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

Weak electrolyte
  Does

dissociates in aqueous solution
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

Raoult’s Law
Vapor pressure
of solvent in a
solution mixture
Mole fraction of
the solvent in
solution
Vapor pressure
of pure solvent
Vapor Pressure Lowering

Raoult’s 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 50.0°C. 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
Molal boiling
point elevation
constant
Molality of
solution
Freezing Point Depression
Change in
freezing point
Molal freezing
point depression
constant
Molality of
solution
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 (π) is the pressure required to stop osmosis.
Osmotic Pressure
Osmotic
pressure
Ideal Gas
Law constant
Temperature
Molarity of solution
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
If the solute is volatile, it will contribute to the vapor pressure:
Ideal Solution
When IMFs of solute is
almost equal to IMFs of
solvent
Such that the solutesolvent interaction is
almost the same as the
solute-solute and the
solvent-solvent
PT is greater than
predicted by Raoult’s law
IMF
A-B
<
IMF
A-A
&
ΔHsol’n > 0
IMF
B-B
PT is less than
predicted by Raoult’s law
IMF
A-B
>
IMF
A-A
&
ΔHsol’n < 0
IMF
B-B
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
ΔP = i[ X solute * P
0
solvent
ΔTb = i[ k b m]
ΔTf = i[ k f m]
Π = i[ MRT ]
]
For example:
NaCl dissolves to Na+ and
Cl- so that the van’t 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 van’t Hoff factor is not really theoretical value but
rather experimental:
PROPERTIES OF COLLOIDS
COLLOIDS – a
dispersed substance is
distributed throughout
a medium.
Particles are larger
than simple molecules
but small enough not
to settle down/out.
SUSPENSION
SOLUTION
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
