The Water Molecule. Draw the Lewis structure. H O H. Covalent bonding. Bent shape

Transcription

1 Water & Solutions 1

2 The Water Molecule Draw the Lewis structure. H O H Covalent bonding. Bent shape 2

3 Water What determines whether a molecule is polar? Is water a polar molecule? d- d+ d+ 1. Oxygen is more electronegative than hydrogen. 2. Molecule is not symmetrical. 3

4 Water: What types of intermolecular forces? Dispersion force Dipole force Hydrogen bond force Water has all three!

5 Hydrogen Bond 5

6 Water: H-Bonding Explains the bulk properties of water: High surface tension Low vapor pressure High specific heat High boiling point High heat of vaporization 6

7 Curved Meniscus Why can you over-fill a glass of water? 7

8 Surface Tension Water molecules are attracted to each other, but not to air. Result: inward pull on the surface. 8

9 Surface Tension The inward pull (force) that minimizes surface area. It causes drops of water to have a spherical shape. 9

10 Surface Tension Since water has strong intermolecular forces, it has high surface tension. demo 10

11 Vapor Pressure: What is it? Due to H-bonding, water molecules have difficulty escaping into the gas phase. Thus water has low vapor pressure. Regents Table H 11

12 Table H: Vapor Pressure VP depends only on: substance temperature 12

13 Specific Heat H H O O H H What is it? How does it depend on IM forces? To raise temperature, molecules must move faster (raise KE), so IM forces must be overcome. Why do coastal regions have milder weather? 13

14 Specific Heat Specific heat of water is: 4.18 J/g o C which is higher than most other substances. Strong intermolecular forces (H-Bonds) 14

15 H vap & Boiling Point Compound CH 4 H 2 O Molar Mass (g/mol) IM Forces dispersion all three BP ( o C) H vap (J/g)

16 Ice For most substances, solids are more dense than liquids. But ice is weird: it floats!! 16

17 Density (g/ml) Water: Density Max. at 4 o C solid liquid Temperature ( o C) 17

18 Ice An open structure 18

19 Electrolyte A substance that conducts electricity when molten or dissolved in water. (vs. nonelectrolyte) What is required for a substance to conduct electricity? 1. Charged species (electron or ion) 2. Mobility (free to move) 19

20 Strong Electrolyte A substance that completely ionizes in water. NaCl(s) NaCl(s) H 2 O H 2 O 100% ionized NaCl(aq) Na + (aq) + Cl - (aq) 20

21 Strong Electrolyte NaCl(s) H 2 O Na + (aq) + Cl - (aq) Hydrated ions 21

22 Weak Electrolyte A substance that is not completely ionized in water NH 3 + H 2 O NH 4 + (aq) + OH - (aq) <<100% ionized 22

23 Electrolytes Strong: Soluble ionic compounds, strong acids, & strong bases (HCl, HNO 3, H 2 SO 4, NaOH, CaBr 2 ) Weak: All weak acids and weak bases (NH 3, acetic acid) Non: molecular compounds (methane, sucrose) 23

24 Water of Hydration Some crystals lock water molecules into the crystal in exact proportions. CuSO 4 5H 2 O Copper(II)sulfate pentahydrate The water can be released by mild heating. 24

25 CuSO 4. 5H2 O Above ~100 0 C, the water of hydration is released.. CuSO 4 5H2 O CuSO H 2 O hydrated anhydrous salt salt 25

26 Hydrates (try naming) MgSO 4. 7H2 O epsom salt CaCl 2. 2H2 O Na 2 B 4 O 7. 10H2 O borax 26

27 Hydrates What is the mass percent of water in Na 2 CO. 3 10H2 O? There are 10 moles of water for every one mole of Na 2 CO. 3 10H2 O. 27

28 Na 2 CO 3. 10H2 O % H 2 O = Mass water Mass hydrate X 100 = 10 mol x 18.0 g mol X g = 62.9% mol include the water! 28

29 Try It!!! What is the mass percent of water in CaCl 2. 2H2 O? 29

30 Terms Hygroscopic: salts that remove water from air. CaCl 2 moist air CaCl 2. 2H2 O Used as drying agents or desiccants. 30

31 Solutions Many chemical reactions take place among particles dissolved in liquid, usually water. 31

32 Aqueous Solutions Solvent: dissolving medium (water) Solute: the dissolved substance e.g. Salt water Water: solvent NaCl: solute Homogeneous mixture 32

33 Types of Solutions Solvent Solute Example Gas Gas Air Liquid Gas Soda Liquid Liquid Ethanol + H 2 O Liquid Solid Salt water Solid Solid Brass 33

34 What Determines Solubility? Molecular attractions (IM forces) between solvent & solute. Water (polar) dissolves polar and ionic substances. Gasoline (nonpolar) dissolves nonpolar substances. 34

35 Aqueous Solutions What dissolves in water? many ionic substances NaCl (Table F) most polar substances NH 3 35

36 Hydration The process of surrounding the solute particles with water. 36

37 Water Solubility In some ionic compounds, attraction between ions is too strong, and they are insoluble. AgCl BaSO 4 Al(OH) 3 Water can t pull the ions apart. 37

