Colligative Properties

What are colligative properties?

Physical properties

Based on # of solute particles, NOT identity!!!

4 properties1) Vapor Pressure Lowering

2) Freezing Point Depression

3) Boiling Point Elevation

4) Osmotic Pressure

1) Vapor Pressure Lowering

Nonvolatile vs. volatile chemicals Nonvolatile

Not likely to be a gasTakes awhile for gas particles to evaporate

Volatile Gas particles evaporate quickly

Vapor PressurePressure exerted from gas molecules on a liquidCharacteristic of a liquid

Volatile vs. Nonvolatile Liquids

1) Volatile LiquidsWEAK intermolecular forcesHigh vapor pressureEx. Gasoline, alcohols, “Whoosh Bottle”

2) Nonvolatile Liquids STRONG intermolecular forces Low vapor pressure

More Vapor Pressure….

INCREASED vapor pressurePure 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 + soluteNumber of particles in the solution is increasedLess 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 ANDFreezing 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.

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?

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?

Homework Colligative Properties Worksheet #1-8

Calculate molality and freezing point ONLY

3) Boiling Point Elevation

Boiling point— 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?

Example 2: A solution contains 50.0g of sucrose (C12H22O11)

dissolved in 500.0g of water. What is the boiling-point elevation?

Example 3: A solution has 450.0g of sucrose (C12H22O11)

dissolved in 250g of water. What is the solution’s boiling point?

Boiling Point and Vapor Pressure

Substances with low vapor pressureMore energy needed for vapor pressure =

atmospheric pressure SOBoiling point INCREASES

** Solutes in a solution LOWER vapor pressure so boiling point INCREASES !

Osmosis

Semipermeable membraneMembrane selects what particles can go through

and blocks other particles

Movement of water through a semipermeable membraneBased on solute concentration (HighLow solute

concentration)

4) Osmotic Pressure

Pressure exerted by solute particles resisting osmosis

INCREASE osmotic pressureHigh solute concentrationWater drawn in

DECREASE osmotic pressure Low solute concentrationWater 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