Freezing Point Depression & Boiling Point Elevation
I. VIDEO
A. Boiling Point Elevation
B. Freezing Point Depression
II. NOTES ON BOILING POINT ELEVATION & FREEZING POINT DEPRESSION
A. Boiling Point Elevation
The boiling point of a solution is the temperature at which its vapor pressure equals the external atmospheric pressure. The presence of the nonvolatile solute lowers the the vapor pressure of a solution because the solutes literally block the solvent molecules from going into gaseous state. As a result, it takes more energy for the vapor pressure to equal the atmospheric thereby elevating the boiling point of the solution.
Therefore,
Δ Tb= Kb m
where
New Boiling Point= Original Boiling Point + Δ Tb
A. Freezing Point Depression
The freezing point depression can be explained in the following manner. Freezing involves a transition from the disordered states to the ordered state. For this to to happen, energy must be removed from the system. Because a solution has greater disorder than the solvent, more energy is needs to be removed from it to create order than in than in the case of a pure solvent. Therefore, the solution has a lower freezing point that its solvent. Note that when a solution freezes, the solid that separates is the pure solvent component.
Therefore,
Δ Tf= Kf m
where
New Freezing Point= Freezing Point - Δ Tf
III. EXERCISES ON BOILING POINT ELEVATION & FREEZING POINT DEPRESSION
