1. Aqueous Reactions Chapter 4 Aqueous Reactions and Solution Stoichiometry John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall, Inc. Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Ch4 Hmwk # 7, 8, 11, 15, 19, 23, 25, 27, 29, 31, 37, 39, 41, 45, 47, 51, 53, 59, 63, 67, 69, 75, 96

2. Aqueous Reactions Aqueous Reactions and Solution Stoichiometry What is aqueous? What is an aqueous reaction? What are some types of aqueous reactions? Why might these be important? Give examples. Aqueous reactions cannot take place without water. What do you already know about water that will help us understand aqueous reactions?

3. Aqueous Reactions Solutions: Homogeneous mixtures of two or more pure substances. The solvent is present in greatest abundance. All other substances are solutes.

4. Aqueous Reactions Dissociation When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them. This process is called dissociation.

5. Aqueous Reactions Electrolytes Substances that dissociate into ions when dissolved in water. Example: NaCl A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Example: C 12 H 22 O 11 Notice, solubility does not imply that something is or is not an electrolyte.

6. Aqueous Reactions Electrolytes and Nonelectrolytes Soluble ionic compounds tend to be electrolytes.

7. Aqueous Reactions Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases.

8. Aqueous Reactions SAMPLE EXERCISE 4.1 Relating Relative Numbers of Anions and Cations to Chemical Formulas The diagram below represents an aqueous solution of one of the following compounds: MgCl 2, KCl, or K 2 SO 4. Which solution does it best represent?

9. Aqueous Reactions Solution Analyze: We are asked to associate the charged spheres in the diagram with ions present in a solution of an ionic substance. Plan: We examine the ionic substances given in the problem to determine the relative numbers and charges of the ions that each contains. We then correlate these charged ionic species with the ones shown in the diagram. Solve: The diagram shows twice as many cations as anions, consistent with the formulation K 2 SO 4. Check: Notice that the total net charge in the diagram is zero, as it must be if it is to represent an ionic substance. SAMPLE EXERCISE 4.1 Relating Relative Numbers of Anions and Cations to Chemical Formulas The diagram below represents an aqueous solution of one of the following compounds: MgCl 2, KCl, or K 2 SO 4. Which solution does it best represent?

10. Aqueous Reactions Electrolytes A strong electrolyte dissociates completely when dissolved in water. A weak electrolyte only dissociates partially when dissolved in water.

11. Aqueous Reactions

12. Aqueous Reactions Strong Electrolytes Are… Strong acids

13. Aqueous Reactions Strong Electrolytes Are… Strong acids Strong bases

14. Aqueous Reactions Strong Electrolytes Are… Strong acids Strong bases Soluble ionic salts

15. Aqueous Reactions Precipitation Reactions When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

16. Aqueous Reactions Metathesis (Exchange) Reactions a.k.a. Double Replacement Rxns Metathesis comes from a Greek word that means “to transpose” AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

17. Aqueous Reactions Metathesis (Exchange) Reactions a.k.a. Double Replacement Rxns Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

18. Aqueous Reactions Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

19. Aqueous Reactions AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) How did we know that silver chloride was a solid??? We checked the solubility table.

20. Aqueous Reactions

21. Aqueous Reactions Predict the products of the following reactions BaCl 2 (aq) + K 2 SO 4 (aq) Fe 2 (SO 4 ) 3 (aq) + LiOH (aq) Mg(NO 3 ) 2 (aq) + CaS (aq) NaCl (aq) + KNO 3 (aq)

22. Aqueous Reactions Predict the products of the following reactions BaCl 2 (aq) + K 2 SO 4 (aq) → BaSO 4 (s) + 2KCl (aq) Fe 2 (SO 4 ) 3 (aq) + 6LiOH (aq) → 2Fe(OH) 3 (s) + 3Li 2 SO 4 (aq) Mg(NO 3 ) 2 (aq) + CaS (aq) → MgS (s) + Ca(NO 3 ) 2 (aq) NaCl (aq) + KNO 3 (aq) → NaNO 3 (aq) + KCl (aq) or NR

23. Aqueous Reactions Solution Chemistry It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

24. Aqueous Reactions Molecular Equation The molecular equation lists the reactants and products in their molecular form. AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

25. Aqueous Reactions Ionic Equation In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

26. Aqueous Reactions Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

27. Aqueous Reactions Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Ag + (aq) + Cl - (aq)  AgCl (s)

28. Aqueous Reactions Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

29. Aqueous Reactions Writing Net Ionic Equations 1.Write a balanced molecular equation. 2.Dissociate all strong electrolytes. 3.Cross out anything that remains unchanged from the left side to the right side of the equation. 4.Write the net ionic equation with the species that remain.

