The change in the boiling point of a solution depends on the solvent and the molal
concentration of the solute is given byΔT = Kb m ΔT → change in the boiling point of a solution
Kb → boiling point elevation constant
m → molality of the solution
A common application of this effect in some parts of the world is in the use of antifreeze solutions in the cooling systems of automobiles in cold climates. "Antifreeze" compounds are usually organic liquids that are miscible with water so that, large freezing point effects can be attained.
In a solution, if the solutes do not dissociate into ions and having negligible vapor pressure (do not evaporate) even at boiling point of solvent, such solutions are called as non–volatile non–electrolyte solutions.
Pure liquids have a set of characteristic physical properties (melting point, boiling points, etc.). Solvents in a solution exhibit these same properties, but differ in the values from those of the pure solvent because of the presence of the solute.
The properties that undergo changes are boiling points , freezing points, vapor pressure, molecules. The properties depend on the collective effect of the number of solute particles, therefore termed as colligative properties (colligative means ‘collectivity’) and the Colligative Properties are :
- Elevation of boiling points
- Depression in freezing points
- Lowering of vapor pressure
- Osmotic pressure
Elevation of boiling points:
The boiling point of a liquid is the temperature at which
its vapor pressure equals the external pressure. Solutions
exhibit higher boiling points than the parent solvent. The
increase in boiling point is dependent upon the number of
solute particles in the solution. The greater the number
of solute particles (i.e. the concentration), the greater
is the boiling point elevation. This can be explained by
considering the vapor pressure, i.e. the vapor pressure of a
solution is said to be lower than that of a pure solvent,
because of which the solution doesn't boil. Therefore,
requires high temperature in order to raise the vapor
pressure of solution which should be equal to the external
pressure and thus, results in the elevation of boiling points.
The vapor pressure curve of the liquid and vapor phases meet at
a triple point. At the normal boiling point of the pure liquid,
the vapor pressure of the solution will be less than 1 atm. But,
in a solution containing solute, a higher temperature is required
to attain a vapor pressure of 1 atmosphere. Thus, the boiling point
of the solution is higher than that of pure liquid.
Depression in freezing point:
The freezing point of a solution is the temperature at
which the first crystals of pure solvent begin to form
in equilibrium with the solution. Solutions exhibit lower
melting points than the parent solvent. The decrease in
melting point is dependent upon the number of solute particles
in the solution. The greater the number of solute particles
(i.e., the concentration), the greater the depression in freezing
point as the solute particles interfere with the crystallization
process. This can be explained by considering the vapor pressure,
i.e., the vapor pressure of a solution is said to be lower
than that of a pure solvent because of which the solution
doesn't freeze. Therefore, requires low temperature in
order to lower the vapor pressure of solution.
In the process of osmosis, solution and solvent are separated by a semipermeable membrane, which allows
only solvent to pass through. Thus more solvent enters the solution than leaving it. As a result, solution volume increases,
so concentration decreases. At equilibrium, the difference in heights of two compartments reflects the osmotic pressure.
Osmotic Pressure:
This property of solutions is perhaps the least familiar of the
colligative properties, but in a sense it is more important than
those already mentioned. A semi–permeable membrane may be
defined as a material that allows molecules of one kind to pass
through it but prevents the passage of other kinds of molecules
or allows the passage of different kinds of molecules at
different rates. Membranes often permit the passage of
solvent molecules and prevent the passage of solute molecules.
This phenomenon is known as osmosis and is of far–reaching
importance in biology, medicine and related areas.
According to Raoult's law, "The partial pressure of any
volatile component of a solution at any temperature is equal to
the product of vapor pressure of the pure component and the mole
fraction of that component in the solution".
Psolvent is the partial pressure of a solvent.
P0solvent is the vapor pressure of a pure solvent.
Xsolvent is the mole fraction of solvent. An
ideal solution is the one that follows the Raoult's
law at any concentration.
Ideal solutions obey Raoult's law
Ideal solution:
In terms of energy, an ideal solution is one in which
the energy released during formation of solution is equal
to the energy used to break the attractive forces within a
solute and solvent before forming a solution.
But most solutions deviate from ideality which leads to
the deviations in vapor pressure of Raoult's law.
These deviations are of two kinds – positive deviations
and negative deviations.
Positive deviations
Positive deviations occur in non–ideal solutions in which the attractions between
solute–solvent molecules are lesser than those between solute–solute and solvent–solvent molecule.
Negative deviations
Negative deviations are due to the reason that intermolecular attractive forces between
solute–solvent molecules are stronger than those between solute–solute and solvent–solvent molecules.
Positive
deviations:
For non–ideal mixtures,
the actual vapor pressure of a solution can be larger than the
ideal value from Raoult's law because of the solute and
the solvent. Since the deviation of vapor pressure is more
than expected from Raoult's law, this deviation is referred
as positive deviation.
Negative deviations:
For non–ideal mixtures, the actual vapor pressure
of a solution can be smaller than the ideal value from
Raoult's law. Since the deviation of vapor pressure
is less than expected from Raoult's law, this deviation
is referred as negative deviation.
The pure solvent has a higher vapor pressure than the solution. This is due to the reason that solute
molecules decrease the surface area which is available for solvent molecules to vaporize. Limited number of molecules tends to
escape leading to the decrease in vapor pressure.
Lowering of vapor pressure by
non–volatile non–electrolyte solutions:
In non–volatile non–electrolyte solutions,
the solute molecules which neither dissociate, nor ionize
occupy the surface of the solution, because of which the solvent
molecules cannot vaporize more readily at
normal temperatures and pressures. Therefore,
the equilibrium between the solvent molecules
in the solution and the solvent molecules above
the surface of the solution (vapor phase) get
disturbed resulting in the lowering of vapor pressure of the solution.
The vapor pressure for solutions
containing (both solute and solvents are) volatile
liquids is given by :
Pactual = X1P01 + X2P02 + .......
The subscripts 1 and 2 represent the two
different volatile liquids in the solution (solute and solvent).