What is a Colligative Property?

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2 What is a Colligative Property? 0 Defined as bulk liquid properties that change when you add a solute to make a solution 0 Colligative properties are based on how much solute is added but NOT the identity of the solute 0 Effect of electrolytes is based on number of ions produced 0 We will discuss the four colligative properties: Vapor pressure lowering Freezing point depression Boiling point elevation Osmotic pressure

3 Ionic vs Molecular Solutes 0 Ionic solutes produce two or more ion particles in solution 0 They affect the colligative properties proportionately more than molecular solutes that do not ionize 1 mol/kg of water for glucose = 1 molal 1 mol/kg of water for NaCl = 2 molal ions 1 mol/kg of water for CaCl 2 = 3 molal ions 0 The effect is proportional to the number of particles of the solute in the solution!

4 Colligative Property #1 Vapor Pressure Lowering 0 When a solute is added to a solvent, the particles of solute are attracted to the solvent particles 0 As a result, the solvent particles have less kinetic energy and tend to escape less into the space above the liquid 0 So, the vapor pressure of the solvent in a solution is decreased by the solute! 0 The more solute is present, the more the vapor pressure of the solvent is lowered! 0 Based on Raoult s Law: 0 P soln = X solvent P solvent

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6 Colligative Property #2 Boiling Point Elevation 0 Recall, boiling point is the temperature at which the vapor pressure of liquid becomes equal to the atmospheric pressure 0 When you add a nonvolatile solute to a solvent, the boiling point goes up 0 This is because the vapor pressure has been lowered! 0 The boiling point will continue to be elevated as you add more solute until you reach saturation 0 Mathematically, boiling point elevation is determined by: DT bp = T elevated T normal = i K bp m 0 Note m is the molality of the solute particles 0 i is the van t Hoff factor represents # of particles ionic solute dissociates into! 0 K bp is the boiling point elevation constant (K bp for H 2 O = 0.51 o C/m) 0 This colligative property is the reason why you cook pasta in salt water and use antifreeze in your car!

7 Example Boiling Point Elevation

8 Colligative Property #3 Freezing Point Depression 0 When you add a solute to a solvent, the freezing point goes down! 0 The freezing point will continue to be lowered as you add more solute until you reach saturation 0 Mathematically, freezing point depression is determined by: DT fp = T normal - T depressed = i K fp m 0 Note m is the molality of the solute particles 0 i is the van t Hoff factor represents # of particles ionic solute dissociates into! 0 K fp is the freezing point depression constant (K fp for H 2 O = 1.86 o C/m) 0 This is why salt is added to roads in winter!

9 Colligative Property #4 Osmotic Pressure 0 Osmosis is the movement of a solvent NOT solute through a semi - permeable membrane 0 A great example of a semi-permeable membrane is a cell wall! 0 Osmotic pressure is the pressure difference needed to stop the flow of solvent across a semi-permeable membrane 0 The osmotic pressure of a solution is proportional to the molar concentration of the solute particles in solution: =i n R T =i MRT V i M T R = van t Hoff factor = molar concentration of solute = temperature in Kelvin = gas law constant ( L atm/mol K)

10 Osmotic Pressure and Biochemistry 0 Biologists and biochemists often take advantage of osmotic pressure when they isolate components of a cell 0 When a cell is added to an aqueous solution that contains a much higher concentration of ions (hypertonic solution) than the liquid within the cell (hypotonic cell), water leaves the cell by flowing through the cell membrane 0The cell membrane shrinks so much that the membrane breaks 0 Called crenation 0 Alternately, when a cell is placed in a solution that has a much smaller ionic strength, water pours into the cell 0The cell expands until the cell membrane bursts 0 Called hemolysis

11 Osmotic Pressure Crenation Hemolysis