30. Aqueous Reactions Writing Net Ionic Equations Write the net ionic equation for each reaction Pb(NO 3 ) 2 (aq) + KI (aq) Li 2 S (aq) + BaBr 2 (aq) NH 4 Cl (aq) + Sr(OH) 2 (aq)

31. Aqueous Reactions Writing Net Ionic Equations Write the net ionic equation for each reaction Pb(NO 3 ) 2 (aq) + 2KI (aq) → PbI 2 (s) + 2K(NO 3 ) (aq) Pb 2+ + 2I - → PbI 2 (s) Li 2 S (aq) + BaBr 2 (aq) → LiBr (aq) + BaS (aq) 2Li + (aq) + S -2 (aq) + Ba 2+ (aq) + 2Br - (aq) → 2Li + (aq) + 2Br - (aq) + Ba 2+ (aq) + S -2 (aq) Notice ….all spectator ions. They all cancel and therefore, no reaction occurs. 2NH 4 Cl (aq) + Sr(OH) 2 (aq) → SrCl 2 (aq) + 2NH 4 OH (aq) 2NH 4 + (aq) + 2Cl - (aq) + Sr 2+ (aq) + 2OH - (aq) → Sr 2+ (aq) + 2Cl - (aq) + 2NH 4 + (aq) + 2OH - (aq) Notice ….all spectator ions. They all cancel and therefore, no reaction occurs.

32. Aqueous Reactions Acids: Substances that increase the concentration of H + when dissolved in water (Arrhenius). Proton donors (Brønsted–Lowry).

33. Aqueous Reactions Acids There are only seven strong acids: Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO 3 ) Sulfuric (H 2 SO 4 ) Chloric (HClO 3 ) Perchloric (HClO 4 ) These substances are consider strong acids because they do not just dissolve in water, they will completely dissociate.

34. Aqueous Reactions Bases: Substances that increase the concentration of OH − when dissolved in water (Arrhenius). Proton acceptors (Brønsted–Lowry).

35. Aqueous Reactions Bases The strong bases are the soluble salts of hydroxide ion: Alkali metals Calcium Strontium Barium

36. Aqueous Reactions Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H + ) to the base.

37. Aqueous Reactions Neutralization Reactions Generally, when solutions of an acid and a base are combined, the products are a salt and water. HCl (aq) + NaOH (aq)  NaCl (aq) + H 2 O (l)

38. Aqueous Reactions Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H 2 O (l) H + ( aq ) + Cl - ( aq ) + Na + ( aq ) + OH - ( aq )  Na + ( aq ) + Cl - ( aq ) + H 2 O ( l )

39. Aqueous Reactions Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H 2 O (l) H + ( aq ) + Cl - ( aq ) + Na + ( aq ) + OH - ( aq )  Na + ( aq ) + Cl - ( aq ) + H 2 O ( l ) H + ( aq ) + Cl - (aq) + Na + ( aq ) + OH - ( aq )  Na + ( aq ) + Cl - ( aq ) + H 2 O ( l ) H+ (aq) + OH- (aq) → H2O (l)

40. Aqueous Reactions Neutralization Reactions What does this have to do with you??? What happens when you have a stomach ache? What do you do about it? Why?

41. Aqueous Reactions Neutralization Reactions What does this have to do with you??? Many antacids contain magnesium hydroxide. It undergoes a neutralization reaction with stomach acid. What acid constitutes the largest quantity of stomach acid? Write a net ionic equation for the neutralization reaction between magnesium hydroxide and hydrochloric acid. Mg(OH) 2 (s) + H + (aq) → Mg 2+ (aq) + H 2 O (l)

42. Aqueous Reactions Neutralization Reactions Observe the reaction between Milk of Magnesia, Mg(OH) 2, and HCl. Can you overdoes on antacid?? What do you think would happen?

