Colligative PropertiesChapter 13.6

Colligative Properties• “Collective”-nature of• Effect on 4 properties: 1) Vapor pressure lowering2)Boiling point elevation3) Freezing point depression4) 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.

• Psolvent = Xsolvent 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

• http://www.kchemistry.com/AddedTopics_1.htm

Boiling Point ElevationA solution boils at a higher temperature than the pure solventΔTb= boiling point elevation

ΔTb = Kb m

Kb = boiling pt elevation constant

m = solution molality

ΔTb= Tb(solution) - Tb(solvent)

Boiling point Elevation

http://www.btinternet.com/~martin.chaplin/collig.html

Freezing point depression

http://www.sparknotes.com/chemistry/solutions/colligative/section1.rhtml

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)

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

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

http://www.youtube.com/watch?v=a2qOZmuhDBI&feature=related

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

Van’t Hoff Factor

http://www.kentchemistry.com/KentsDemos.htm

http://www.youtube.com/watch?v=SX5V7kzmGeU