Colligative Properties HONORS CHEMISTRY Mr. McIntosh Objective: Students will calculate BP elevation and FP depression.

Colligative Properties These properties DO NOT depend on the type of solute, instead they depend on the number of solute particles. The colligative properties are Boiling Point Elevation Freezing Point Depression Vapor Pressure Reduction

Vapor Pressure Reduction When the nonvolatile solute is added to a pure solvent, the solute molecules take up space at the surface of the liquid. This prevents solute molecules from leaving the liquid. Because the reduction in vapor pressure does not depend on the identity of the solute, VP reduction is a colligative property.

Boiling Point Elevation A solutions BP is higher than a pure solvent. Why use anti-freeze in the summer time? Ethylene Glycol is a solute which when added to water will raise the temperature at which it boils.

Boiling Point Elevation The presence of a solute raises the boiling point. In other words……. The Boiling Point of water is 100 Sugar water may boil at 110. The greater the concentration of solute (the more particles), the more it raises the BP.

Calculating BP Elevation ΔTb = Kbm Kb is a constant called the molal boiling point elevation constant. The value of Kb depends on the solvent. Waters Kb is 0.52C˚/m.

BP Practice Problem By how much will the BP of water be elevated if 100g of C12H22O11 is added to 500g of water? Molality of solution X BP constant. .29 moles sucrose .29mole/.500kg = .58molal .58 x 0.52C˚-m= 0.30˚C

Freezing Point Depression Why do we lay salt in the winter to de-ice the streets? How does it work? By adding solute to a solvent you will lower the freezing point. Why do you add antifreeze to a car in the winter? Antifreeze is added to a cars cooling system in the winter because it lowers the temperature at which the water will freeze.

Freezing Point Depression Like BP elevation, FP depression is directly proportional to amount of the solute. The greater the concentration of solute (the more particles), the more it lowers the FP. In other words……. The Freezing Point of water is 0°C Salt water may freeze at -15°C.

Calculating Freezing Point Depression ΔTf = Kfm Kf is a constant called the molal freezing point depression constant. The value of Kf depends on the solvent. Waters Kf is 1.86 C˚/m.

FP Practice problem What is the freezing point of an aqeous solution of .100g of C2H6O2 dissolved in .200kg of water? .0016 moles C2H6O2/.200kg water= .008 molal. .008 x 1.86 C˚/m= .015 ̊C Answer is - .015 ̊C

BP Elevation Question #1 Which will raise the Boiling Point of water more, adding… 1M CCl4 2M CCl4 Hint: 1M CCl4 means there is 1 mole of CCl4/Liter of water. 2M CCl4 will raise the BP more than 1M CCl4, because there is more particles present. It’s a higher concentration. The more particles, the HIGHER the BP and the LOWER the FP.

FP Depression Question #1 Which will lower the Freezing Point of water more, adding… 1M NaCl or 1M CH3OH Hint: What types of compounds are each of these? Covalent substances (made of all non-metals) in water stay whole. When an ionic compound (metal and a non-metal) dissolves in water it splits apart into 2 or more pieces. 2 for the price of one.

FP Depression Question #1 cont. In other words… NaCl → Na + Cl (1 particle becomes 2 particles in water) CH3OH → CH3OH ( 1 particle stays 1 particle in water) FP depression and BP elevation is directly proportional to the amount present. 2 Pieces in water is more than 1 so you will have a LOWER FP with NaCl added to water.

FP Depression Question #2 Which will lower the FP of water the most? 1M NaCl 1M CaCl2 NaCl → Na + Cl- (Splits into 2 pieces) CaCl2 → Ca + 2Cl- (Splits into 3 pieces)

Class work Colligative Properties Worksheet 1-7