Colligative Properties

What are colligative properties? Physical properties Based on # of solute particles, NOT identity!!! 4 properties Vapor Pressure Lowering Freezing Point Depression Boiling Point Elevation Osmotic Pressure

1) Vapor Pressure Lowering Nonvolatile vs. volatile chemicals Nonvolatile Not likely to be a gas Takes awhile for gas particles to evaporate Volatile Gas particles evaporate quickly Vapor Pressure Pressure exerted from gas molecules on a liquid Characteristic of a liquid

Volatile vs. Nonvolatile Liquids WEAK intermolecular forces High vapor pressure Ex. Gasoline, alcohols, “Whoosh Bottle” Nonvolatile Liquids STRONG intermolecular forces Low vapor pressure

More Vapor Pressure…. INCREASED vapor pressure Pure solvent Particles can easily move between the liquid and gas phase to create an equal amount of particles in each phase (equilibrium) DECREASED vapor pressure Solvent + solute Number of particles in the solution is increased Less water/solvent can evaporate

2) Freezing Point Depression “difference between freezing point of a pure solvent and a solution containing a nonelectrolyte.” Difference between Freezing point of pure solvent AND Freezing point of solvent + solute/substance **Adding a solute/substance LOWERS the freezing point.

Molality (m) Another way to represent a solution’s concentration Not as common as molarity Used when determining colligative properties of solutions (boiling point elevation, freezing point depression)

Molality (m) Molality (m) = Moles solute mass of solvent (kg)

Example 1 A solution has 17.1g of sucrose (C12H22O11) dissolved in 125g of water. Calculate the molal concentration of the solution. 0.400mC12H22O11

Calculating Freezing-Point Depression Δtf = iKfm Δtf = change in freezing temperature (°C) m = molality of solution Kf = molal freezing point lowering constant (°C/m) For water, 1.86 °C/m

Van Hoff factor (i) Constant in the equation for freezing-point depression calculations Most of the time, we will ignore this factor. So equation become…. Δtf = -Kfm Indicates how much dissociation has occurred in the solution 1 = compound does not dissocation 2 = compound dissociates into 2 ions (ex. LiCl) 3 = compound dissociates into 3 ions (ex. CaCl2)

Example 1: 152g of sodium sulfate, Na2SO4, are dissolved in 875g H2O. What is the freezing point of the solution? -6.8°C

Example 2: What is the freezing point depression (Δtf) of water in a solution of 17.1 sucrose (C12H22O11) in 200g of water? What is the actual freezing point of the solution? Δtf = -0.465 °C Freezing point of solution = -0.465 °C

Homework Colligative Properties Worksheet #1-8 Calculate molality and freezing point ONLY

3) Boiling Point Elevation Temperature where a liquid’s vapor pressure = atmosphere pressure **Boiling point changes result from changes in vapor pressure. “difference between boiling point of a pure solvent and a solution containing a nonelectrolyte” Difference between Boiling point of pure solvent AND Boiling point of solvent + solute/substance

Calculating the Boiling Point Elevation ΔTb = iKbm ΔTb = change in boiling temperature (°C) m = molality of solution Kb = molal boiling point lowering constant (°C/m) For water, 0.52 °C/m

Van Hoff factor (i) Constant in the equation for freezing-point depression calculations Most of the time, we will ignore this factor. So equation become…. ΔTb = -Kbm Indicates how much dissociation has occurred in the solution 1 = compound does not dissocation 2 = compound dissociates into 2 ions (ex. LiCl) 3 = compound dissociates into 3 ions (ex. CaCl2)

Example 1: If 90.0g of nonionizing glucluse (C6H12O6) are dissolved in 255g of water, what is the resulting solution’s boiling point? 101.04 degrees C

Example 2: A solution contains 50.0g of sucrose (C12H22O11) dissolved in 500.0g of water. What is the boiling-point elevation? 0.15 degrees C

Example 3: A solution has 450.0g of sucrose (C12H22O11) dissolved in 250g of water. What is the solution’s boiling point? 102.7 degrees C

Boiling Point and Vapor Pressure Substances with low vapor pressure More energy needed for vapor pressure = atmospheric pressure SO Boiling point INCREASES ** Solutes in a solution LOWER vapor pressure so boiling point INCREASES !

Osmosis Semipermeable membrane Membrane selects what particles can go through and blocks other particles Movement of water through a semipermeable membrane Based on solute concentration (HighLow solute concentration)

4) Osmotic Pressure Pressure exerted by solute particles resisting osmosis INCREASE osmotic pressure High solute concentration Water drawn in DECREASE osmotic pressure Low solute concentration Water driven out

Electrolytes and Colligative Properties Electrolytes GREATLY influence colligative properties. Electrolytes---dissociate into ions in solution Ions increase the NUMBER of solute particles in a solvent.

Homework Colligative Properties Worksheet