1. Chapter 13 Properties of Solutions Dr. Subhash C. Goel South GA StateCollege Douglas, GA 31533 © 2012 Pearson Education, Inc.

2. Solutions Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent. © 2012 Pearson Education, Inc.

3. Solutions The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles. © 2012 Pearson Education, Inc.

4. Solutions How Does a Solution Form? As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them. © 2012 Pearson Education, Inc.

5. Solutions How Does a Solution Form? If an ionic salt is soluble in water, it is because the ion–dipole interactions are strong enough to overcome the lattice energy of the salt crystal. © 2012 Pearson Education, Inc.

6. Solutions Energy Changes in Solution Simply put, three processes affect the energetics of solution: –Separation of solute particles, –Separation of solvent particles, –New interactions between solute and solvent. © 2012 Pearson Education, Inc.

7. Solutions Energy Changes in Solution The enthalpy change of the overall process depends on  H for each of these steps. © 2012 Pearson Education, Inc.

8. Solutions Why Do Endothermic Processes Occur? Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered. © 2012 Pearson Education, Inc.

9. Solutions Why Do Endothermic Processes Occur? Yet we know that in some processes, like the dissolution of NH 4 NO 3 in water, heat is absorbed, not released. © 2012 Pearson Education, Inc.

10. Solutions Enthalpy Is Only Part of the Picture The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system. © 2012 Pearson Education, Inc.

11. Solutions Enthalpy Is Only Part of the Picture So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered. © 2012 Pearson Education, Inc.

12. Solutions Dissolution vs Reaction Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved. It may have reacted. © 2012 Pearson Education, Inc.

13. Solutions Dissolution vs Reaction Dissolution is a physical change—you can get back the original solute by evaporating the solvent. If you can’t get it back, the substance didn’t dissolve, it reacted. © 2012 Pearson Education, Inc.

14. Solutions Types of Solutions Saturated –In a saturated solution, the solvent holds as much solute as is possible at that temperature. –Dissolved solute is in dynamic equilibrium with solid solute particles. Unsaturated –If a solution is unsaturated, less solute than can dissolve in the solvent at that temperature is dissolved in the solvent. © 2012 Pearson Education, Inc.

15. Solutions Types of Solutions Supersaturated –In supersaturated solutions, the solvent holds more solute than is normally possible at that temperature. –These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask. © 2012 Pearson Education, Inc.

16. Solutions Factors Affecting Solubility Chemists use the axiom “like dissolves like.” –Polar substances tend to dissolve in polar solvents. –Nonpolar substances tend to dissolve in nonpolar solvents. © 2012 Pearson Education, Inc.

17. Solutions Factors Affecting Solubility The more similar the intermolecular attractions, the more likely one substance is to be soluble in another. © 2012 Pearson Education, Inc.

18. Solutions Factors Affecting Solubility Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not. © 2012 Pearson Education, Inc.

19. Solutions Factors Affecting Solubility Vitamin A is soluble in nonpolar compounds (like fats). Vitamin C is soluble in water. © 2012 Pearson Education, Inc.

20. Solutions Gases in Solution In general, the solubility of gases in water increases with increasing mass. Larger molecules have stronger dispersion forces. © 2012 Pearson Education, Inc.

21. Solutions Gases in Solution The solubility of liquids and solids does not change appreciably with pressure. But the solubility of a gas in a liquid is directly proportional to its pressure. © 2012 Pearson Education, Inc.

22. Copyright © Cengage Learning. All rights reserved.12 | 22 Henry’s law describes the effect of pressure on gas solubility: The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the solution. This is expressed mathematically in the equation S = k H P where S = gas solubility k H = Henry’s law constant for the gas P = partial pressure of the gas over the solution

23. Exercise 1 A Henry’s Law Calculation Calculate the concentration of CO 2 in a soft drink that is bottled with a partial pressure of CO 2 of 4.0 atm over the liquid at 25  C. The Henry’s law constant for CO 2 in water at this temperature is 3.4  10  2 mol/L-atm.

24. Exercise 2 A Henry’s Law Calculation Calculate the concentration of O 2 in water in moles per liter at 1.00 atm pressure and 20  C. The mole fraction of O 2 in air is 0.209 moles. (Remember the sum of the mole fractions of all gases in a mixture is equal to 1) Henry’s law constant for O 2 in water at this temperature is 1.3  10  3 mol/L-atm.