38 Examples: AgNO 3, AgCl, Na 2 S 38

39 Try It Write the formula and determine if it is soluble in water. sodium carbonate calcium carbonate ammonia (NH 3 ) hydrogen chloride methane (CH 4 ) 39

40 Rate of Dissolution Nature of solvent & solute Temperature Stirring (agitation) Surface area 40

41 Saturated Solution max. amount of solute that will dissolve in a given solvent at a specific temp. less more Unsaturated Supersaturated 41

42 Solubility of NaCl in Water At 25 o C, solubility is 36.2 g per 100 g water. How do you make a saturated solution? How can you tell if a solution is saturated or unsaturated? NaCl(s) NaCl(aq) 42

43 Solubility & Temperature What happens to the solubility of a solid substance if the temperature is increased? 43

44 Always write solubility as: g solute g solvent and state temp. Example at 70 o C: 48 g KCl 100 g H 2 O 44

45 Effect of T on Solubility Regents Table G 1.What is the solubility of KNO 3 in water at 50 o C? 2.Can you dissolve 62 g of NaCl in 200 g of water at 70 o C? 45

46 Gas Solubility and Temp. What is the first thing you notice when you start to heat water? Note the gases on Table G (HCl, NH 3, SO 2 ) 46

47 Solubility of O 2 in Water.006 Thermal pollution temperature o 80 C 47

48 Effect Of Pressure on Gas Solubility What s the first thing you notice when you open a bottle of soda? How do you keep a soda fizzy? 48

49 Gas Solubility vs. Pressure Solubility of a gas is proportional to pressure of that gas. More collisions of gas molecules with liquid. 49

50 Gas Solubility vs. Pressure What is the bends? 50

51 Summary: Solubility State Solid & Liquid Gas Increase Temp. (Table G) Increase solubility Decrease solubility Increase Pressure No effect Increase solubility 51

52 Concentration of Solutions Amount of solute in a solution. (dilute vs. concentrated) Percent (by mass or volume) PPM (parts per million) Molarity (M) Why so many terms??? 52

53 Percent (mass or volume) Commonly used by non-chemists, e.g. food industry. % = mass (or vol) of solute mass (or vol) of solution X 100 %(m/m) or %(v/v) Units must be the same!!! 53

54 Percent What is %(v/v) if 85 ml of ethanol is diluted to 0.25 L with water?? 0.25 L is 250 ml %(v/v) = 85 ml 250 ml X 100 = 34 % ethanol (v/v) 54

55 Percent: Try It!!! What is the mass % of NaCl if 33 g of NaCl is diluted to 175 g with water? How many ml of H 2 O 2 are in 2.0 L of 3%(v/v) solution? 55

56 PPM (parts Per Million) Used for very dilute solutions. Example: The allowed limit for Cu in drinking water is 0.20 ppm. PPM = mass solute (g) mass solution (g) X 106 (similar to percent) 56

57 PPM (parts Per Million) If 3.4 mg NaCl is dissolved in 2.0 L of water, what is the concentration in ppm? Solute: 3.4 mg = 3.4 x 10-3 g Solution: 2.0 L = 2000 ml = 2000 g ppm = 3.4 x 10-3 g 2000 g x 10 6 = 1.7 ppm 57

58 PPM (parts Per Million) Try It!!! What is the ppm conc. of 2.7 g of sugar in a 3000 L tank of water? 58

59 Molarity (M) Molarity = M = moles of solute liters of solution For example, 1.23 M NaCl is: 1.23 moles of NaCl 1 L of solution 59

60 Molarity (M = mol/l) To make a 0.50 M aqueous solution of NaCl: Dissolve 0.50 mol NaCl (29 g) in a total volume of 1.0 L of solution. 60

61 Molarity (M = mol/l) How many grams of KBr are needed to make 250 ml of 0.24 M solution? Molarity = moles of solute liters of solution 0.24 M = X moles 0.25 L X = mol x 119 g mol = 7.1 g 61

62 Molarity as a Conversion Factor How many grams of KBr are needed to make 250 ml of 0.24 M solution? molarity 250 ml x 10-3 L 1 ml x 0.24 mol KBr 1 L x 119 g KBr mol KBr = 7.1 g of KBr 62

63 Molarity: Try It 1.How many grams of CaCl 2 are in 250. ml of 2.0 M CaCl 2? 2.What is the molarity of a solution that has 40. g of NaCl in a volume of 220. ml? (best done as M formula) 63

64 Dilutions.50 M stock Often it is necessary to make a more dilute solution from a.050 M 100 ml concentrated ( stock ) solution. For example, how do you make 100 ml of M NaCl from a stock solution of 0.50 M NaCl? 64

65 Dilution Equation Moles of solute before dilution equals moles of solute after dilution. moles solute M = L solution (V) moles solute = M x V moles before dilution = moles after dilution M 1 x V 1 = M 2 x V 2 65

