Colligative properties of solutions
The Effects of Solutes on Solvents
Objectives
• When you complete this presentation, you will be
able to
o define a colligative property
o explain how colligative properties are dependent
on the number of particles in solution
o describe the effects on the numbers of particles
in a solution on the vapor pressures, freezing
points, boiling points, and osmotic pressures of
those solutions
Introduction
• The physical properties of a solution differ from
the properties of the pure solvent.
o Salt added to water decreases the freezing point
of the solution.
o Copper(II) sulfate added to water changes the
color.
• When a property depends on the number of
particles of solute in a solution, that is a colligative
property.
Introduction
• Colligative properties depend only on the number
of solute particles, not on identity of the solute.
• Different solutes produce different numbers of
solute particles when they are dissolved.
• Ionic compounds produce more particles per mol
of solute dissolved than do molecular compounds.
Introduction
• For example:
• NaCl produces two mols of particles for each
mol of salt dissolved:
+
»1 mol of Na
and
»1 mol of Cl
• Glucose, C6H12O6, produces one mol of particles
for each mol of glucose dissolved.
Introduction
• There are many different kinds of colligative
properties.
• Important colligative properties of solutions that
we will study include:
• vapor-pressure lowering
• freezing-point depression
• boiling-point elevation
• osmotic pressure increase
Vapor-pressure lowering
• Vapor pressure is the pressure exerted by a vapor
that is in equilibrium with its liquid in a closed
system.
• When we add a solute to pure liquid, we reduce
the number of particles of liquid that are available
to be converted to vapor.
pure liquid
solution
Vapor-pressure lowering
• In the pure liquid, there are 8 solvent particles
available to go into vapor.
• In the solution, there are only 6 solvent particles
available to go into vapor.
pure liquid
solution
Vapor-pressure lowering
• If we have more solute, there are even fewer
solvent particles available.
pure liquid
solution
Vapor-pressure lowering
• If we have fewer solute, there are more solvent
particles available.
pure liquid
solution
Vapor-pressure lowering
• The decrease in vapor pressure is proportional to
the number of particles in solution.
• Adding one mol of NaCl produces twice the vapor
pressure lowering of adding one mol of glucose.
pure liquid
solution
Freezing-point depression
• Adding a solute to a solvent lowers the freezing
point of the solution.
• The lowering is proportional to the number of
particles of the solute added to the solvent.
• 1 mole of glucose added to 1 kg of water results
in a fp lowering of 1.86°C.
• 1 mole of NaCl added to 1 kg of water results in
a fp lowering of 3.72°C.
Freezing-point depression
• This is used for • keeping the roads ice-free in the winter
•
home-made ice cream
•
adding antifreeze to your car
Boiling-Point elevation
• Adding a solute to a solvent also raises the boiling
point of the solution.
• The elevation is also proportional to the number
of particles of the solute added to the solvent.
• 1 mole of glucose added to 1 kg of water results
in a bp elevation of 0.51°C.
• 1 mole of NaCl added to 1 kg of water results in
a bp elevation of 1.02°C.
Boiling-Point elevation
• This is used for • adding to your car’s cooling system for the
summer
Osmotic Pressure
• Osmosis is the spontaneous movement of
solvent molecules through a partially
permeable membrane into an area of higher
solute concentration.
• This process is used extensively in biological
systems.
o Biological membranes are semipermeable.
Osmotic Pressure
In an osmotic cell, we have two solutions of different
concentrations divided be a semipermeable membrane.
Osmotic Pressure
Pure water will move through the semipermeable
membrane from the cell on the right into the cell on the
left.
Osmotic Pressure
This causes the water level on the left to rise and the water
level on the right to fall.
Osmotic Pressure
This causes the water level on the left to rise and the water
level on the right to fall.
Osmotic Pressure
At a certain point, the pressure from the height of the
water on the left will cause water to flow back into the cell
on the right and the system will come to an equilibrium.
Osmotic Pressure
The difference in liquid levels between the two cells is a
measure of the osmotic pressure of the system.
Osmotic Pressure
A system with a greater number of solute particles in one
cell will have a greater osmotic pressure.
Osmotic Pressure
A system with a greater number of solute particles in one
cell will have a greater osmotic pressure.
Osmotic Pressure
If we raise the solution level of the left cell only, there will
be more pressure to move pure water to the right than
there is pressure to move water to the left.
Osmotic Pressure
If we raise the solution level of the left cell only, there will
be more pressure to move pure water to the right than
pressure to move water to the left.
Osmotic Pressure
Water will move from the left cell to the right cell until the
equilibrium is reestablished.
Osmotic Pressure
This is called reverse osmosis and is used around the world
to purify water.
Osmotic Pressure
Membranes are fragile and expensive, so the process is
used only where other purification systems will not work.
Summary
• The physical properties of a solution differ from
the properties of the pure solvent.
• When a property depends on the number of
particles of solute in a solution, that is a colligative
property.
• Colligative properties depend only on the number
of solute particles, not on identity of the solute.
• Ionic compounds produce more particles per mol
of solute dissolved than do molecular compounds.
Summary
• Important colligative properties of solutions
include vapor-pressure lowering, freezing-point
depression, and boiling-point elevation.
• When we add a solute to pure liquid we o reduce the vapor pressure that is exerted over
the surface of the liquid.
o lower the freezing point of the liquid.
o raise the boiling point of the liquid.
o increase the osmotic pressure of the liquid.