25. Solutions Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature. © 2012 Pearson Education, Inc.

26. Solutions Temperature The opposite is true of gases. –Solubility of gases in water decreases with increasing temperature. –Warm lakes have less O 2 dissolved in them than cool lakes. © 2012 Pearson Education, Inc.

27. Solutions Ways of Expressing Concentrations of Solutions © 2012 Pearson Education, Inc.

28. Solutions Mass Percentage Mass % of A = mass of A in solution total mass of solution  100 © 2012 Pearson Education, Inc.

29. Solutions Parts per Million and Parts per Billion ppm = mass of A in solution total mass of solution  10 6 Parts per million (ppm) Parts per billion (ppb) ppb = mass of A in solution total mass of solution  10 9 © 2012 Pearson Education, Inc.

30. Solutions Exercise 3 (a) A solution is made by dissolving 13.5 g of glucose (C 6 H 12 O 6 ) in 0.100 kg of water. What is the mass percentage of solute in this solution? (b) A 2.5-g sample of groundwater was found to contain 5.4  g of Zn 2+. What is the concentration of Zn 2+ in parts per million?

31. Solutions moles of A total moles of all components X A = Mole Fraction (X) In some applications, one needs the mole fraction of solvent, not solute— make sure you find the quantity you need! © 2012 Pearson Education, Inc.

32. Solutions moles of solute liters of solution M = Molarity (M) You will recall this concentration measure from Chapter 4. Since volume is temperature- dependent, molarity can change with temperature. © 2012 Pearson Education, Inc.

33. Solutions moles of solute kilograms of solvent m = Molality (m) Since both moles and mass do not change with temperature, molality (unlike molarity) is not temperature- dependent. © 2012 Pearson Education, Inc.

34. Solutions Exercise 4 Calculation of Molality A solution is made by dissolving 4.35 g glucose (C 6 H 12 O 6 ) in 25.0 mL of water at 25  C. Calculate the molality of glucose in the solution. Water has a density of 1.00 g/mL.

35. Solutions Exercise 5 Calculation of Mole Fraction and Molality An aqueous solution of hydrochloric acid contains 36% HCl by mass. (a)Calculate the mole fraction of HCl in the solution. (b) Calculate the molality of HCl in the solution.

36. Solutions Changing Molarity to Molality If we know the density of the solution, we can calculate the molality from the molarity, and vice versa. © 2012 Pearson Education, Inc.

37. Solutions Exercise 6 Calculation of Molarity using density of solution A solution with a density of 0.876 g/mL contains 5.0 g of toluene (C 7 H 8 ) and 225 g of benzene. Calculate the molarity of the solution.

38. Copyright © Cengage Learning. All rights reserved.12 | 38 Colligative properties of solutions are properties that depend on the concentration of the solute molecules or ions in solution but not on the chemical identity of the solute. 1.Vapor-pressure lowering 2.Boiling-point elevation 3.Freezing-point lowering 4.Osmotic pressure

39. Solutions Vapor Pressure Because of solute–solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase. © 2012 Pearson Education, Inc.

40. Copyright © Cengage Learning. All rights reserved.12 | 40 Therefore, vapor pressure of a solution, P, is less than the vapor pressure of the pure solvent, P°. Raoult’s Law When the solute is nonvolatile, the vapor pressure of a solution is the mole fraction of the solvent times the vapor pressure of pure solvent.

41. Solutions Exercise 7 Calculation of Vapor Pressure of a Solution Glycerin (C 3 H 8 O 3 ) is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25  C. Calculate the vapor pressure at 25  C of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure of pure water at 25  C is 23.8 torr (Appendix B), and its density is 1.00 g/mL. 12 | 41 Copyright © Cengage Learning. All rights reserved.

42. Solutions © 2015 Pearson Education, Inc. Boiling-Point Elevation Since vapor pressures are lowered for solutions, it requires a higher temperature to reach atmospheric pressure. Hence, boiling point is raised.

43. Solutions © 2015 Pearson Education, Inc. Freezing-Point Depression The construction of the phase diagram for a solution demonstrates that the freezing point is lowered while the boiling point is raised.

