Fun With Colligative Properties:
·
Try some
practice worksheets
Not
surprisingly, some of the properties of solutions change when you change the
concentration. Concentrated
solutions generally tend to be thicker, darker in color, and denser.
Colligative property: Any
property of a solution that changes when the concentration of the solution
changes.
Examples of colligative
properties and how they work:
·
Boiling point
elevation:
o
As you increase the concentration of a solution, the boiling point of
the solution increases.
o
The reason for this is that adding solute decreases the vapor pressure
of a solution because there are fewer molecules at the surface of the solution
that can vaporize. Because liquids
boil when their vapor pressure is equal to the ambient atmospheric pressure,
you need to heat them more to make them boil.
o
The relationship between concentration and boiling point is described
by the equation:
∆Tb
= Kbm
Where:
§
∆Tb is the change in boiling point
§
Kb is the ebullioscopic constant (0.520 C/m for
water)
§
m is the effective molality of the solute
o
“Effective molality” refers to the molality of particles in the solution.
§
For covalent compounds, the effective molality is the same as the
regular molality because the molecules don’t break into smaller pieces
when you put them in water.
·
For example, if you have a 0.75 m solution of CS2, the
effective molality is 0.75 m.
§
For ionic compounds, the effective molality is equal to the molality
times the number of ions in the compound.
·
For example, if you have a 0.75 m solution of NaCl, the effective
molality is 1.50 m (0.75 x 2)
·
More examples, if needed.
o
Sample problems:
·
What is the boiling point of a 1.75 m solution of H2CO? 100.910
C
·
What is the boiling point of a 1.75 m solution of NaCl? 101.820
C
·
What is the boiling point of a 1.75 m solution of Ca3(PO4)2? 104.550
C
·
Melting point
depression:
o
Solutions melt at lower temperatures than pure liquids because the
solute molecules disturb the intermolecular forces between the solvent
molecules. As a result, less energy
is needed to break up the solid.
o
The relationship between concentration and melting point is determined
by the equation:
∆Tf
= Kfm
Where:
§
∆Tf is the change in freezing point
§
Kf is the cryoscopic constant (1.860 C/m for
water)
§
m is the effective molality of the solute
o
The rules for determining effective molality are the same as for
boiling point. The only thing
different is the constant used.
§
Example: What is the
melting point of a 1.75 m solution of NaCl? -6.510
C
·
Vapor pressure decrease
o
The more concentrated a solution, the lower the vapor pressure (for the
reasons we described above with the BP elevation):
·
Osmotic pressure increase
o
Osmotic pressure is the force with which a pure solvent moves across a semi-permeable
barrier into a container that holds a solution (explain what a semi-permeable
barrier is).
o
This occurs because solvents tend to diffuse across barriers in ways
that will decrease the difference in concentration. The bigger the difference in
concentration (because of high solute concentration), the stronger the force of
osmosis.
·
Conductivity of electricity
o
Electrolytes: Solutions that are able to
conduct electricity.
§
These are solutions in which ionic compounds are the solute –
recall that ionic compounds are good at conducting electricity when dissolved
in water.
o
The more concentrated the solution of an ionic compound, the better it
is able to conduct electricity.
§
The larger the number of mobile ions present, the larger the number of electrons
that can be moved from one place to another.
© 2009 Cavalcade Publishing and Ian Guch,
may not be reproduced without permission.
For more information, email Ian Guch at misterguch@chemfiesta.com