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Colligative Properties

Colligative Properties. Chapter 13.6. Colligative Properties. “Collective”-nature of Effect on 4 properties: 1) Vapor pressure lowering 2)Boiling point elevation 3) Freezing point depression 4) Osmotic pressure. Electrolytes. Solutes dissolved in a solution can separate into ions

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Colligative Properties

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  1. Colligative Properties Chapter 13.6

  2. Colligative Properties • “Collective”-nature of • Effect on 4 properties: • 1) Vapor pressure lowering • 2)Boiling point elevation • 3) Freezing point depression • 4) Osmotic pressure

  3. Electrolytes • Solutes dissolved in a solution can separate into ions • These ions conduct electricity • Strong electrolytes (soluble • salts, strong acids and strong • bases) conduct current well • because they dissociate completely

  4. Raoult’s Law • Vapor pressure of the solvent over a solution equals the mole fraction of solvent times the vapor pressure of the pure solvent; nonvolatile nonelectrolyte reduces vp. • Psolvent = XsolventP°solvent • Recognize that X is always <1 therefore, the new pressure is ALWAYS lower.

  5. Vapor Pressure Lowering • So when a solute is added to a solvent, less solvent can escape and vapor pressure is lowered • ΔP = X solute P°solvent • Read Sample Problem 13.6; Put Follow-up Problem on Dry Erase boards • http://www.kchemistry.com/AddedTopics_1.htm

  6. Boiling Point Elevation A solution boils at a higher temperature than the pure solvent ΔTb= boiling point elevation ΔTb = Kb m Kb = boiling pt elevation constant m = solution molality ΔTb= Tb(solution) - Tb(solvent)

  7. Boiling point Elevation http://www.btinternet.com/~martin.chaplin/collig.html

  8. Freezing point depression http://www.sparknotes.com/chemistry/solutions/colligative/section1.rhtml

  9. Freezing Point Depression • ΔTf = Kfm • ΔTf = freezing point depression • Kf = molal freezing point depression constant • m = molal concentration of the solute • ΔTf = Tf(solvent) – Tf(solution)

  10. Osmotic Pressure • Osmotic pressure is created when two solutions are separated by a semipermeable membrane. The solvent can pass but not solute. • Osmotic pressure: applied pressure required to prevent the net movement of water from solvent. • II = nsoluteRT = MRT • Vsoln

  11. Overarching Colligative Pts! http://www.youtube.com/watch?v=a2qOZmuhDBI&feature=related • Each property rests on the inability of solute particles to cross between two phases. The solute reduces the solution’s ability to change phase-freeze, boil, vaporize or move across a semipermeable membrane. • Note: van’t Hoff factor

  12. Van’t Hoff factor • Strong electrolytes dissociate completely giving 2+ particles of solute per ionic compound. Example: MgCl2 dissolves into 3 particles unlike glucose. • So: “i” = measured value for electrolyte solution • Expected value for nonelectrolyte • “i” = changes “m” on all colligative properties

  13. Van’t Hoff Factor http://www.youtube.com/watch?v=SX5V7kzmGeU http://www.kentchemistry.com/KentsDemos.htm

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