1. Please Pick Up a Copy of Raoult’s Law and Vapor Pressure problem set Distillation problem set

2. Gas-Solution Processes Colligative Properties Raoult’s Law Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

3. Colligative Properties  Reading assignment: Zumdahl: Chapter 16.10-16.11, 17.5-17.7  The properties of a solution depends on the amount of dissolved solute in the solution. Properties which depend on the amount of solute, but not the identity of the compound are called colligative properties.

4. 7/13/2015 Temp (°C)P (torr) 109.2 2017.5 3031.8 4055.3 5092.5 Vapor Pressure Water On the graph where is the boiling point located? What does a vapor pressure of water graph look like?

5. Vapor Pressure of Water 0 20 40 60 80 100 01020304050 Temperature (°C) Pressure (torr) What will be the vapor pressure when the temperature is 100 °C?

6. 7/13/2015 Vapor Pressure of Water Which of the dozen water molecules shown are evaporating? What factors determine whether a molecule will evaporate?

7. 7/13/2015 Which Is More Volatile? Water or Isopropanol Vapor Pressure of Water & Isopropanol

8. 7/13/2015 Vapor Pressure of a Solution What factors determine the total vapor pressure of the solution?

9. 7/13/2015 Raoult’s Law  The vapor pressure of an ideal solution is equal to the sum of the vapor pressure of each component times its mole fraction. A P   B P   total P  A   B   

10. 7/13/2015 P° A is the vapor pressure of pure A. A P   B P   total P  A   B    If the solvent or solute is non-volatile, then P ° for that species is zero.

11. 7/13/2015  Non-ideal solutions have vapor pressures higher or lower than predicted. A P   B P   total P  A   B    Raoult’s Law

12. 7/13/2015 Raoult’s Law Application  Distillation: The separation of one component from a solution by boiling and condensation of the vapor.

13. 7/13/2015 Benzene-Toluene Distillation  One mole of benzene is mixed with three moles of toluene.  What will be the vapor pressure of the solution at 88  C. Benzene (C 6 H 6 ) and toluene (C 6 H 5 CH 3 ) form an ideal solution. P  benzene is 960 torr at 88  C P  toluene is 380 torr at 88  C What other questions could be asked?

14. 7/13/2015 Benzene-Toluene Distillation  One mole of benzene is mixed with three moles of toluene.  What will be the composition of the vapor that is condensed?

15. 7/13/2015 Distillation  Distillation separates one compound from another because the vapor and the solution have different compositions.  How could you make a solution containing two volatile components very pure?

16. 7/13/2015 Fractional Distillation

17. 7/13/2015 Not All Compounds can be Separated by Distillation  Non-ideal solutions that have the same composition in solution and the vapor are called azeotropes. Isopropanol (BP 82  C) and water (BP 100  C) forms a 80.4  C azeotrope of composition  isopropanol = 0.878,  water = 0.122

18. 7/13/2015 Colligative Properties  A physical property of a solution which depends upon the concentration of the solute, but not its nature or identity.  Examples: Vapor pressure lowering Boiling point elevation

19. 7/13/2015 Vapor Pressure Lowering Why does the dissolution of a non-volatile solute result in a lower solution vapor pressure than the pure solvent? Describe the cause based on a mathematical argument and a mechanical argument.

20. 7/13/2015 Under what conditions does boiling occur? Temperature ( o C) 0 200 400 600 800 020406080100120 Vapor Pressure of Water Solutions Pressure (torr) Water Sugar Water

21. 7/13/2015 Temperature ( o C) 0 200 400 600 800 020406080100120 Vapor Pressure of Water Solutions Pressure (torr) Water Sugar Water What is the vapor pressure of a 0.10 mole fraction sugar in water solution at 100 °C?

22. 7/13/2015 Temperature ( o C) 0 200 400 600 800 020406080100120 Vapor Pressure of Water Solutions Pressure (torr) Water Sugar Water If the vapor pressure of a solution is lowered, why is the boiling point elevated?

23. 7/13/2015 Boiling Point Elevation   T is the increase in boiling point  k b is the boiling point (ebullioscopic) constant ·k b = 0.512 K  m -1 for water  m is the molality of the solution  T k mi b 

24. 7/13/2015 Boiling Point Elevation  Sugar water does not conduct electricity.  k b = 0.512 K  m -1 for water  What is the boiling point of 0.10 mole fraction sugar water solution?  T k mi b 

25. 7/13/2015 Boiling Point Elevation  i is the multiplicative factor to account for ions if a compound does not ionize then there is one particle per molecule and i = 1. if a compound breaks into two ions then i = 2, etc.  T k mi b 

26. 7/13/2015 Rank the following solutions in terms of increasing boiling point water 0.2 m salt water 0.3 m sugar water  T k mi b 

27. 7/13/2015 Freezing Point Depression  The dissolution of a solute in a solvent results in a lower freezing point for the solution.  T k mi f 

28. 7/13/2015 Freezing Point Depression How does an impurity change the freezing point of the triangular molecules?

29. 7/13/2015 Freezing Point Depression   T is the decrease in freezing point  k f is the freezing point (cryoscopic) constant ·k f = 1.86 K  m -1 for water  Applications  T k mi f 

30. 7/13/2015 Osmotic Pressure  A pressure differential due to solvent molecules passing through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.  mRT

31. 7/13/2015 Osmotic Pressure

32. 7/13/2015 Osmotic Pressure

33. 7/13/2015 Osmotic Pressure

34. 7/13/2015 Osmotic Pressure   mRT

35. 7/13/2015 Osmotic Pressure Applications   mRT Molecular weight determination Biochemical Why is physiological 0.9% saline solution important in intravenous transfusions?

36. 7/13/2015 Historical Perspective  Colligative properties were important to the early chemists because it provided information about the molecular weight or degree of ionization of the solute.