Colligative Properties The Chemistry of Crowding

Colligative Explained All substances have unique physical and chemical properties. When substances are mixed together, their properties are modified by the interactions with other particles. Properties that depend on the number of particles mixed together (and not the identity) are called Colligative Properties.

And They Are… There are four main Colligative Properties: Vapor Pressure Suppression Boiling Point Elevation Freezing Point Depression Osmotic Pressure They arise from the interactions of the individual particles.

Holding It Back Vapor Pressure is Suppressed Particles near the surface vaporize Solute with low vapor pressure gets in the way. Results in a lowering of vapor pressure.

Calculating Vapor Pressure Raoult’s Law: P solution = P° A Χ A + P° B Χ B + P° C Χ C + … P° A = vapor pressure of pure substance Χ A = mole fraction of substance P° of salts ~ 0, so P soln = P° A Χ A

Boiling Points Solute particles get in the way of moving solution particles, robbing them of K.E. Requires more energy to get particles up into the gas phase (vaporize). ΔT = T b(soln) – T b(solvent) [Can be decreased if more volatile solute is added]

Freezing Points Solute particles get in the way of particles coalescing into crystals. Requires removal of more energy to get particles to settle into crystals (freeze). ΔT = T f(soln) – T f(solvent) [Cannot be increased, regardless of solute added]

Factors Affecting Δ T Δ T is dependent upon the number of particles dissolved in a solvent, not the identity of the solute. Number of particles = moles New Concentration: Molality m = moles solute kg solvent

More Δ T Formulas! Boiling Point Elevation: ΔT = iK b m Freezing Point Depression: ΔT = -iK f m K b = ebullioscopic constant (boiling point elevation constant) K f = cryoscopic constant (freezing point depression constant) i = van ’t Hoff Factor = number of particles formed by the solute when dissolved.

Calculating Δ T What is the boiling point of a solution containing 5.00 g NaCl in mL of water? K b = 0.512°C·kg/mol = °C/m ΔT = iK b m m = 5.00 g NaCl / 58.44g/mol = m kg ΔT = (2)(0.512°C·kg/mol)(0.171 m) = 0.175°C T b = 100.0°C °C = = 100.2°C

It’s Water Power! Osmotic Pressure (Π) The difference in pressure between a pure solvent and a solution connected by a semi-permeable membrane. Can be though of as the pressure required to stop osmosis!

It’s Air Power?!? Osmotic Pressure (Π) is related to solute concentration and the absolute temperature. Because of this, the formula to calculate osmotic pressure is…the ideal gas law… Π = nRTi / V Or, if you know the molarity of the solution, Π = MRTi [This is, of course, for Ideal Solutions]