43. Aqueous Reactions Gas-Forming Reactions These metathesis reactions do not give the product expected. The expected product decomposes to give a gaseous product (CO 2 or SO 2 ). CaCO 3 (s) + HCl (aq)  CaCl 2 (aq) + CO 2 (g) + H 2 O (l) NaHCO 3 (aq) + HBr (aq)  NaBr (aq) + CO 2 (g) + H 2 O (l) SrSO 3 (s) + 2 HI (aq)  SrI 2 (aq) + SO 2 (g) + H 2 O (l)

44. Aqueous Reactions Gas-Forming Reactions This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! Just as in the previous examples, a gas is formed as a product of this reaction: Na 2 S (aq) + H 2 SO 4 (aq)  Na 2 SO 4 (aq) + H 2 S (g)

45. Aqueous Reactions A Carbonated Beverage is Another Example How is a beverage carbonated? What happens to the dissolved gas? H 2 O (l) + CO 2 (g) ↔ H 2 CO 3 (aq) H 2 O (l) + CO 2 (g) ↔ 2H + (aq) + CO 3 -2 (aq)

46. Aqueous Reactions Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. (It becomes more positive) A reduction occurs when an atom or ion gains electrons. (It becomes more negative)

47. Aqueous Reactions Oxidation-Reduction Reactions One cannot occur without the other.

48. Aqueous Reactions Oxidation Numbers To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

49. Aqueous Reactions Oxidation Numbers Elements in their elemental form have an oxidation number of 0. The oxidation number of a monatomic ion is the same as its charge.

50. Aqueous Reactions Oxidation Numbers Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions.  Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1.  Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.

51. Aqueous Reactions Oxidation Numbers Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions.  Fluorine always has an oxidation number of −1.  The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.

52. Aqueous Reactions Oxidation Numbers The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.

53. Aqueous Reactions Oxidation Numbers Find the oxidation numbers for each of the following species. CO 3 2- H 2 S S 8 SCl 2 SO 4 2- Cr 2 O 7 2-

54. Aqueous Reactions CO 3 2- C = +4 and O = -2 H 2 S H = +1 and S = -2 S 8 S = 0 SCl 2 S = +2 and Cl = -1 SO 4 2- S = +6 and O = -2 Cr 2 O 7 2- Cr = +6 and O = -2

55. Aqueous Reactions Examine each Reaction below and Identify the oxidized and reduced reagents 2K (s) + 2H 2 O (l) → 2KOH (aq) + H 2 (g) Fe 2 O 3 (s) + 2Al (s) → 2Fe (l) + Al 2 O 3 (s) Si (s) + 2Cl 2 (g) → SiCl 4 (l)

56. Aqueous Reactions Examine each Reaction below and Identify the oxidized and reduced agents 2K (s) + 2H 2 O (l) → 2KOH (aq) + H 2 (g) K (s) → K + (aq), it was oxidized; Hydrogen was reduced in water to form hydrogen gas. Fe 2 O 3 (s) + 2Al (s) → 2Fe (l) + Al 2 O 3 (s) Aluminum metal is oxidized; iron is reduced Si (s) + 2Cl 2 (g) → SiCl 4 (l) Si is oxidized while Cl is reduced

57. Aqueous Reactions Which three of the following reactions are Red-Ox? What is oxidized and what is reduced? Zn (s) + 2NO 3 - (aq) + 4H + (aq) → Zn +2 (aq) + 2NO 2 (g) + 2H 2 O (l) Zn(OH) 2 (s) + H 2 SO 4 (aq) → ZnSO 4 (aq) + 2H 2 O (l) Ca (s) + 2H 2 O (l) → Ca(OH) 2 (s) + H 2 (g) 4Fe(OH) 2 (s) + 2H 2 O (l) + O 2 (g) → 4Fe(OH) 3 (aq)

58. Aqueous Reactions Which three of the following reactions are Red-Ox? What is oxidized and what is reduced? Zn (s) + 2NO 3 - (aq) + 4H + (aq) → Zn +2 (aq) + 2NO 2 (g) + 2H 2 O (l) Red-ox: zinc is oxidized, nitrogen is reduced Zn(OH) 2 (s) + H 2 SO 4 (aq) → ZnSO 4 (aq) + 2H 2 O (l) Not Red-ox Ca (s) + 2H 2 O (l) → Ca(OH) 2 (s) + H 2 (g) Red-ox: calcium is oxidized, hydrogen is reduced 4Fe(OH) 2 (s) + 2H 2 O (l) + O 2 (g) → 4Fe(OH) 3 (aq) Red-ox: Iron is oxdized, oxygen is reduced

59. Aqueous Reactions Displacement Reactions a.k.a. Single Replacement Reactions In displacement reactions, ions oxidize an element. The ions, then, are reduced.

