Chapter 14.2 : Colligative Properties

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Chapter 14.2 : Colligative Properties

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

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