Unit – IV A
Boiling Point Elevation:
Boiling Point of a Liquid:
- Boiling point is defined as the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure.
- Boiling point of a liquid is a characteristic property and can be treated as criteria for purity of liquid.
- It increases with the increase in external pressure.
- Liquids having greater intermolecular forces have high boiling points.
Elevation of Boiling Point of a Liquid:
- The vapour pressure of solution of non volatile solute is always less than the vapour pressure of pure solvent.
- At the tempearature of boiling point of pure solvent, solution will not boil because its vapour pressure of solution is less than vapour pressure of pure solvent. Thus vapour pressure of solution is less than the external pressure. To boil the solution we have to increases vapour pressure of the solution to make it equal with external pressure. It is achieved by increasing the temperature of the solution. Thus there is elevation of boiling point of liquid.
- Let Tbo be the boiling point of pure solvent and Tb be the boiling point of solution. The increase in the boiling point ΔTb = Tb – Tbo is known as elevation of boiling point.
The elevation of boiling point (ΔTb) is directly proportional to the lowering of vapour pressure (Δp).
Thus ΔTb α Δp.
4. Experiments have shown that for dilute solutions the elevation of boiling point (ΔTb) is directly proportional to the molal concentration of the solute in a solution. Thus, the elevation of boiling point also depends on the number of solute molecules rather than their nature.
ΔTb α m
ΔTb = Kb m ………….. (1)
- Here m (molality) is the number of moles of solute dissolved in 1 kg of solvent and the constant of proportionality, Kb is called Boiling Point Elevation Constant or Molal Elevation Constant (Ebullioscopic Constant). The unit of Kb is K kg mol-1.
- The molal elevation of boiling point constant is defined as the elevation of boiling point produced when one mole of solute is dissolved in 1 kg of solvent.
- The experimental method to determine the molecular mass of non-volatile solute by determining boiling points of pure solvent and solution of known concentration is called ebullioscopy.