SN2 Colligative Properties Chapter 11 PDF

Topic 3: Colligative Properties Chapter 11 Learning Objective: 11.5 Boiling Point Elevation and Freezing Point Depression 11.7 Colligative Properties of Electrolyte Solutions Instructor: Liliane Halab 1 ...

Topic 3: Colligative Properties Chapter 11 Learning Objective: 11.5 Boiling Point Elevation and Freezing Point Depression 11.7 Colligative Properties of Electrolyte Solutions Instructor: Liliane Halab 1  Colligative Properties Colligative properties are physical properties of a solution that depend on the number of solute particles dissolved in the solvent (colligative means “depending on the collection”) and not on the nature of the particles. Boiling-point elevation Freezing-point depression We must consider 2 type of solutions: 1. Solute is a nonelectrolyte (non-dissociating) 2. Solute is an electrolyte (dissociating) 2  Boiling Point and Freezing Point of Solutions The normal boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure. Boiling-point elevation (ΔTb): boiling point of solution is higher than the boiling point of pure solvent. The normal freezing point is the temperature at which a liquid turns into a solid at atmospheric pressure. Freezing-point depression (ΔTf): freezing point of solution is lower than the freezing point of pure solvent. 3  The Vapor Pressure of Solutions with nonvolatile Solute Vapor pressure of PURE solvent Vapor pressure of SOLUTION = solute + (volatile liquid): solvent (solute is nonvolatile): To vaporize: molecules from liquid solute particles interfere with the ability must reach surface and overcome of the solvent to vaporize. intermolecular interactions to escape The rate of vaporization is diminished, liquid. and the rate of condensation is greater. => lowers solvent’s vapor pressure At any given temperature, the vapor pressure of the solution is lower than the vapor pressure of the pure solvent. 4  Boiling-Point Elevation of Solutions Since a solution decreases the vapor pressure of the solvent, the solution must be heated at higher temperature than the boiling point of the solvent to reach the atmospheric pressure. Therefore, a nonvolatile solute raises the boiling point. ΔTb = boiling-point elevation (°C) Tsolution = boiling-point of solution (°C) Tsolvent = boiling-point of pure solvent (°C) i = van’t Hoff factor (moles of particles in solution per moles of solute dissolved) Kb = molal boiling-point elevation constant for the solvent (°C·Kg/mol) m = molality of solution (mol/Kg) 5  Freezing-Point Depression of Solutions When a solute is dissolved in a solvent, the freezing point of the solution is lower than that of the pure solvent. Solutes hinder freezing & facilitate melting. Result: rate freezing = rate melting at lower T than in pure solvent Ice in equilibrium Ice in equilibrium with liquid water with liquid water containing a dissolved solute ΔTf = Tsolvent ‒ Tsolution = i Kf m ΔTf = freezing-point depression (°C) Tsolution = freezing-point of solution (°C) Tsolvent = freezing-point of pure solvent (°C) i = van’t Hoff factor (moles of particles in solution per moles of solute dissolved) Kf = molal freezing-point depression constant for solvent (°C·Kg/mol) m = molality of solution (mol/Kg) 6 7  van’t Hoff Factor van’t Hoff Factor, i, relates to the number of particles released when a substance dissolves The relationship between the moles of solute dissolved and the moles of particles in solution is usually expressed as: moles of particles in solution i = moles of solute dissolved The expected (predicted) value for i can be determined for a salt by noting the number of ions per formula unit (assuming complete dissociation and that ion pairing does not occur). NaCl i=2 KNO3 i=2 Na3PO4 i=4 Glucose (C6H12O6) i = 1 8  Ion Pairing in electrolyte solutions At a given instant a small percentage of the cation and the anion in the ionic compound are paired and thus count as a single particle. Ion pairing is most important in concentrated solutions. As the solution becomes more dilute, the ions are farther apart, and less ion pairing occurs. Ion pairing occurs to some extent in all electrolyte solutions. Ion pairing is most important for highly charged ions. Ex. 0.0010 m NaCl solution => iobserved = 1.97 0.05 m NaCl solution => iobserved = 1.9 9 van’t Hoff Factor 10 Practical Applications of Boiling Point Elevation and Freezing Point Depression of Solutions Salt lowers freezing point on icy roads (by interrupting the ice crystal formation, it has to get colder to freeze salt water. Salt in pasta water make it boil at higher temperature and cook pasta faster. 11  Example: Boiling-Point Elevation of Solution A solution was prepared by dissolving 25.0 g glucose (non-dissociating) in 200. g water. The molar mass of glucose is 180.16 g/mol. What is the boiling point of the resulting solution (in °C)? Kb of water is 0.51°C · kg/mol. 12  Example: Freezing point of electrolyte solution Calculate the freezing point for 0.050 m MgCl2 aqueous solution. (Assume complete dissociation) Kf of water is 1.86°C·Kg/mol 13 These properties can be used to determine the molar mass of an unknown solute. Example: Calculating molar mass of solute A chemist is trying to identify a human hormone that controls metabolism by determining its molar mass. A sample weighing 0.546 g was dissolved in 15.0 g benzene, and the freezing-point depression was determined to be 0.240°C. Calculate the molar mass of the hormone. (Kf of benzene is 5.12°C · Kg/mol) 14  Suggested readings-problems Topic 3 Read Chapter 11 (sections 11.5 and 11.7) from Zumdahl (same for all editions) Work on suggested problems: from Zumdahl, 11th edition: Colligative properties of non-electrolyte solutions: 87, 91, 137a Colligative properties of electrolyte solutions: 99(a,c), 100(a-d), 101, 107, 109 Or from Zumdahl, 10th edition: Colligative properties of non-electrolyte solutions: 73, 77, 115a Colligative properties of electrolyte solutions: 85(a,c), 86(a-d), 87, 91, 93 Or from Zumdahl, 9th edition Colligative properties of non-electrolyte solutions: 67, 71, 107a Colligative properties of electrolyte solutions: 79(a,c), 80(a-d), 81, 85, 87 Answers to the even Number from the 11th edition: 100: a) and d) is pure H2O, b), c) and e) is CaCl2 solution