Chapter 14.2 : Colligative Properties
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Chapter 14.2 : Colligative Properties

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    Chapter 14.2 : Colligative Properties Chapter 14.2 : Colligative Properties Presentation Transcript

    • Colligative Properties of solutions
      Chapter 14.2
    • Objectives:
      List four colligative properties, and explain why they are classified as colligative properties.
      Calculate freezing-point depression, boiling-point elevation, and solution molality of nonelectrolytic solutions.
      Calculate the expected changes in freezing point and boiling point of an electrolytic solution.
      Discuss causes of the differences between expected and experimentally observed colligative properties of electrolytic solutions.
    • Colligative Properties of Solutions
      Properties that depend on the concentration of solute particles but not on their identity.
      Examples
      Vapor-pressure lowering
      Freezing-point depression
      Boiling-point elevation
    • Vapor-Pressure Lowering
      Nonvolatile solute raises boiling point and lowers freezing point of solution
      Nonvolatile substance : one that has little tendency to become a gas under existing conditions
      Look at figures 14.6 and 7 on page 436 & 437
      Solute molecules crowd surface of solution, lowering tendency of water molecules to escape to liquid phase.
    • Freezing-point depression
      tf , is the difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent, and it is directly proportional to the molal concentration of the solution.
      Molal freezing-point constant(Kf)
      The freezing-point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolytic solute
      tf=Kfm
    • Boiling-point Elevation
      tb, is the difference between the boiling points of the pure solvent and a solution of a nonelectrolyte in that solvent, and it is directly proportional to the molal concentration of the solution.
      Molal boiling-point constant(Kb)
      The freezing-point depression of the solvent in a 1-molal solution of a nonvolatile, nonelectrolytic solute
      tb=Kbm
    • Osmotic Pressure
      Semipermeable Membranes
      Allow the movement of some particles while blocking the movement of others
      Osmosis
      The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration
      Osmotic Pressure
      External pressure that must be applied to stop osmosis
      Life Processes
      Cell membranes are semipermeable
      Shrink when placed in solution of higher concentration
    • Electrolytes and Colligative Properties
      Electrolytes in solution enhance the Colligative properties.
      Because they break up into ions
      Example: 0.1m NaCl will double the amount the freezing point lower than a nonelectrolyte
      0.1 m CaCl2 solution will triple the amount the boiling point elevates than a nonelectrolyte.
      Why? More particles are produced
      NaCl (s) Na+ (aq) + Cl- (aq)
      CaCl2 (s) Ca2+(aq) + 2 Cl- (aq)
      2 total moles produced
      1 mol
      1 mol
      1 mol
      3 total moles produced
      2 mol
      1 mol
      1 mol