44. Solutions © 2015 Pearson Education, Inc. Boiling-Point Elevation and Freezing-Point Depression The change in temperature is directly proportional to molality (using the van’t Hoff factor).

45. Solutions © 2015 Pearson Education, Inc. The van’t Hoff Factor (i) What is the van’t Hoff factor? It takes into account dissociation in solution! Theoretically, we get 2 particles when NaCl dissociates. So, i = 2. In fact, the amount that particles remain together is dependent on the concentration of the solution.

46. Solutions © 2015 Pearson Education, Inc. Note K b is the molal boiling-point elevation constant, a property of the solvent.  T b is added to the normal boiling point of the solvent. K f is the molal freezing-point depression constant of the solvent.  T f is subtracted from the normal freezing point of the solvent.

47. Solutions Boiling-Point Elevation and Freezing-Point Depression Note that in both equations,  T does not depend on what the solute is, but only on how many particles are dissolved.  T b = i.K b  m  T f = i.K f  m © 2012 Pearson Education, Inc.

48. Solutions Exercise 8 Calculating Of Boiling-Point Elevation and Freezing-Point Depression Automotive antifreeze consists of ethylene glycol, CH 2 (OH)CH 2 (OH), a nonvolatile nonelectrolyte. Calculate the boiling point and freezing point of a 25.0 mass % solution of ethylene glycol in water.

49. Solutions Colligative Properties of Electrolytes Since the colligative properties of electrolytes depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes. © 2012 Pearson Education, Inc.

50. Solutions Exercise 9 Freezing-Point Depression in Aqueous Solutions List the following aqueous solutions in order of their expected freezing point: 0.050 m CaCl 2, 0.15 m NaCl, 0.10 m HCl, 0.050 m CH 3 COOH, 0.10 m C 12 H 22 O 11.

51. Solutions Osmosis Some substances form semipermeable membranes, allowing some smaller particles to pass through, but blocking other larger particles. In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so. © 2012 Pearson Education, Inc.

52. Solutions Osmosis In osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the area of lower solvent concentration (higher solute concentration). © 2012 Pearson Education, Inc.

53. Solutions Osmotic Pressure The pressure required to stop osmosis, known as osmotic pressure, , is  = ( ) RT = MRT nVnV where M is the molarity of the solution. If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic. © 2012 Pearson Education, Inc.

54. Solutions Osmosis in Blood Cells If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic. Water will flow out of the cell, and crenation results. © 2012 Pearson Education, Inc.

55. Solutions Osmosis in Cells If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic. Water will flow into the cell, and hemolysis results. To prevent crenation or hemolysis of red blood cell, the IV solution must be isotonic with the intracellular fluids of the blood cells. © 2012 Pearson Education, Inc.

56. Solutions Exercise 10 Calculating Involving Osmotic Pressure The average osmotic pressure of blood is 7.7 atm at 25  C. What molarity of glucose (C 6 H 12 O 6 ) will be isotonic with blood?

57. © 2012 Pearson Education, Inc. Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward Determination of Molar Mass Colligative properties of the solutions can be used to determine molar mass of the solute.

58. © 2012 Pearson Education, Inc. Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward Exercise11 Molar Mass from Boiling Point Elevation A solution of an unknown nonvolatile nonelectrolyte was prepared by dissolving 0.250 g of the substance in 40.0 g of CCl 4. The boiling point of the resultant solution was 0.357  C higher than that of the pure solvent. Calculate the molar mass of the solute.

59. © 2012 Pearson Education, Inc. Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward Exercise12 Molar Mass from Osmotic Pressure The osmotic pressure of an aqueous solution of a certain protein was measured to determine the protein’s molar mass. The solution contained 3.50 mg of protein dissolved in sufficient water to form 5.00 mL of solution. The osmotic pressure of the solution at 25  C was found to be 1.54 torr. Treating the protein as a nonelectrolyte, calculate its molar mass.

60. Solutions Colloids Suspensions of particles larger than individual ions or molecules, but too small to be settled out by gravity, are called colloids. © 2012 Pearson Education, Inc.

61. Solutions Colloids in Biological Systems Some molecules have a polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic (water- hating) end. © 2012 Pearson Education, Inc.

62. Solutions Colloids in Biological Systems These molecules can aid in the emulsification of fats and oils in aqueous solutions. © 2012 Pearson Education, Inc.