Freezing Point Depression & Boiling Point Elevation

 

Two of the most familiar of the colligative changes comes from adding chemicals to solutions to change the boiling point or freezing point.  These are more common than one would expect as we will discuss.  Since the addition of a non-volatile solute changes the vapor pressure of the solution to a lower value the state equilibrium lines must also move.  The rate at which the different states of matter change their vapor pressure is highest for gases and lowest for solids.

Freezing point depression is the temperature change (lowering) of the freezing point of a solvent caused by adding a [usually] ionic solute.  This process is demonstrated anytime that someone makes ice cream or puts salt on the driveway to melt it.  The ionic solid interferes with the attraction between the water molecules causing them to have a lower vapor pressure.  As the temperature is lowered, the vapor pressure of the liquid decreases faster than the vapor pressure of the solid and eventually the solution will freeze, however it is at a lower temperature.  The formula for calculating the change in freezing point is ΔTf = mKf where ΔTf is the change in the freezing point temperature, m is the molality and Kf is the freezing point constant (1.853˚C Kg/mol for water).

            Another colligative property change that can be made is the elevation of the boiling point. By adding a chemical like ethylene glycol (anti-freeze) to the water in your vehicle’s radiator, it not only protects the engine from freezing, but it also stops the water from boiling at the vehicles high operating temperature. Boiling point elevation is the temperature change (raising) of the boiling point of a solvent caused by adding a solute.  If the vapor pressure is lowered, the relative number of particles becoming a gas is decreased.  In order to reach an equilibrium between vapor pressure and atmospheric pressure, it becomes necessary to add more energy (heat) to force the particles at the surface of the liquid to release to the gaseous state.  This increase results in the liquid boiling at a higher temperature.  The formula ΔTb = mKb calculates the change in the boiling point ΔTb by multiplying the molality, m, by the boiling point constant Kb (0.515˚C K/mol for water).

 

Consider adding 334g of sodium chloride to 500mL of water, by how much does the freezing point change?

 

The gfm for NaCl is 58.5g

 

334gNaCl/58.5g/mol NaCl = 4mol NaCl

 

500mL of H2O is 500g or 0.500Kg of H2O

 

4mol NaCl/0.500Kg H2O = 8m NaCl in H2O or 8mol/Kg NaCl in H2O

 

ΔTf = mKf

 

ΔTf = 8m(1.853˚C Kg/mol) = 14.8˚C

 

This means that the new freezing point for this salt water solution is –14.8˚C

 

 

Consider adding 1Kg of antifreeze (ethylene glycol, C2H6O2) to 4L of water in a car’s radiator, what is the new boiling point?

 

The gfm for ethylene glycol is 62g/mol

 

4L of H2O = 4Kg of H2O

 

1Kg of C2H6O2 = 1,000g of C2H6O2

 

1,000g C2H6O2/62g/mol = 16.13mol

 

16.13mol C2H6O2/4Kg H2O = 4.03m

 

ΔTb = mKb

 

ΔTb = 4.03m(0.515˚C Kg/mol) = 2.08˚C

 

This means that the water in the radiator will now boil at 102.08˚C instead of 100.00˚C.