66 before after demo M 1 x V 1 = M 2 x V 2 How do you make 100 ml of M NaCl from a stock solution of 0.50 M NaCl? 0.50 M x V 1 = M x 100 ml V 1 = 10. ml Thus take 10. ml of stock and dilute to 100 ml with water. 66

67 Dilution: Try It How do you prepare 150 ml of 0.10 M NaCl from a stock solution of 1.5 M NaCl? 67

68 Colligative Properties Properties of solutions that depend only on the concentration of dissolved particles. Boiling point elevation (B.P. solution > solvent) Freezing point depression (F.P. solution < solvent) 68

69 Boiling Point Elevation What is boiling? when vapor pressure of a substance equals the external pressure. Regents Table H 69

70 Boiling Point Elevation A solution has a higher boiling point than a pure solvent. Solvent molecules ( )are attracted to solute molecules ( ), so less solvent molecules enter the vapor phase. 70

71 Boiling Point Elevation Which raises the BP of water more, adding 0.1 mol of NaCl or adding 0.1 mol of sucrose? TRICKY NaCl because it gives twice as many particles in solution as sucrose. NaCl(s) H 2 O Na + (aq) + Cl - (aq) 71

72 Boiling Point Elevation Which raises the BP of 1000 g of water more, adding 0.1 mol of NaCl or 0.1 mol of CaCl 2? CaCl 2 because it gives more particles in solution than NaCl. CaCl 2 (s) H 2 O Ca +2 (aq) + 2Cl - (aq) 72

73 Boiling Point Elevation Adding one mole of each substance to a liter of water gives how many moles of particles? Rank the BP. Solute (1 mol) FeCl 3 CH 3 OH Moles of Particles BP Rank Mg(NO 3 ) 2 73

74 Freezing Point Depression Likewise a solute lowers the freezing point of a solvent. 74

75 Freezing & Boiling Point Change BP thermometers BP (increase) FP FP (decrease) solvent solution 75

76 Freezing Point Depression 1. Why do you put salt on your driveway to melt ice? 2. Is NaCl or CaCl 2 better for melting ice? 3. Why is ethylene glycol (antifreeze) added to the water in a car radiator? 4. Put in order of increasing F.P.: water, 1M NaClO 3 (aq), 1M sugar(aq), 1M FeCl 2 (aq) 76

77 Colligative Properties Solutions have different properties compared to pure solvents: boiling point is higher freezing point is lower Depends only on the number of particles dissolved. 77

78 78

79 Warm-up How many grams of CO 2 are in a 3700 ml flask at 25 o C and 670 kpa? 79

80 Warm-up Draw the Lewis structure for water. What is its molecular shape? Is it polar? What type(s) of molecular attractions hold water molecules together in the solid and liquid state? 80

81 Warm-up What is surface tension? Does water have high or low surface tension? Why? 81

82 Warm-up Is the vapor pressure of water relatively high or low? Why? What is the VP of water at 60 o C and 100 o C? 82

83 Warm-up How does sweating allow you to exercise? Why does ice float? 83

84 Warm-up What is an electrolyte? What is required for a substance to carry an electrical current? strong weak non 84

85 Warm-up For the compound MgSO 4. 7H 2 O name it what is the percent water? 85

86 Warm-up What is % water in MgSO 4. 7H 2 O? How do IM forces for water explain? high BP Low vapor pressure (VP)? What is VP water at 85 o C? 86

87 Warm-up Define solute, solvent, & solution. 87

88 Warm-up In a flask filled with water, you see sodium sulfate on the bottom. Is the solution unsaturated or saturated? Given a particular solute/solvent, what factors affect the rate of dissolution? 88

89 Warm-up How do you tell if an ionic compound is soluble in water? Are the following soluble? silver bromide iron(iii) nitrate For one that is soluble, write a chemical equation for its dissolution. 89

90 Warm-up How do you tell if a molecular compound is soluble in water? Is SO 2 soluble in water? 90

91 Warm-up How many grams of NH 4 Cl will dissolve in 320 ml water at 65 o C? 91

92 Warm-up Consider dissolving CO 2 in water. Under which conditions would the most CO 2 dissolve? 25 o C and 1.0 atm. 50 o C and 1.0 atm. 25 o C and 2.0 atm. 50 o C and 2.0 atm. 92

93 Warm-up Define with a formula: mass percent parts per million molarity 93

94 Warm-up What is the ppm concentration if 2.2 g NaOH is dissolved in 1.8 L solution? 94

95 Warm-up How many grams of NaCl are in 250g of 15% (m/m) solution? 95

96 Warm-up How do you make 420. ml of 0.60 M KBr? Starting with this solution, how do you make 100. ml of M KBr? 96

97 Warm-up How many grams of calcium chloride are needed to make 300. ml of 0.85 M solution? 97

98 Warm-up How do you prepare 500.mL of 1.5 M NaOH aqueous solution? Starting with this solution, how do you prepare 200.mL of 0.20 M NaOH? 98

99 Warm-up What is a colligative property? Arrange these aqueous solutions in order of increasing F.P.: water, 0.2M NaNO 3 0.2M ethanol 0.2M Fe(OH) 3 99