Freezing Point Depression

1. Freezing Point Depression Boiling Point Elevation

2. Melting Point is the temperature at which the crystal structure of a solid breaks down with increasing entropy (degree of disorder). It is the same as the freezing point but an opposite process. Generally, the freezing point occurs at the same temperature as the melting point.

3. At the melting point there is enough Kinetic energy to cause particles in the crystal structure to break free from the forces holding them fixed in ridged position in the crystal structure. If solute particles are introduced in the solvent this increases the measure of disorder in the beaker. 0oC -5oC The red particles keep the crystal structure from forming.

4. As you decrease the temperature of the solution in order to reach the freezing point of the solution, the temperature will need to be even lower to get the two ‘solutions to separate themselves to ‘freeze' turn into solids). So the freezing point is depressed as solute is added. -10oC

5. Vapor-Pressure Lowering Boiling-Point Elevation ∆Tb = Kbm 0 P1 = X1 P 1 Freezing-Point Depression ∆Tf = Kfm p = MRT Osmotic Pressure (p) Colligative Properties of Nonelectrolyte Solutions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

6. actual number of particles in soln after dissociation van’t Hoff factor (i) = number of formula units initially dissolved in soln Colligative Properties of Electrolyte Solutions 0.1 m NaCl solution 0.1 m Na+ ions & 0.1 m Cl- ions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. 0.1 m NaCl solution 0.2 m ions in solution i should be 1 nonelectrolytes 2 NaCl CaCl2 3

7. The ‘coldness’ in math: Δ Tfreezing point = (Tinitial – Tfinal) i Kf m i = particles produced when dissolved in solvent Kf = freezing point constant (should be negative) m = moles per Kg of solvent

8. 0 ΔTf = T f – Tf 0 T f is the freezing point of the pure solvent 0 T f > Tf ΔTf = iKfm Freezing-Point Depression T f is the freezing point of the solution ΔTf > 0 m is the molality of the solution Kf is the molal freezing-point depression constant (0C/m)

9. Change in Freezing Point • Which would you use for the streets of Bloomington to lower the freezing point of ice and why? Would the temperature make any difference in your decision? • sand, SiO2 • Rock salt, NaCl • Ice Melt, CaCl2

10. Change in Freezing Point Common Applications of Freezing Point Depression Ethylene glycol – deadly to small animals Propylene glycol

11. Calculate the Freezing Point of a solution of 90 grams of glucose (MW = 180) a non-electrolyte, dissolved in 750 grams of water. The Kf for water = -1.86 Celsius/molality 90 g 180g/mole = 0.5 moles glucose 0.5 moles 0.750 Kg water = 0.667 molality 0 – Tf = (-1.86 C/m)(1)(.667m) Tf = -1.24 oC

12. What would be the Freezing Point of an aqueous solution of Al2(SO4)3 if the concentration was the same as the above problem, m = 0.667 Al2(SO4)3 + H2O ----> 2 Al+3(aq) + 3 SO4-2 (aq) Tf - 0 = (5) (-1.86) (.667) Tf = -6.2 oC

13. Change in Boiling Point Common Applications of Boiling Point Elevation

14. Boiling point can be defined in terms of the vapor pressure of the solvent. It is the temperature at which the vapor pressure of the liquid or solvent in a solution is equal to the external pressure. When adding particles to a solvent, the number of solvent particles on the surface, able to evaporate, decreases! Therefore, to get a solvent to boil, more heat needs to be added to raise more and more particles into vapor.

15. 0 ΔTb = Tb – T b 0 T b is the boiling point of the pure solvent 0 Tb > T b ΔTb = iKbm Boiling-Point Elevation T b is the boiling point of the solution ΔTb > 0 m is the molality of the solution Kb is the molal boiling-point elevation constant (0C/m)

16. Ethylene Glycol is an organic liquid called anti-freeze which is added to water to make an aqueous solution of Ethylene Glycol and water. This is to prevent water in the radiator of the vehicle from boiling over. It elevates the boiling point of water. At the same time, anti-freeze will depress the freezing point of that same water to prevent freeze up in the winter, hence the name of the solution, "anti-freeze".

17. Tb - Tb0 = i Kb m Same math is used: Calculate the boiling point of a solution of 0.0222 m glucose, a non-electrolyte in water. The Kb for water = 0.512 C/m. Tb0 = 100o C. Tb - 100 = (0.512) ( 0.0222) = .0114 Tb = 100 + .0114 = 100.0114 C

18. What would be the boiling point of a solution of the same concentration (.0222m) of an electrolyte solute, Ca3(PO4)2 Kb = 0.512 and m = 0.0222

19. Practice Problems • What is the boiling point of a solution of a non-electrolyte • ethylene glycol which is .05 m in water. • The Kb = 0.512 C/m • 2. If we used .05 molal solution of Na2SO4 • what would be the boiling point? • The Kb = 0.512 C/m

20. Osmotic Pressure (p) Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one. A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules. Osmotic pressure (p) is the pressure required to stop osmosis. more concentrated dilute

21. Osmotic Pressure (p) High P Low P p = iMRT M is the molarity of the solution i= mole # of soute R is the gas constant (0.0821 L atm/mole K) T is the temperature (in K)

22. A cell in an: isotonic solution hypotonic solution hypertonic solution (too much) (not enough) It’s all about the salt……

23. Chemistry In Action: Desalination