What is Elevation in boiling point?

Question

The boiling of a liquid may be defused by the temperature at which its vapour pressure which is equal to atmospheric pressure. The effect of addition in a non-volatile solute on the boiling point shown and its solution containing non-volatile solute with temperature are represented by the curves AB and CD respectively. It is evident by the curves temperature the vapour pressure of solutions is lower than that of the pure solvent and thus, the vapour pressure.

Curve for solution runs below that of the pure solvent. At temperature T₀, the vapour pressure of the pure solvent becomes equal to the atmospheric pressure. Thus, the atmospheric pressure and therefore, it is necessary to heat the solution to a higher temperature sayT₁ in the atmospheric pressure. Thus, it is clear that the solution in higher temperature than the pure solvent. Evidence T₁ - T₀ (or Δ T_b)is the elevation in boiling point vapour pressure (Δp), the elevation in the boiling point is also proportional to the solute concentration. Thus,

ΔT_b ∝ Δp

According to Raoult's law, Δp ∝ xB

∴ ΔT_b ∝ xB

 

 

 

 

If W_A is the mass of solvent in kg, then   is equal to molality (m) of the solution

ΔTb = kM_A m

Here, k and M_A are constants and hence their product, i.e. kM_A is replaced by another constant K₂.

ΔTb = Kb m, where Kb is called boiling point-elevation constant or molal elevation constant or molal ebullioscopic constant.

As elevation in boiling point depends upon the relative number of moles of solute and solvent but does not depend upon the nature of solute, so it is a colligative property.