Elevation in boiling point-
source : sciencing
Elevation in boiling point-
Boiling point of a liquid is a characteristic temperature at which its vapour pressure becomes equal to one atmosphere or 760 mm or 76 cm pressure(atmospheric pressure).
Addition of a nonvolatile solute in a solvent lowers its vapour pressure and thus more heat is required to increase the vapour pressure of solution upto one atmosphere.Hence the boiling point of the solvent is elevated when nonvolatile solute is added. It means boiling point of solution is greater than boiling point of pure solvent.This increase in boiling point is known as elevation in boiling point or the difference in boiling point of the solution and pure solvent is termed elevation in boiling point .
source : socratic.org
source : edu.glogster.com
Elevation in boiling point = ΔTb = boiling point of solution -boiling point of pure solvent
When a graph is plotted between vapour pressure and temperature for a pure solvent and two solutions of different concentrations. The curves of the solutions always lie below the curve of the pure solvent.
source : eXe
Po=vapour pressure of pure solvent
P1= vapour pressure of solution I
P2= vapour pressure of solution II
T0=boiling point of pure solvent
T1= boiling point of solution I
T2= boiling point of solution II
Line P0 I represents the atmospheric pressure
If solutions are very dilute then these curves may be taken as straight lines near the boiling point .
Hence ,
ΔGIK and ΔGHJ are similar
So,
GI/GH =GK/GJ
(T2-T0) / (T1-T0) = (P0-P2) / (P0-P1)
ΔT2 /ΔT1= ΔP2 /ΔP1
ΔTb ∝ ΔP
According to Raoult’s law,
(p0-ps) / p0 = wM/Wm
(p0-ps) = [wM/Wm] x p0
p0 (vapour pressure of pure solvent) and molecular weight of solvent (M) are constant.
So,
(p0-ps) = w / mW
Δp ∝ w/ m W
Δp ∝ ΔTb ∝ w/mW
Thus ,
ΔTb ∝ w/ m W
ΔTb = K w/ m W
K = Elevation constant
when w/m = 1 mole, W= 1 gram then,
ΔTb = K
when w/m = 1 mole, W= 100 gram then,
ΔTb = K /100 = Kb‘
Kb‘ is molecular or molar elevation constant
ΔTb = 100 Kb‘ w/m W
Definition of Molecular or molar elevation constant-
If w/m = 1 mole, W =100 gram then ΔTb = Kb‘
” Molar elevation constant is equal to the elevation in boiling point which is produced when one mole of nonvolatile ,non electrolyte solute is dissolved in 100 gram of solvent.
Definition of molal elevation constant-
If w/m = 1 mole, W =1000 gram then ΔTb = Kb
” Molal elevation constant is equal to the elevation in boiling point which is produced when one mole of nonvolatile ,non electrolyte solute is dissolved in 1000 gram of solvent.
We know,
w x 1000/m W = molality
So,
ΔTb = molality x Kb
Molal elevation constant can be calculated as ,
Kb = R Tb2/1000 Lv
R = gas constant (in calorie), Tb= boiling point of solvent in Kelvin
Lv = Latent heat of vaporisation (calorie /gram)
IMPORTANT POINTS TO REMEMBER-
1) Elevation of boiling point is a colligative property because it depends upon the molality of solution (means number of moles present in definite amount of solvent).
2) Equimolal solutions of different nonvolatile and non electrolyte substances dissolved in the same solvent show the same elevation in boiling point (colligative property).
