Colligative Properties

1. Colligative Properties Vapor Pressure Freezing and Boiling Points Osmotic Pressure

2. Objectives • SWBAT • Explain why boiling point can be elevated or freezing point can be depressed. • Use ΔTb = i Kbm to solve boiling point or freezing point problems.

3. Drill • We will finish the Ksp problem from yesterday.

4. Website to Try • www.chemprofessor.com/colligative.htm

5. Definitions Po= vapor pressure of the pure solventP = vapor pressure of the solvent in a solution Boiling point - The temperature at which the vapor pressure of a liquid is equal to the pressure on the liquid. Normal (Standard) Boiling Point – The temperature at which the vapor pressure of a liquid is equal to standard pressure (1.00 atm = 760 mmHg = 760 torr = 101.325 kPa)

6. Vapor Pressure Reduction • Nonvolatile Substance- a substance that doesn’t have a tendency to vaporize (i.e. sugar) • When a nonvolatile substance is added to a pure substance, solute molecules block the solvent molecules from evaporating • However, the solvent molecules can still change from gas to liquid

7. Vapor Pressure Reduction • Because more molecules are leaving the gas state than entering it, there are less molecules of gas • This in turn lowers the pressure of the gas, known as vapor pressure reduction • The reduction of vapor pressure does not depend on the solute used

8. Vapor Pressure and IMF “If the liquid is part of a solution, the non-volatile solute molecules act to disrupt the evaporation process by attracting the solvent molecules, since they have intermolecular forces at least as strong as the liquid molecules, they impede evaporation. The presence of the solute does not affect the condensation process, because, since it is non-volatile, it is not present in the vapor phase.” www.chemprofessor.com/colligative.htm

9. Boiling Point Elevation • The boiling point of any substance is the temperature where the vapor pressure is equal to the atmospheric pressure (usually) • Because the vapor pressure is reduced when a nonvolatile substance is added, a higher temperature is required to make the two pressures match • (new boiling point) – (original boiling point) = the boiling point elevation • This is shown as ΔTb

10. Boiling Point Elevation Solute particles weaken IMF in the solvent.

11. Adding a Solute • There are two reasons why the addition of a solute raises the boiling point of a solution (i.e. why the vapor pressure of the solvent in a solution is lower than the vapor pressure of a pure solvent):

12. Reason #1 1. The solute particles occupy space at the surface. This consequently slows the rate at which the solvent molecules in the liquid phase can escape into the gas phase

13. Reason #2 • P < Po P = Xsolvent Po Raoult's Law - The vapor pressure of the solvent above a solution is equal to the product of the mole fraction of the solvent and the vapor pressure of the pure solvent: • The solute particles introduce a new set of attractive forces with the solvent molecules.

14. The vapor pressure of the “solvent with the solute” is lower than the vapor pressure of the pure solvent.   • In order to raise the vapor pressure of the “solvent with the solute” to atmospheric pressure, the temperature must be raised.

15. Effect of Adding Solute to a Pure Solvent http://library.thinkquest.org/C006669/media/Chem/img/antiBP.gif

16. Boiling Point Elevation http://www.brynmawr.edu/Acads/Chem/Chem103lc/colligative_lecture.html

17. Calculating ΔTb • ΔTb = i Kbm • m is the molality of the solution • Kb is a constant that is different for every solvent • i is the Van’t Hoff factor, which is the number of particles into which the solute dissociates • Common Kb’s are located in Figure 15-25 in your book

18. Freezing Point Depression • Freezing Point Depression is the change in temperature to where the solid vapor pressure equals the liquid vapor pressure • ΔTf = i Kfm

19. Freezing Point Depression

20. Freezing Point Depression http://www.brynmawr.edu/Acads/Chem/Chem103lc/colligative_lecture.html

21. Definition • Osmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.

22. Π = iMRT i is the dimensionlessvan 't Hoff factor M is the molarity R=0.08206 L · atm · mol-1 · K-1 is the gas constant T is the thermodynamic (absolute) temperature Thermodynamic (absolute) temperature is the temperature measured or calculated on an absolute scale.

23. Osmotic Pressure • When two solutions of different concentrations are separated by a semi-permeable membrane, osmosis occurs to result in the distribution of solute molecules • What happens if you have a u-shaped tube, with a membrane in the middle, and different concentrations on either side?

24. If, assuming that the two sides have different concentrations, what happens to this solution?

25. The solution will actually become uneven. This will continue until the concentrations are equal, or the osmotic pressure is reached

26. Osmotic Pressure • Osmotic pressure is the pressure required to stop osmosis in the previous example • It is generally represented as the Greek letter pi • Osmotic pressure plays very important roles in the chemistry of the body

27. Osmosis Equation • π = i M R T

28. http://www.brynmawr.edu/Acads/Chem/Chem103lc/colligative_lecture.htmlhttp://www.brynmawr.edu/Acads/Chem/Chem103lc/colligative_lecture.html

29. Try this Osmotic Pressure website • http://web.fccj.org/~ethall/2046/ch11/op.htm

30. http://www.chemprofessor.com/colligative_files/image019.jpg

31. Osmosis in Real Life http://www.chrisrepetsky.com/wp-content/uploads/2009/10/553px-Osmotic_pressure_on_blood_cells_diagram.svg.png