1. Lecture Notes Alan D. Earhart Southeast Community College Lincoln, NE Chapter 4 Reactions in Aqueous Solution John E. McMurry Robert C. Fay CHEMISTRY Fifth Edition Copyright © 2008 Pearson Prentice Hall, Inc.

2. Chapter 4/2 Some Ways That Chemical Reactions Occur Acid-Base Neutralization Reactions: Processes in which an acid reacts with a base to yield water plus a salt. H 2 O(l) + NaCl(aq)HCl(aq) + NaOH(aq) Precipitation Reactions: Processes in which soluble reactants yield an insoluble solid product that falls out of solution. 2KNO 3 (aq) + Pb I 2 (s)Pb(NO 3 ) 2 (aq) + 2K I (aq)

3. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/3 Some Ways That Chemical Reactions Occur Oxidation-Reduction (Redox) Reactions: Processes in which one or more electrons are transferred between reaction partners (atoms, molecules, or ions). MgCl 2 (aq) + H 2 (g)Mg(s) + 2HCl(aq)

4. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/4 Electrolytes in Aqueous Solution Electrolytes: Substances which dissolve in water to produce conducting solutions of ions. H2OH2O Na 1+ (aq) + Cl 1- (aq)NaCl(s) A solution of NaCl conducts electricity because of the movement of charged particles (ions).

5. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/5 Electrolytes in Aqueous Solution Nonelectrolytes: Substances which do not produce ions in aqueous solutions. H2OH2O C 12 H 22 O 11 (aq)C 12 H 22 O 11 (s) A solution of C 12 H 22 O 11 does not conduct electricity because it contains no charged particles (ions).

6. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/6 Electrolytes in Aqueous Solution Weak Electrolytes: Compounds that dissociate to a small extent into ions when dissolved in water. H 1+ (aq) + CH 3 CO 2 1- (aq)CH 3 CO 2 H(aq) Strong Electrolytes: Compounds that dissociate to a large extent into ions when dissolved in water. K 1+ (aq) + Cl 1- (aq)KCl(aq)

7. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/7 Electrolytes in Aqueous Solution Strong Acids: hydrochloric acid, hydrobromic acid, hydroiodic acid, perchloric acid, nitric acid, sulfuric acid.

8. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/8 Electrolytes in Aqueous Solution Ionic Compounds

9. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/9 Electrolytes in Aqueous Solution Weak acids

10. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/10 Electrolytes in Aqueous Solution Molecular Compounds (other than any strong or weak electrolytes)

11. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/11 Aqueous Reactions and Net Ionic Equations 2KNO 3 (aq) + Pb I 2 (s)Pb(NO 3 ) 2 (aq) + 2K I (aq) Molecular Equation: All substances in the chemical equation are written using their complete formulas as if they were molecules. strong electrolytesprecipitate

12. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/12 Aqueous Reactions and Net Ionic Equations Ionic Equation: All of the strong electrolytes are written as ions. 2K 1+ (aq) + 2NO 3 1- (aq) + Pb I 2 (s) Pb 2+ (aq) + 2NO 3 1- (aq) + 2K 1+ (aq) + 2 I 1- (aq) 2KNO 3 (aq) Pb(NO 3 ) 2 (aq) 2K I (aq)

13. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/13 Aqueous Reactions and Net Ionic Equations Spectator Ions: Ions that undergo no change during the reaction and appear on both sides of the reaction arrow. 2K 1+ (aq) + 2NO 3 1- (aq) + Pb I 2 (s) Pb 2+ (aq) + 2NO 3 1- (aq) + 2K 1+ (aq) + 2 I 1- (aq)

14. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/14 Aqueous Reactions and Net Ionic Equations Pb I 2 (s)Pb 2+ (aq) + 2 I 1- (aq) Net Ionic Equation: Only the ions undergoing change are shown.

15. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/15 Precipitation Reactions and Solubility Guidelines 1.A compound is probably soluble if it contains one of the following cations: Group 1A cation: Li 1+, Na 1+, K 1+, Cs 1+ Ammonium ion: NH 4 1+ 2.A compound is probably soluble if it contains one of the following anions: Halide: Cl 1-, Br 1-, I 1- except Ag 1+, Hg 2 2+, and Pb 2+ compounds Nitrate (NO 3 1- ), perchlorate (ClO 4 1- ), acetate (CH 3 CO 2 1- ), sulfate (SO 4 2- ) except Ba 2+, Hg 2 2+, and Pb 2+ sulfates

16. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/16 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO 3 and Na 2 CO 3 are mixed.

17. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/17 Precipitation Reactions and Solubility Guidelines Write the chemical formulas of the products (use proper ionic rules). Ag 2 CO 3 AgNO 3 (aq)+Na 2 CO 3 (aq)+NaNO 3 CB + ADAB + CD double replacement reaction Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO 3 and Na 2 CO 3 are mixed. 1.

18. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/18 Precipitation Reactions and Solubility Guidelines Molecular Equation: Balance the equation and predict the solubility of each possible product. Ag 2 CO 3 (s) + 2NaNO 3 (aq)2AgNO 3 (aq) + Na 2 CO 3 (aq) Contains a group 1A cation. Neither the cation nor the anion is in the solubility list. Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO 3 and Na 2 CO 3 are mixed. 2.

19. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/19 Precipitation Reactions and Solubility Guidelines Ionic Equation: Dissociate the soluble ionic compounds. Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO 3 and Na 2 CO 3 are mixed. 3. Ag 2 CO 3 (s) + 2Na 1+ (aq) + 2NO 3 1- (aq) 2Ag 1+ (aq) + 2NO 3 1- (aq) + 2Na 1+ (aq) + CO 3 2- (aq) 2NaNO 3 (aq) 2AgNO 3 (aq)Na 2 CO 3 (aq)

20. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/20 Precipitation Reactions and Solubility Guidelines Write the molecular, ionic, and net ionic equations for the reaction that occurs when aqueous solutions of AgNO 3 and Na 2 CO 3 are mixed. Net Ionic Equation: Eliminate the spectator ions from the ionic equation. Ag 2 CO 3 (s)2Ag 1+ (aq) + CO 3 2- (aq) 4. Ag 2 CO 3 (s) + 2Na 1+ (aq) + 2NO 3 1- (aq) 2Ag 1+ (aq) + 2NO 3 1- (aq) + 2Na 1+ (aq) + CO 3 2- (aq)

21. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/21 Acids, Bases, and Neutralization Reactions H 3 O 1+ (aq) + Cl 1- (aq)HCl(aq) + H 2 O(aq) Acid (Arrhenius): A substance that dissociates in water to produce hydrogen ions, H 1+ : H 1+ (aq) + A 1- (aq)HA(aq) In water, acids produce hydronium ions, H 3 O 1+ : H 1+ (aq) + Cl 1- (aq)HCl(aq)

22. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/22 Acids, Bases, and Neutralization Reactions Ammonia, commonly called “ammonium hydroxide” is a base: M 1+ (aq) + OH 1- (aq)MOH(aq) Na 1+ (aq) + OH 1- (aq)NaOH(aq) NH 4 1+ (aq) + OH 1- (aq)NH 3 (aq) + H 2 O(aq) Base (Arrhenius): A substance that dissociates in water to produce hydroxide ions, OH 1- :

23. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/23 Acids, Bases, and Neutralization Reactions Weak acids and weak bases are weak electrolytes. Strong acids and strong bases are strong electrolytes.

24. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/24 Acids, Bases, and Neutralization Reactions MA + H 2 OHA + MOH These acid-base neutralization reactions are double- replacement reactions just like the precipitation reactions: or MA + HOHHA + MOH WaterAcidBaseSalt

25. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/25 Write the chemical formulas of the products (use proper ionic rules for the salt). 1. Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH) 2. H2OH2OHBr(aq)+Ba(OH) 2 (aq)+BaBr 2 SaltAcidBaseWater

26. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/26 Molecular Equation: Balance the equation and predict the solubility of the salt in the products. 2. Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH) 2. 2H 2 O(l) + BaBr 2 (aq)2HBr(aq) + Ba(OH) 2 (aq) Use the solubility rules.

27. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/27 Ionic Equation: Dissociate a strong acid and the soluble ionic compounds. 3. Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH) 2. 2H 2 O(l) + Ba 2+ (aq) + 2Br 1- (aq) 2H 1+ (aq) + 2Br 1- (aq) + Ba 2+ (aq) + 2OH 1- (aq) BaBr 2 (aq) 2HBr(aq)Ba(OH) 2 (aq)

28. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/28 2H 2 O(l) + Ba 2+ (aq) + 2Br 1- (aq) Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH) 2. Net Ionic Equation: Eliminate the spectator ions from the ionic equation. 4. 2H 1+ (aq) + 2Br 1- (aq) + Ba 2+ (aq) + 2OH 1- (aq) or H 2 O(l)H 1+ (aq) + OH 1- (aq) 2H 2 O(l)2H 1+ (aq) + 2OH 1- (aq)

29. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/29 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. Write the chemical formulas of the products (use proper ionic rules for the salt). 1. SaltAcidBaseWater H2OH2OHF(aq) + NaOH(aq)+NaF

30. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/30 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. Molecular Equation: Balance the equation and predict the solubility of the salt in the products. 2. H 2 O(l) + NaF(aq)HF(aq) + NaOH(aq) Use the solubility rules.

31. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/31 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. Ionic Equation: Dissociate a strong acid and the soluble ionic compounds. 3. H 2 O(l) + Na 1+ (aq) + F 1- (aq)HF(aq) + Na 1+ (aq) + OH 1- (aq) NaF(aq) NaOH(aq)

32. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/32 Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF. Net Ionic Equation: Eliminate the spectator ions from the ionic equation. 4. H 2 O(l) + Na 1+ (aq) + F 1- (aq)HF(aq) + Na 1+ (aq) + OH 1- (aq) H 2 O(l) + F 1- (aq)HF(aq) + OH 1- (aq)

33. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/33 Oxidation-Reduction (Redox) Reactions 2Fe 2 O 3 (s)4Fe(s) + 3O 2 (g) Rusting of iron: an oxidation of Fe 4Fe(s) + 3CO 2 (g)2Fe 2 O 3 (s) + 3C(s) Manufacture of iron: a reduction of Fe

34. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/34 Oxidation-Reduction (Redox) Reactions 1.An atom in its elemental state has an oxidation number of 0. Rules for Assigning Oxidation Numbers Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron- poor. NaH2H2 Br 2 SNe Oxidation number 0

35. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/35 Oxidation-Reduction (Redox) Reactions 2.A monatomic ion has an oxidation number identical to its charge. Na 1+ +1 Ca 2+ +2 Al 3+ +3 Cl 1- O 2- -2

36. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/36 Oxidation-Reduction (Redox) Reactions b)Oxygen usually has an oxidation number of -2. HO 1- -2+1 HHCa +2 3.An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion. a)Hydrogen can be either +1 or -1. OHO +1 HHO +1 -2 H +1

37. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/37 Oxidation-Reduction (Redox) Reactions HCl +1 c)Halogens usually have an oxidation number of -1.3. Cl O +1 -2

38. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/38 Oxidation-Reduction (Redox) Reactions Cr 2 O 7 2- -2+1x 4.The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion. x = +6 2(x) + 7(-2) = -2 (net charge) H2SO3H2SO3 x-2 x = +4 2(+1) + x + 3(-2) = 0 (net charge)

39. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/39 Identifying Redox Reactions Reducing Agent Causes oxidation Gains one or more electrons Undergoes reduction Oxidation number of atom decreases Oxidizing Agent Causes reduction Loses one or more electrons Undergoes oxidation Oxidation number of atom increases

40. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/40 Identifying Redox Reactions Reducing Agent Oxidizing Agent 2Fe 2 3O2(g)3O2(g)+4Fe(s)O3O3 (s)(s) +30 oxidation reduction -20

41. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/41 3 Identifying Redox Reactions Reducing Agent Oxidizing Agent 4Fe(s)C(s) +2Fe 2 O 3 +3(g)(g)(s)(s)O2O2 C reduction 0+3 +40 oxidation

42. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/42 The Activity Series of the Elements 2Ag 1+ (aq) + Cu(s)2Ag(s) + Cu 2+ (g) Cu 2+ (aq) + 2Ag(s)Cu(s) + 2Ag 1+ (g) Which one of these reactions will occur?

43. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/43 The Activity Series of the Elements The elements that are higher up in the table are more likely to be oxidized. Thus, any element higher in the activity series will reduce the ion of any element lower in the activity series.

44. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/44 The Activity Series of the Elements 2Ag 1+ (aq) + Cu(s)2Ag(s) + Cu 2+ (g) Cu 2+ (aq) + 2Ag(s)Cu(s) + 2Ag 1+ (g) Which one of these reactions will occur?

45. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/45 The Activity Series of the Elements 2Ag 1+ (aq) + Cu(s)2Ag(s) + Cu 2+ (g) Cu 2+ (aq) + 2Ag(s)Cu(s) + 2Ag 1+ (g) Which one of these reactions will occur?

46. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/46 Balancing Redox Reactions: The Oxidation-Number Method

47. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/47 Balancing Redox Reactions: The Oxidation-Number Method Balance the following net ionic equation in acidic solution: Cr 3+ (aq) + I O 3 1- (aq) I 1- (aq) + Cr 2 O 7 2- (aq) Balance the equation for all atoms other than H and O. 2Cr 3+ (aq) + I O 3 1- (aq) I 1- (aq) + Cr 2 O 7 2- (aq)

48. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/48 Balancing Redox Reactions: The Oxidation-Number Method Assign oxidation numbers to all atoms. 2Cr 3+ (aq) + I O 3 1- (aq) I 1- (aq) + Cr 2 O 7 2- (aq) -2-2+6+3+5

49. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/49 Balancing Redox Reactions: The Oxidation-Number Method Decide which atoms have changed oxidation number, and by how much. 2Cr 3+ I 1- (aq) +O 7 2- (aq)(aq) +O 3 1- (aq) I +5+3 reduction Cr 2 +6 oxidation

50. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/50 Balancing Redox Reactions: The Oxidation-Number Method Make the total increase in oxidation number equal to the total decrease in oxidation number. 2Cr 3+ I 1- (aq) +O 7 2- (aq)(aq) +O 3 1- (aq) I +5+3 Cr 2 +6 Net decrease = -6Net increase = +6

51. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/51 Balancing Redox Reactions: The Oxidation-Number Method Balance the equation for by adding H 2 O, and then balance for H by adding H 1+. 2Cr 3+ (aq) + I O 3 1- (aq) + 4H 2 O(l) 8H 1+ (aq) + I 1- (aq) + Cr 2 O 7 2- (aq)

52. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/52 Balancing Redox Reactions: The Half-Reaction Method

53. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/53 Balancing Redox Reactions: The Half-Reaction Method Balance the following net ionic equation in acidic solution: Cr 3+ (aq) + I O 3 1- (aq) I 1- (aq) + Cr 2 O 7 2- (aq)

54. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/54 Balancing Redox Reactions: The Half-Reaction Method Write the two unbalanced half-reactions. I O 3 1- (aq) I 1- (aq) Cr 3+ (aq)Cr 2 O 7 2- (aq)

55. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/55 Balancing Redox Reactions: The Half-Reaction Method Balance both half-reactions for all atoms except O and H. I O 3 1- (aq) I 1- (aq) 2Cr 3+ (aq)Cr 2 O 7 2- (aq)

56. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/56 Balancing Redox Reactions: The Half-Reaction Method Balance each half-reaction for O by adding H 2 O, and then balance for H by adding H 1+. I O 3 1- (aq) + 6H 1+ (aq) 3H 2 O(l) + I 1- (aq) 2Cr 3+ (aq) + 7H 2 O(l)14H 1+ (aq) + Cr 2 O 7 2- (aq)

57. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/57 Balancing Redox Reactions: The Half-Reaction Method Balance each half-reaction for charge by adding electrons to the side with greater positive charge. I O 3 1- (aq) + 6H 1+ (aq) 6e - + 3H 2 O(l) + I 1- (aq) 2Cr 3+ (aq) + 7H 2 O(l) + 6e - 14H 1+ (aq) + Cr 2 O 7 2- (aq) oxidation: reduction:

58. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/58 Balancing Redox Reactions: The Half-Reaction Method Multiply each half-reaction by a factor to make the electron count the same in both half-reactions. I O 3 1- (aq) + 6H 1+ (aq) 6e - + 3H 2 O(l) + I 1- (aq) 2Cr 3+ (aq) + 7H 2 O(l) + 6e - 14H 1+ (aq) + Cr 2 O 7 2- (aq) oxidation: reduction:

59. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/59 Balancing Redox Reactions: The Half-Reaction Method Add the two balanced half-reactions together and cancel species that appear on both sides of the equation. I O 3 1- (aq) + 6H 1+ (aq) 6e - + 3H 2 O(l) + I 1- (aq) 2Cr 3+ (aq) + 7H 2 O(l) + 6e - 14H 1+ (aq) + Cr 2 O 7 2- (aq) oxidation: reduction: I O 3 1- (aq) + 2Cr 3+ (aq) + 4H 2 O(l) 8H 1+ (aq) + I 1- (aq) + Cr 2 O 7 2- (aq)

60. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/60 MnO 2 (s) + BrO 3 1- (aq)MnO 4 1- (aq) + Br 1- (aq) Balancing Redox Reactions: The Half-Reaction Method Balance the following net ionic equation in basic solution:

61. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/61 Balancing Redox Reactions: The Half-Reaction Method Write the two unbalanced half-reactions. Br O 3 1- (aq) Br 1- (aq) MnO 2 (s)MnO 4 1- (aq)

62. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/62 Balancing Redox Reactions: The Half-Reaction Method Balance both half-reactions for all atoms except O and H. Br O 3 1- (aq) Br 1- (aq) MnO 2 (s)MnO 4 1- (aq)

63. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/63 Balancing Redox Reactions: The Half-Reaction Method Br O 3 1- (aq) + 6H 1+ (aq)3H 2 O(l) + Br 1- (aq) MnO 2 (s) + 2H 2 O(l)4H 1+ (aq) + MnO 4 1- (aq) Balance each half-reaction for O by adding H 2 O, and then balance for H by adding H 1+.

64. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/64 Balancing Redox Reactions: The Half-Reaction Method Br O 3 1- (aq) + 6H 1+ (aq) + 6e - 3H 2 O(l) + Br 1- (aq) MnO 2 (s) + 2H 2 O(l)3e - + 4H 1+ (aq) + MnO 4 1- (aq) Balance each half-reaction for charge by adding electrons to the side with greater positive charge.

65. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/65 Balancing Redox Reactions: The Half-Reaction Method Multiply each half-reaction by a factor to make the electron count the same in both half-reactions. 2 Br O 3 1- (aq) + 6H 1+ (aq) + 6e - 3H 2 O(l) + Br 1- (aq) MnO 2 (s) + 2H 2 O(l)3e - + 4H 1+ (aq) + MnO 4 1- (aq)

66. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/66 2MnO 2 (s) + H 2 O(l) + BrO 3 1- (aq) 2H 1+ (aq) + 2MnO 4 1- (aq) + Br 1- (aq) Balancing Redox Reactions: The Half-Reaction Method Br O 3 1- (aq) + 6H 1+ (aq) + 6e - 3H 2 O(l) + Br 1- (aq) 2MnO 2 (s) + 4H 2 O(l)6e - + 8H 1+ (aq) + 2MnO 4 1- (aq) Add the two balanced half-reactions together and cancel species that appear on both sides of the equation.

67. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/67 Balancing Redox Reactions: The Half-Reaction Method Since the reaction occurs in a basic solution, “neutralize” the excess H 1+ by adding OH 1- and cancel any water (if possible) 2MnO 2 (s) + H 2 O(l) + BrO 3 1- (aq) + 2OH 1- (aq) 2OH 1- (aq) + 2H 1+ (aq) + 2MnO 4 1- (aq) + Br 1- (aq) 2H 2 O 2MnO 2 (s) + BrO 3 1- (aq) + 2OH 1- (aq) H 2 O(l) + 2MnO 4 1- (aq) + Br 1- (aq)

68. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/68 Redox Titrations If the unknown concentration is the potassium permanganate solution, MnO 4 1-, it can be slowly added to a known amount of oxalic acid, H 2 C 2 O 4, until a faint purple color persists. Titration: A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known. 5H 2 C 2 O 4 (aq) + 2MnO 4 1- (aq) + 6H 1+ (aq) 10CO 2 (g) + 2Mn 2+ (aq) + 8H 2 O(l)

69. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/69 Redox Titrations 5H 2 C 2 O 4 (aq) + 2MnO 4 1- (aq) + 6H 1+ (aq) 10CO 2 (g) + 2Mn 2+ (aq) + 8H 2 O(l) A solution is prepared with 0.2585 g of oxalic acid, H 2 C 2 O 4. 22.35 mL of an unknown solution of potassium permanganate are needed to titrate the solution. What is the concentration of the potassium permanganate solution? Moles of H 2 C 2 O 4 Mass of H 2 C 2 O 4 Moles of KMnO 4 Molarity of KMnO 4 Mole RatioMolarity of KMnO 4 Molar Mass of H 2 C 2 O 4

70. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/70 Redox Titrations 5H 2 C 2 O 4 (aq) + 2MnO 4 1- (aq) + 6H 1+ (aq) 10CO 2 (g) + 2Mn 2+ (aq) + 8H 2 O(l) Moles of H 2 C 2 O 4 available: 90.04 g 1 mol = 0.002871 mol H 2 C 2 O 4 0.2585 g H 2 C 2 O 4 Moles of KMnO 4 reacted: 5 mol H 2 C 2 O 4 2 mol KMnO 4 = 0.001148 mol KMnO 4 0.002871 mol H 2 C 2 O 4 x x

71. Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 4/71 1 L 1000 mL Redox Titrations 5H 2 C 2 O 4 (aq) + 2MnO 4 1- (aq) + 6H 1+ (aq) 10CO 2 (g) + 2Mn 2+ (aq) + 8H 2 O(l) Concentration of KMnO 4 solution: = 0.05136 M KMnO 4 22.35 mL x 0.001148 mol KMnO 4