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Chapter 4: Aqueous Reactions and Solution Stoichiometry.

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1 Chapter 4: Aqueous Reactions and Solution Stoichiometry

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

3 Dissolution of NaCl in Water

4 Electrolytes Substances that dissociate into ions when dissolved in water. A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so.

5 Electrolytes and Nonelectrolytes Soluble ionic compounds tend to be electrolytes.

6 Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases.

7 Electrolytes A strong electrolyte dissociates completely when dissolved in water. A weak electrolyte only dissociates partially when dissolved in water.

8 Electrolytes vs. Nonelectrolytes

9 Strong Electrolytes Are… Strong acids

10 Strong Electrolytes Are… Strong acids Strong bases

11 Strong Electrolytes Are… Strong acids Strong bases Soluble ionic salts

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

13 Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means to transpose AgNO 3 (aq) + KCl (aq) AgCl (s) + KNO 3 (aq)

14 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)

15 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)

16 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.

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

18 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)

19 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)

20 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)

21 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 didnt 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)

22 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.

23 Writing Net Ionic Equations

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

25 Introduction to Aqueous Acids

26 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 )

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

28 Introduction to Aqueous Bases

29 Bases The strong bases are the soluble salts of hydroxide ion: Alkali metals Calcium Strontium Barium

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

31 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)

32 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 )

33 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 )

34 Neutralization Reactions Observe the reaction between Milk of Magnesia, Mg(OH) 2, and HCl.

35 Dissolution of Mg(OH) 2 by an Acid

36 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)

37 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)

38 Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. A reduction occurs when an atom or ion gains electrons.

39 Oxidation-Reduction Reactions One cannot occur without the other.

40 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.

41 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.

42 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.

43 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.

44 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.

45 Displacement Reactions In displacement reactions, ions oxidize an element. The ions, then, are reduced.

46 Oxidation of Silver Ions

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

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

49 Activity Series

50 Molarity Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. Molarity is one way to measure the concentration of a solution. moles of solute volume of solution in liters Molarity (M) =

51 Mixing a Solution

52 2005 D. O'ReillyChapter 4 Central Science52 Making a Solution of Known Concentration

53 Sample Exercise 4.11 Calculating Molarity Calculate the molarity of a solution made by dissolving 23.4 g of sodium sulfate (Na 2 SO 4 ) in enough water to form 125 mL of solution. Solution Solve: The number of moles of Na 2 SO 4 is obtained by using its molar mass: Converting the volume of the solution to liters: Thus, the molarity is 1.32M

54 Sample Exercise 4.11 Calculating Molarity Calculate the molarity of a solution made by dissolving 5.00 g of glucose (C 6 H 12 O 6 ) in sufficient water to form exactly 100 mL of solution. Practice Exercise

55 Answer: M Sample Exercise 4.11 Calculating Molarity Calculate the molarity of a solution made by dissolving 5.00 g of glucose (C 6 H 12 O 6 ) in sufficient water to form exactly 100 mL of solution. Practice Exercise

56 Sample Exercise 4.12 Calculating Molar Concentrations of Ions What are the molar concentrations of each of the ions present in a M aqueous solution of calcium nitrate? Practice Exercise What is the molar concentration of K + ions in a M solution of potassium carbonate?

57 Sample Exercise 4.13 Using Molarity to Calculate Grams of Solute How many grams of Na 2 SO 4 are required to make L of M Na 2 SO 4 ?

58 Sample Exercise 4.13 Using Molarity to Calculate Grams of Solute How many grams of Na 2 SO 4 are required to make L of M Na 2 SO 4 ?

59 (a)How many grams of Na 2 SO 4 are there in 15 mL of 0.50 M Na 2 SO 4 ? (b) How many milliliters of 0.50 M Na 2 SO 4 solution are needed to provide mol of this salt? Practice Exercise

60 Answers: (a) 1.1 g, (b) 76 mL (a)How many grams of Na 2 SO 4 are there in 15 mL of 0.50 M Na 2 SO 4 ? (b) How many milliliters of 0.50 M Na 2 SO 4 solution are needed to provide mol of this salt? Practice Exercise

61 Dilution

62 2005 D. O'ReillyChapter 4 Central Science62 We recognize that the number of moles are the same in dilute and concentrated solutions. So: M dilute V dilute = moles = M concentrated V concentrated Molarity by Dilution

63 Sample Exercise 4.14 Preparing A solution by Dilution How many milliliters of 3.0 M H 2 SO 4 are needed to make 450 mL of 0.10 M H 2 SO 4 ?

