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 These ions conduct electricity Strong electrolytes (soluble salts, strong acids and strong bases) conduct current well because they dissociate completely

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. P solvent = X solvent P° solvent Recognize that X is always <1 therefore, the new pressure is ALWAYS lower.

 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

 Boiling Point Elevation A solution boils at a higher temperature than the pure solvent ΔT b = boiling point elevation ΔT b = K b m K b = boiling pt elevation constant m = solution molality ΔT b = T b(solution) - T b(solvent)

Boiling point Elevation

 Freezing point depression

Freezing Point Depression ΔT f = K f m ΔT f = freezing point depression K f = molal freezing point depression constant m = molal concentration of the solute ΔT f = T f(solvent) – T f(solution)

 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 = n solute RT = MRT V soln

Overarching Colligative Pts! 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

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

Van’t Hoff Factor