Colligative Properties HONORS CHEMISTRY Mr Mc Intosh Objective

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

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

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

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

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

Calculating BP Elevation n ΔT b n Kb is = K bm a constant called the molal boiling point elevation constant. n The value of Kb depends on the solvent. n Waters Kb is 0. 52 C˚/m.

BP Practice Problem n By how much will the BP of water be elevated if 100 g of C 12 H 22 O 11 is added to 500 g of water? n Molality of solution X BP constant. n. 29 moles sucrose n. 29 mole/. 500 kg =. 58 molal n. 58 x 0. 52 C˚-m= 0. 30˚C

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

Calculating Freezing Point Depression n ΔT f n Kf is = K fm a constant called the molal freezing point depression constant. n The value of Kf depends on the solvent. n Waters Kf is 1. 86 C˚/m.

FP Practice problem n What is the freezing point of an aqeous solution of. 100 g of C 2 H 6 O 2 dissolved in. 200 kg of water? n. 0016 moles C 2 H 6 O 2/. 200 kg water=. 008 molal. n. 008 x 1. 86 C˚/m=. 015 C n Answer is -. 015 C

BP Elevation Question #1 n n Which will raise the Boiling Point of water more, adding… 1 M CCl 4 2 M CCl 4 n Hint: 1 M CCl 4 means there is 1 mole of CCl 4/Liter of n 2 M CCl 4 will raise the BP more than 1 M CCl 4, because there is more particles present. It’s a higher concentration. The more particles, the HIGHER the BP and the LOWER the FP. 1. n water.

FP Depression Question #1 1. n n n Which will lower the Freezing Point of water more, adding… 1 M Na. Cl or 1 M CH 3 OH 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. n n n n In other words… Na. Cl → Na + Cl (1 particle becomes 2 particles in water) CH 3 OH → CH 3 OH ( 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 Na. Cl added to water.

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

Class work n Colligative Properties Worksheet 1 -7