Colligative properties of solutions

1
Colligative properties of solutions

• The Effects of Solutes on Solvents

2
Objectives

• When you complete this presentation, you will be
  able to
• define a colligative property
• explain how colligative properties are dependent
  on the number of particles in solution
• 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

3
Introduction

• The physical properties of a solution differ from
  the properties of the pure solvent.
• Salt added to water decreases the freezing point
  of the solution.
• 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.

4
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.

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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.

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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

7
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.

8
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.

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Vapor-pressure lowering

• If we have more solute, there are even fewer
  solvent particles available.

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Vapor-pressure lowering

• If we have fewer solute, there are more solvent
  particles available.

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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.

12
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 1.86C.
• 1 mole of NaCl added to 1 kg of water results in
  a fp lowering of 3.72C.

13
Freezing-point depression

• This is used for -
• keeping the roads ice-free in the winter

• home-made ice cream

• adding antifreeze to your car

14
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 0.51C.
• 1 mole of NaCl added to 1 kg of water results in
  a bp elevation of 1.02C.

15
Boiling-Point elevation

• This is used for -
• adding to your cars cooling system for the summer

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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.
• Biological membranes are semipermeable.

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Osmotic Pressure
In an osmotic cell, we have two solutions of
different concentrations divided be a
semipermeable membrane.
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Osmotic Pressure
Pure water will move through the semipermeable
membrane from the cell on the right into the cell
on the left.
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Osmotic Pressure
This causes the water level on the left to rise
and the water level on the right to fall.
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Osmotic Pressure
This causes the water level on the left to rise
and the water level on the right to fall.
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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.
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Osmotic Pressure
The difference in liquid levels between the two
cells is a measure of the osmotic pressure of the
system.
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Osmotic Pressure
A system with a greater number of solute
particles in one cell will have a greater osmotic
pressure.
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Osmotic Pressure
A system with a greater number of solute
particles in one cell will have a greater osmotic
pressure.
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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.
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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.
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Osmotic Pressure
Water will move from the left cell to the right
cell until the equilibrium is reestablished.
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Osmotic Pressure
This is called reverse osmosis and is used around
the world to purify water.
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Osmotic Pressure
Membranes are fragile and expensive, so the
process is used only where other purification
systems will not work.
30
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.

31
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 -
• reduce the vapor pressure that is exerted over
  the surface of the liquid.
• lower the freezing point of the liquid.
• raise the boiling point of the liquid.
• increase the osmotic pressure of the liquid.