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Colligative properties

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colligative properties, solutions, vapor pressure, boiling and freezing point changes, osmotic pressure

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Colligative properties

  1. 1. Solutions Colligative Properties • Changes in colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. • Among colligative properties are Vapor pressure lowering Boiling point elevation Melting point depression Osmotic Pressure
  2. 2. 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.
  3. 3. Solutions Vapor Pressure Therefore, the vapor pressure of a solution is lower than that of the pure solvent.
  4. 4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Vapor Pressures of Pure Water and a Water Solution Chapter 13 Section 2 Colligative Properties of Solutions
  5. 5. Solutions Boiling Point Elevation and Freezing Point Depression Nonvolatile solute- solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.
  6. 6. Solutions Boiling Point Elevation The change in boiling point is proportional to the molality of the solution: Tb = Kb  m where Kb is the molal boiling point elevation constant, a property of the solvent.Tb is added to the normal boiling point of the solvent.
  7. 7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Sample Problem E What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and 400.0 g of water? The molar mass of the solute is 62.0 g. Chapter 13 Section 2 Colligative Properties of Solutions Boiling-Point Elevation, continued
  8. 8. Solutions Freezing Point Depression • The change in freezing point can be found similarly: Tf = Kf  m • Here Kf is the molal freezing point depression constant of the solvent. Tf is subtracted from the normal freezing point of the solvent.
  9. 9. 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. Tb = Kb  m Tf = Kf  m
  10. 10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Sample Problem C What is the freezing-point depression of water in a solution of 17.1 g of sucrose, C12H22O11, in 200. g of water? What is the actual freezing point of the solution? Chapter 13 Section 2 Colligative Properties of Solutions Freezing-Point Depression, continued
  11. 11. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Chemistry: The Central Science, Eleventh Edition By Theodore E. Brown, H. Eugene LeMay, Bruce E. Bursten, and Catherine J. Murphy With contributions from Patrick Woodward Sample Exercise 13.9 Calculation of Boiling-Point Elevation and Freezing-Point Lowering Calculate the freezing point of a solution containing 0.600 kg of CHCl3 and 42.0 g of eucalyptol (C10H18O), a fragrant substance found in the leaves of eucalyptus trees. Practice Exercise
  12. 12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Osmotic Pressure • A semipermeable membrane allows the passage of some particles while blocking the passage of others. • The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration is osmosis. • Osmotic pressure is the external pressure that must be applied to stop osmosis. Chapter 13 Section 2 Colligative Properties of Solutions
  13. 13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 13 Section 2 Colligative Properties of Solutions Osmotic Pressure
  14. 14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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).
  15. 15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Osmotic Pressure The pressure required to stop osmosis, known as osmotic pressure, , is n V  = ( )RT = MRT 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.
  16. 16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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.
  17. 17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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.
  18. 18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Osmosis in Cells Hypotonic Isotonic Hypertonic
  19. 19. Solutions Colligative Properties of Electrolytes Since these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes.
  20. 20. Solutions Colligative Properties of Electrolytes However, a 1 M solution of NaCl does not show twice the change in freezing point that a 1 M solution of methanol does.
  21. 21. Solutions van’t Hoff Factor One mole of NaCl in water gives rise to two moles of ions. (or very close to two moles)
  22. 22. Solutions van’t Hoff Factor Some Na+ and Cl− re- associate for a short time, so the true concentration of particles is somewhat less than two times the concentration of methanol.
  23. 23. Solutions The van’t Hoff Factor • Re-association is more likely at higher concentration. • Therefore, the number of particles present is concentration dependent.
  24. 24. Solutions The van’t Hoff Factor We modify the previous equations by multiplying by the van’t Hoff factor, i Tf = Kf  m  i
  25. 25. Solutions The van’t Hoff Factor Determine the expected boiling point of a solution made by dissolving 25.0g of barium chloride in 0.150kg of water.

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