Fun With Colligative Properties:

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·         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.