Elevation of boiling point

Introduction

Elevation of boiling point

The temperature at which a liquid starts to boil is known as its boiling point. This is the temperature when the liquid starts to turn into vapours.

This temperature is dependent on two factors: vapour pressure of the liquid, and pressure of its external surroundings. The former is the quality of the solvent; the latter, the quality of its atmosphere. The conditions under which both these pressures become equal is called the boiling point of a solvent.

Elevation of boiling point is described as a rise in temperature required to attain the boiling point of a solution. This phenomenon is dependent upon the addition of a solute into a pure solvent.

This article will further discuss topics like: the properties of boiling point; elevation in boiling point derivation; the properties of elevation of boiling point; and, calculating and defining a boiling point elevation formula.

The properties of boiling point

• The boiling point of a liquid decreases with the decrease in external or atmospheric pressure.
• The boiling point of water, measured at sea level, is 100 degrees Celsius. Atmospheric pressure at sea level is 1 atm.
• The boiling point of water, measured at a higher altitude, like in mountainous regions, would be a lot lower than 100 degrees Celsius. This is because atmospheric pressure decreases with the rise in altitude. And so, the required vapour pressure to reach the boiling point would also be less.
• Boiling point and vapour pressure are values that share an inversely proportional relationship. A rise in vapour pressure results in a lowering of boiling point. Similarly, a fall in vapour pressure would lead to an elevation of boiling point.

Elevation in boiling point derivation

• The vapour pressure of a liquid is directly proportional to its temperature. The higher the temperature, the higher the vapour pressure.

• The vapour pressure of a non-volatile solute is zero. Therefore, when added to a solvent, this non-volatile solute decreases the vapour pressure of the resulting solution. 

A liquid in its pure solvent form boils at a certain temperature. However, after the addition of a non-volatile solute the vapour pressure of this liquid decreases. This results in the need to increase the temperature to enable the vapour pressure to increase where it equals the atmospheric pressure. This is the point where this new liquid starts to boil. This increase in temperature is the method of elevation of boiling point. 

 The properties of elevation of boiling point

• Elevation of boiling point is independent of the type of solute in the solution.
• Elevation of boiling point is dependent on the quantity of the solute in the solution, that is, the solute to solvent ratio. It is thus a colligative property.
• Elevation of boiling point is dependent on molality.
• When the solution is highly saturated, the elevation of the boiling point is higher. 
• When the solution has a small quantity of solute, the elevation of the boiling point is lower.

Calculating and defining a boiling point elevation formula

Boiling point elevation formula

ΔTb = Kb . m 

OR

ΔTb = Kb . bB

ΔTb = Kb . bsolute . i

Thermodynamic representation of boiling point elevation formula:

ΔTb = (RTb2M / ΔHv) . bsolute . i

Definitions and Conditions for boiling point elevation formula and elevation in boiling point derivation:

• ΔTb denotes elevation of boiling point. This is the difference between the boiling point of the solution (Tb (solution)) and the boiling point of the original liquid in pure form (Tb (pure solvent))
•  Kb denotes what is known as the ebullioscopic constant/molal elevation constant.
• The value of molal elevation constant Kb is a pre-defined value that differs for every solvent.
• Kb can also be calculated using the formula Kb = RTb2M / ΔHv.

Here,

• R is the gas constant
• Tb is the boiling point for the pure solvent
• M denotes the molar mass of the pure solvent
• ΔHv denotes the heat of vaporisation or the energy required to turn every mole of the solvent into vapours

• bB denotes the molal concentration of the solute in the solution.
• m denotes the molality of the solvent.
•  The van’t Hoff factor denoted as i can be placed in the formula bB = bsolute . i
• Using the van’t Hoff factor, another formula can be used for elevation in boiling point derivation. This formula is, ΔTb = Kb . bsolute . i
• When we replace Kb with the formula Kb = RTb2M / ΔHv, in the equation ΔTb = Kb . bsolute . i, we get a resultant formula, ΔTb = (RTb2M / ΔHv) . bsolute . i
•  Assumption: The solute used is a non-volatile solute.
• In case the solute used is volatile, elevation in boiling point derivation can be done through the help of a phase diagram of the solution.

 Conclusion

The elevation of boiling point is seen when a pure solvent turns into a solution through the addition of a solute. It is a colligative property and can be understood in several ways, such as the combination of the Clausius and Clapeyron Model and Raoult’s Law. This chapter has taught the reader the following: the various chemical and physical properties of boiling point, of the elevation of boiling point; elevation in boiling point derivation; and the boiling point elevation formula; along with the various symbolic representations.