Elevation of Boiling Point Definition/Equation/Molal Boiling Point Elevation

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The boiling point of the solvent is the temperature at which vapor pressure is equal to that of atmospheric pressure. When a non-volatile solute is added to the solvent, its vapor pressure decreases. An elevation of boiling point is the difference between the boiling points of a solution containing a non-volatile solute and the boiling point of the pure solvent.

Elevation of boiling point
T = Boiling point of the solution
Tb = Boiling point of pure solvent
ΔTb = Elevation of boiling point

A liquid boils at a temperature when its vapor pressure equals to atmospheric pressure.

Elevation of Boiling point curve

Elevation of boiling point curve
µ = chemical potential

At the boiling point, the vapor-liquid curves meet and the chemical potentials are equal.

In a pure solvent at a given temperature and pressure, there exists an equilibrium between the vapor phase and the liquid phase. If the system is at 1 atm pressure, then the chemical potential of the pure component is equal to the standard chemical potential. With the addition of the non-volatile solute, the vapor pressure of the solvent will be lowered and the escaping tendency of the solvent molecule will be diminished, so more solvent molecules from the vapor phase will pass into the liquid phase to restore equilibrium.

Thus, we will have to heat the liquid to a temperature higher than the boiling point of pure liquid to increase the escaping tendency of the solvent molecules from the liquid phase.

image 28
ΔHvap,m = molar heat of vaporization of the solvent from the solution.
(X2) = mole fraction

This implies that with the addition of the non-volatile solute, the boiling point (T) of the solution should increase.

Molal Boiling Point Elevation

Molal boiling point elevation is the elevation in the boiling point of the solvent is caused by 1 mole of non-volatile solute dissolved per kg (1000 g) of the solvent.

ΔTb = kb × m

The proportionality constant, Kb is called the molal boiling point elevation constant. It is a constant that is equal to the change in the boiling point for a 1-molal solution of a nonvolatile molecular solute. For example, the Boiling point of water is 100 oc, when 1 mole of solute like NaCl, sugar, urea, etc is dissolved in 1 kg of water then its boiling point increases by 0.51 oc. Its value can be evaluated experimentally.

The mole fraction (X2) and the molality (m2) of the solute in a dilute solution are related by

image 18

Where w1 is the mass of the solvent expressed in gram and m1 is its molar mass.

image 20

ΔHvap,m = molar heat of vaporization of the solvent from the solution

So, from equations (i) and (ii)

image 21

The term within parenthesis on RHS of equation iii includes the parameters of a pure solvent and hence its characteristic of that substance. Let it be represented by Kb and defined as,

image 23
So, equation iii becomes,

ΔTb = kb × m2 = kbm…………………..(v)

Where m is unity, ΔTb = kb = molal boiling elevation

unit of kb= kmolal-1

Molal boiling point elevation of some compound

SolventNormal boiling point (oc )molal boiling point elevation constant,kb(oc/m)
Acetic acid111.93.07
Camphor207.45.61
Naphthalene217.75.80
Phenol181.83.60
Water100.00.512

Molecular Weight Determination

The molar mass of the involatile solute can be calculated if we know the elevation of boiling point (ΔTb) caused by the mass of the solute dissolved in a mass of the solvent, whose elevation constant kb.

image 25
Here,
w2 = mass of solute, w1= mass of solvent.

Boiling points of some compounds

CompoundBoiling pointCompoundBoiling point
Water100oCHCl– 82.05oC / 188.1K
Methanol64.7oC HBr– 66oC / 207.1K
Ethanol 78.37oC HF19.5oC
Acetone 56oC NH3– 33.34oC / 239.8K
Toluene 100.6oC SiH4– 112oC / 161.1K
Alcohol 78.37oC Milk100.5oC
Benzene 80.1oC CO2– 78.46oC / 194.7K
Cyclohexane 80.75oC NaCl1465oC
Diethyl ether 34.6oC NaHCO3851oC
Ethylene glycol 197oC Dichloromethane39.6oC
Isopropyl alcohol 82.5oC Iodine184.3oC
Benzoic acid 249.2oC Helium– 268.9oC / 4.22K
Sulphuric acid 337oC Argon– 185.8oC / 87.3K
Acetic acid 118oC Oil300oC
Methane– 161.6oC / 111.5KOxygen– 183oC / 90.19K
Ethane – 89oC / 184.1K Hydrogen– 252.9oC / 20.28K
Propane – 42oC / 231.1K Nitrogen– 195.8oC / 77.36K
Butane – 1oC / 272.2K Fluorine– 188.1oC / 85.04K
Pentane 36.2oC Chlorine– 34.04oC / 239.1K
Hexane 69oC Br258.8oC
Heptane 98.42oC CCl476.72oC
Octane 125.6oC H2S– 60oC / 213K
Ethyl acetate 77.1oC CH3OCH324oC / 249K
Diethyl ether 34.6oC CO– 191.5oC / 81.65K
1-butanol 117.7oC NaOH1388oC
1-propanol 97oC Cyclohexane80.75oC

FAQs

What is the elevation of boiling point?

The difference between the boiling points of a solution containing a non-volatile solute and the boiling point of the pure solvent is called the elevation of boiling point.

What is molal boiling point elevation?

Molal boiling point elevation is the elevation in the boiling point of the solvent is caused by 1 mole of non-volatile solute dissolved per kg (1000 g) of the solvent.

Boiling point

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