Elevation of Boiling Point

Elevation of boiling point definition

When you add a solute to a solvent, it raises its boiling point. The boiling point of a mixture containing a non-volatile solute is greater than that of the pure solvent. A sodium chloride (salt) plus water solution, for example, has a higher boiling point than pure water.

The solute-to-solvent ratio determines the elevation of a boiling point, but not the identity of the solute. This aspect means that the amount of solute added to a solution determines how high the boiling point of the solution rises; the boiling point increases in proportion to the solute concentration in the solution.

What Causes the Elevation of the Boiling Point?

• The degree to which a liquid’s vapor pressure equals the ambient environment’s pressure is the boiling point. 
• Non-volatile chemicals have very low vapor pressures and do not readily evaporate (assumed to be zero). When one combines a non-volatile solute with a pure solvent, the vapor pressure of the resultant solution is lower than that of the pure solvent. As a result, it is necessary to apply more heat for the solution to boil. 
• Boiling point elevation is when the boiling point of a solution rises. When the concentration of the additional solute increases, the solution’s vapor pressure drops, and the boiling point rises.
• When one introduces a solute to a solvent, the vapor pressure drops. This phenomenon occurs as a result of the solute dissolving solvent molecules. 
• It occurs in both electrolytic and non-electrolytic solutions when the solute replaces part of the solvent molecules at the liquid’s surface. Because there are fewer solvent molecules at the surface, less will be evaporation, lowering the vapor pressure. 
• A greater temperature is necessary to equalize the vapor pressure with atmospheric pressure; thus, it is essential to consider a higher boiling point.

Boiling Point Elevation Formula

For a solution containing a non-volatile solute:

The boiling point of a solution equals the boiling point of the pure solvent plus the elevation of the boiling point (Tb). The solute concentration in the solution determines the increase in boiling point (Tb). 

It is possible to calculate the extent of the boiling point elevation. One can use the following equation to determine it. We get the following equation from the elevation in boiling point derivation:

ΔTb = i*Kb*m

Where,

• i is the Van’t Hoff factor
• Kb is the ebullioscopic constant
• m is the molality of the solute

Notably, when the solute concentration is really high, this formula becomes less exact. In addition, this formula does not apply to volatile solvents.

The ebullioscopic constant (Kb) is commonly represented as degrees Celsius per molal or degrees Celsius per kilogramme of mol-1. One can use the boiling point elevation technique to calculate the degree of dissociation of the solute and the molar mass of the solute.

Solved Example

In 200 grams of benzene, 10 grams of a non-volatile, non-dissociating solute are dissolved. The resultant solution boils at 81.2 degrees Celsius. Determine the solute’s molar mass.

Let x be the number of moles of the solute. An ebullioscopic constant of 2.53 °C/molal and a boiling point of 80.1 °C is required for Pure Benzene. The following connection may be deduced from the boiling point elevation formula:

(81.2 oC – 80.1 oC) = (1)*(2.53 oC.kg.mol-1)(x/0.2 kg)

x = (1.1 oC*0.2 kg)/(2.53 oC.kg.mol-1)

x = 0.0869 moles

Because 0.0869 moles of the solute have a mass of 10 grams, one mole of the solute has a mass of 10/0.0869 grams or 115.07 grams. As a result, the solute’s molar mass is 115.07 grams per mole.

The Boiling Point Elevation and Vapor Pressure Relationship

The rising of the boiling point can be described in terms of vapor pressure. 

The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a particular temperature is known as vapor pressure. In simple words, it is a quantity of the solution molecules’ proclivity to escape by entering the gas phase. 

When the vapor pressure of a liquid equals the air pressure, it boils.

Kb = RTb2M/ΔHv, 

Where,

R is the gas constant

Tb is the boiling temperature of the pure solvent [in K], 

M is the molar mass of the solvent

ΔHv is the heat of vaporization per mole of the solvent.

Conclusion

The article has shared everything that one needs to learn about the elevation of boiling point. It gives a detailed definition of boiling point elevation and its formula. Besides, the solved example enables easy understanding of the topic.