71 CHEM 102, Fall 2012, LA TECH Weak acid Equilibria Example Determine the pH of a 0.10 M benzoic acid solution at 25 o C if K a = 6.28 x 10 -5 HBz (aq) + H 2 O (l) H 3 O + (aq) + Bz -(aq) The first step is to write the equilibrium expression K a = K a = [H 3 O + ][Bz - ] [HBz]

72. 16-72 CHEM 102, Fall 2012, LA TECH Weak acid Equilibria HBz H 3 O + Bz - HBz H 3 O + Bz - Initial conc., M 0.10 0.00 0.00 Change,  M -x x x Eq. Conc., M0.10 - x x x [H 3 O + ] = [Bz - ] = x We’ll assume that [Bz - ] is negligible compared to [HBz]. The contribution of H 3 O + from water is also negligible.

73. 16-73 CHEM 102, Fall 2012, LA TECH Weak Acid Equilibria Solve the equilibrium equation in terms of x K a = 6.28 x 10 -5 = x = (6.28 x 10 -5 )(0.10) H 3 O + = 0.0025 M H 3 O + = 0.0025 M pH= 2.60 x 2 0.10

74. 16-74 CHEM 102, Fall 2012, LA TECH pH from K a or K b 1.00 M solution of HCN; K a = 4.9 x 10 -10 First write the dissociation equilibrium equation: First write the dissociation equilibrium equation: HCN(aq) + H 2 O(l) H 3 + O(aq) + CN - (aq) [HCN] [H + ] [CN - ] [HCN] [H + ] [CN - ] Ini. Con. 1.00 M 0.0 M 0.00 M Chg. Con -x x x Eq. Con. 1.0 - x x x

75. 16-75 CHEM 102, Fall 2012, LA TECH [H 3 + O ][CN - ] x 2 [H 3 + O ][CN - ] x 2 K a = ---------------=---------------- K a = ---------------=---------------- [HCN] 1.0 - x [HCN] 1.0 - x 1.0 - x ~ 1.00 since x is small 1.0 - x ~ 1.00 since x is small x 2 x 2 K a = -----------; K a =4.9 x 10 -10 = x 2 K a = -----------; K a =4.9 x 10 -10 = x 2 1.0 1.0 x = 4.9 x 10 -10 = 2.21 x 10 -5 x = 4.9 x 10 -10 = 2.21 x 10 -5 pH = -log [H + ] pH = -log [H + ] pH = -log(2.21 x 10 -5 ) pH = -log(2.21 x 10 -5 ) pH = 4.65 pH = 4.65 Weak Acid Equilibria

76. 16-76 CHEM 102, Fall 2012, LA TECH The Conjugate Partners of Strong Acids and Bases The conjugate acid/base of a strong base/acid has no net effect on the pH of a solution The conjugate base of a weak acid hydrolyze in water and basic or pH of a solution > 7.00 E.g. Na + C 2 H 3 O 2 - sodium acetate The conjugate acid of a weak base hydrolyze in water and acidic or pH of a solution < 7.00 E.g NH 4 Cl

77. 16-77 CHEM 102, Fall 2012, LA TECH 12) Calculate the [H + ], [OH - ] and pH of 0.90 M HC 2 H 3 O 2 ; Ka= 1.8 x 10 -5. a) Dissociation equilibria: b) ICE setup: c) [H + ] and [OH - ]: d) pH:

78. 16-78 CHEM 102, Fall 2012, LA TECH 13) Calculate the [H + ], [OH - ] pOH and pH 5.0 M NH 3 ; K b = 1.8 x 10 -5 a) Dissociation equilibria: b) ICE setup: c) [H+] and [OH-]: d) pOH and pH:

79. 16-79 CHEM 102, Fall 2012, LA TECH 14) Calculate the pH of a 0.015 M solution of lactic acid. The K a for lactic acid is 1.4 x 10 -4.

80. 16-80 CHEM 102, Fall 2012, LA TECH 15) Calculate the pH of a 0.14 M solution of an acid with K a = 6.2 x 10 -8 (pH = 4.03)