64 Sample Exercise 4.14 Preparing A solution by Dilution How many milliliters of 3.0 M H 2 SO 4 are needed to make 450 mL of 0.10 M H 2 SO 4 ?

65 (a)What volume of 2.50 M lead(II) nitrate solution contains mol of Pb 2+ ? (b) How many milliliters of 5.0 M K 2 Cr 2 O 7 solution must be diluted to prepare 250 mL of 0.10 M solution? (c) If 10.0 mL of a 10.0 M stock solution of NaOH is diluted to 250 mL, what is the concentration of the resulting stock solution? Practice Exercises

66 Answers: (a) L = 20.0 mL, (b) 5.0 mL, (c) 0.40 M (a)What volume of 2.50 M lead(II) nitrate solution contains mol of Pb 2+ ? (b) How many milliliters of 5.0 M K 2 Cr 2 O 7 solution must be diluted to prepare 250 mL of 0.10 M solution? (c) If 10.0 mL of a 10.0 M stock solution of NaOH is diluted to 250 mL, what is the concentration of the resulting stock solution? Practice Exercises

67 Using Molarities in Stoichiometric Calculations

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

69 Titration

70 Acid Base Titration

71 2005 D. O'ReillyChapter 4 Central Science71 Titrations

72 2005 D. O'ReillyChapter 4 Central Science72 Suppose we know the molarity of a NaOH solution and we want to find the molarity of an HCl solution. We know: molarity of NaOH, volume of HCl. What do we want? Molarity of HCl. What do we do? Take a known volume of the HCl solution, measure the mL of NaOH required to react completely with the HCl. Titrations

73 2005 D. O'ReillyChapter 4 Central Science73 What do we get? Volume of NaOH. We know molarity of the NaOH, we can calculate moles of NaOH. Next step? We also know HCl + NaOH NaCl + H 2 O. Therefore, we know moles of HCl. Can we finish? Knowing mol(HCl) and volume of HCl (20.0 mL above), we can calculate the molarity. Titrations

74 Sample Exercise 4.15 Using Mass Relations In a Neutralization Reaction How many grams of Ca(OH) 2 are needed to neutralize 25.0 mL of M HNO 3 ?

75 Sample Exercise 4.15 Using Mass Relations In a Neutralization Reaction How many grams of Ca(OH) 2 are needed to neutralize 25.0 mL of M HNO 3 ?

76 (a)How many grams of NaOH are needed to neutralize 20.0 mL of M H 2 SO 4 solution? (b) How many liters of M HCl(aq) are needed to react completely with mol of Pb(NO 3 ) 2 (aq), forming a precipitate of PbCl 2 (s)? Answers: (a) g, (b) L Practice Exercise

77 Sample Exercise 4.16 Determining the Quality of Solute by Titration The quantity of Cl – in a municipal water supply is determined by titrating the sample with Ag +. The reaction taking place during the titration is The end point in this type of titration is marked by a change in color of a special type of indicator. (a) How many grams of chloride ion are in a sample of the water if 20.2 mL of M Ag + is needed to react with all the chloride in the sample? (b) If the sample has a mass of 10.0 g, what percent Cl – does it contain?

78

79 A sample of an iron ore is dissolved in acid, and the iron is converted to Fe 2+. The sample is then titrated with mL of M MnO 4 – solution. The oxidation-reduction reaction that occurs during titration is as follows: (a) How many moles of MnO 4 – were added to the solution? (b) How many moles of Fe 2+ were in the sample? (c) How many grams of iron were in the sample? (d) If the sample had a mass of g, what is the percentage of iron in the sample? Answers: (a) –3 mol MnO 4 – (b) –3 mol Fe 2+, (c) g, (d) 33.21% Practice Exercise


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