60. Aqueous Reactions Displacement Reactions In this reaction, silver ions oxidize copper metal. Cu (s) + 2 Ag + (aq)  Cu 2+ (aq) + 2 Ag (s)

61. Aqueous Reactions Displacement Reactions The reverse reaction, however, does not occur. Cu 2+ (aq) + 2 Ag (s)  Cu (s) + 2 Ag + (aq) x

62. Aqueous Reactions Activity Series

63. Aqueous Reactions Use the Activity Series to predict the products of the following reactions. If the reactions does take place, write the net ionic equation for the reaction. Lastly, identify the species being oxidized and reduced. Al (s) + HBr (aq) Fe (s) + KI (aq) Zn (s) + Ni(NO 3 ) 2 (aq)

64. Aqueous Reactions 2Al (s) + 6HBr (aq) → 2AlBr 3 (aq) + 3H 2 (g) 2Al (s) + 6H + (aq) → 2Al 3+ (aq) + 3H 2(g) Aluminum is oxidized and Hydrogen is reduced. Fe (s) + KI (aq) No Reaction Zn (s) + Ni(NO 3 ) 2 (aq) → Zn(NO 3 ) 2 (aq) + Ni (s) Zn (s) + Ni 2+ (aq) → Zn 2+ (aq) + Ni (s) Zinc is oxidized and Nickel is reduced.

65. Aqueous Reactions Molarity Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. (For example, it would be much more dangerous to spill a high concentration of hydrochloric acid on your hand than a low concentration) Molarity is one way to measure the concentration of a solution. moles of solute volume of solution in liters Molarity (M) =

66. Aqueous Reactions Mixing a Solution How many grams of silver nitrate are required to make 100 mL of a 0.5 M solution? How many grams of NaOH are required to make 50 mL of an 8.2 M solution?

67. Aqueous Reactions Mixing a Solution How many grams of silver nitrate are required to make 100 mL of a 0.5 M solution? 9 grams How many grams of NaOH are required to make 50.0 mL of an 8.2 M solution? 16 grams

68. Aqueous Reactions Dilution M 1 V 1 = M 2 V 2

69. Aqueous Reactions Dilution You have 1L of a 1.00 M stock solution of hydrochloric acid. You need to make 500 mL of a 0.20M solution. How much of the stock solution is needed? You need to make 100 mL of 0.50 M nitric acid. The storage bottle reads 6.0M. How much of the 6.0M nitric acid will you need?

70. Aqueous Reactions Dilution You have 1L of a 1.00 M stock solution of hydrochloric acid. You need to make 500 mL of a 0.20M solution. How much of the stock solution is needed? 100. mL or 0.100 L You need to make 100 mL of 0.50 M nitric acid. The storage bottle reads 6.0M. How much of the 6.0M nitric acid will you need? 8.3 mL

71. Aqueous Reactions Using Molarities in Stoichiometric Calculations How many grams of Ca(OH) 2 are needed to neutralize 20.0 mL of a 0.150 M H 2 SO 4 solution? 0.222 g (see board for work)

72. Aqueous Reactions Using Molarities in Stoichiometric Calculations How many liters of 0.500 M HCl are needed to react completely with 0.100 mol Pb(NO 3 ) 2, forming the precipitate PbCl 2 ? 0.400L (see board for work)

73. Aqueous Reactions Titration The analytical technique in which one can calculate the concentration of a solute in a solution.

74. Aqueous Reactions Titration

75. Aqueous Reactions Titration Example A 1.034-g sample of impure oxalic acid (H 2 C 2 O 4 ) is dissolved in water and an acid- base indicator is added. The sample required 34.47 mL of 0.485 M NaOH to reach the equivalence point. What is the mass of oxalic acid and what is its mass percent in the sample? 0.752 g H 2 C 2 O 4 and 72.7% H 2 C 2 O 4 (see board for work)

76. Aqueous Reactions Titration A 0.263-g sample of sodium carbonate requires 28.35 mL of aqueous HCl for titration to the equivalence point. What is the molarity of the HCl? 0.175 M HCl

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