Boiling Point Elevation

Last modified by

Dasha

on

Sep 29, 2022, 12:51:58 AM

Created by

Dasha

on

May 12, 2016, 2:28:35 PM

$DeltaT_b = K_b*n/M * i$

$(n) "moles of solute"$

$(M) "Mass of solvent"$

$(K_b) "ebullioscopic constant"$

$(i)"van't Hoff factor"$

The Math / Science

The solvent boiling point elevation^[1] is a colligative property^[3] equation that calculates the new boiling temperature of the solvent after it has been mixed in with a solute. The addition of solutes causes the solvent to become impure, causing the boiling point to elevate. However, for the boiling point of the solvent to be raised, the solute must not contribute to the vapor pressure and must remain in the solvent throughout the phase change. The impurity of the solvent causes a decrease in it's chemical potential^[6], which is the amount of molar Gibbs free energy^[4],[5] one mole of solvent contributes to the mix. A high chemical potential drives the reaction towards product formation, however increasing solute molarity, increases boiling point and decreases chemical potential.

The formula for the change in boiling point of a solvent is:

ΔTb = Kb • m • i

where:

ΔT_b is the change in solvent boiling temperature of the solvent in units of (^oC).
K_b is the ebullioscopic constant, unique for each solvent in units of (^oC kg/mol), a full list can be found here.
m is the solute molality in units of moles of solute per kg of solvent (mol/kg)
i is the van't Hoff factor^[7] which takes dissociation of the solute in to account, for a solute that doesn't dissociate i=1, for dissociable solutes the value of i will depend on how many particle the solute dissociates in to, more information can be found here.