COLLIGATIVE PROPERTIES AND COLLOIDS

Colligative Properties

collective The property is governed not by the chemical

identity (or chemical properties) but by the number of particles of solute.

These properties include: Boiling point elevation

Vapor pressure lowering Freezing point depression Osmotic pressure

Types of solutes

Strong electrolyte Completely dissociates in aqueous solution

Weak electrolyte Does not completely dissociate in aqueous solution

Non-electrolytes Does not dissociate in aqueous solution

Nonvolatile and Non-electrolyte

Simplest case 1 particle of solute 1 particle in solution Solute will not become gas, solute will not solidify

Vapor Pressure Lowering

Vapor Pressure Lowering

Raoults Law

Vapor pressure of solvent in a solution mixture

Vapor pressure of pure solvent

Mole fraction of the solvent in solution

Vapor Pressure Lowering

Raoults Law

Vapor Pressure Lowering

Calculate the vapor pressure lowering (P) when 10.0 mL of glycerol (C3H8O3) is added to 500. mL of water at 50.0C. At this temperature, the vapor pressure of pure water is 92.5 torr and its density is 0.988 g/mL. The density of glycerol is 1.26 g/mL

0.461 torr

Boiling Point Elevation

Because vapor pressure is lowered at a particular temperature, we expect the vapor pressure at the normal boiling point to be lower than 1 atm.

In order for solution to boil (to reach 1 atm of vapor pressure), we need a higher temperature than the original boiling point

Boiling Point Elevation

Freezing Point Depression

BP Elevation and FP Depression

Boiling Point Elevation

Change in boiling point

Molality of solution Molal boiling

point elevation constant

Freezing Point Depression

Change in freezing point

Molality of solution Molal freezing

point depression constant

BP Elevation and FP Depression

You add 1.00 kg of ethylene glycol (C2H6O2) antifreeze to your car radiator which contains 4450 g of water. What are the boiling and freezing points of the solution?

Kf of water 1.86 C/m Kb of water 0.512 C/m

101.85 C and -6.73 C

Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one.

A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules.

Osmotic pressure () is the pressure required to stop osmosis.

Osmotic Pressure

Osmotic pressure

Molarity of solution

Temperature

Ideal Gas Law constant

Osmotic Pressure

isotonic solution

hypotonic solution

hypertonic solution

SWEATING

SPORTS DRINK

Volatile Solutes

If the solute is volatile, it will contribute to the vapor pressure:

In the liquid phase

Volatile Solutes

In the vapor phase

Putting them together

Ideal Solution

If the solute is volatile, it will contribute to the vapor pressure:

When IMFs of solute is almost equal to IMFs of solvent

Such that the solute-solvent interaction is almost the same as the solute-solute and the solvent-solvent

PT is greater than predicted by Raoults law

PT is less than predicted by Raoults law

IMF A-B

IMF A-A

IMF B-B < &

IMF A-B

IMF A-A

IMF B-B > &

Hsoln > 0 Hsoln < 0

Volatile Solutes

Take equimolar amounts of benzene (benz) and toluene (tol) with vapor pressure of pure compounds at 95.1 and 28.4 torr

Volatile Solutes

In the vapor phase

The main component is the more volatile one!

Strong Electrolytes

1 particle of solute dissociates into more than 1 particle in solution

Tf = i k f m[ ]

Tb = i kbm[ ]

= i MRT[ ]

P = i Xsolute *Psolvent0[ ] For example:

NaCl dissolves to Na+ and Cl- so that the vant Hoff factor is now 2.

For nonelectrolytes i=1

For weak electrolytes is usually not a whole number

Strong Electrolytes

Strong electrolytes are not ideal! Because of ionic atmosphere

Strong Electrolytes

So vant Hoff factor is not really theoretical value but rather experimental:

PROPERTIES OF COLLOIDS

SUSPENSION SOLUTION

COLLOIDS a dispersed substance is distributed throughout a medium.

Particles are larger than simple molecules but small enough not to settle down/out.

Brownian Motion and Tyndall Effect

Brownian Motion physical phenomenon where minute particles immersed in a fluid move about randomly.

Tyndall Effect light scattering due to dispersed particles