Boiling Point Elevation



 

Learning Objective

  • Discuss the effects of a solute on the boiling point of a solvent

Key Points

    • One of the colligative properties of a solution is boiling point elevation.
    • The amount that the boiling point increases in the presence of solute can be calculated by using the boiling point elevation constant and the molality of the solution.
    • The addition of solute dilutes the solvent molecules and makes it harder for them to escape into the gaseous phase.

Terms

  • boiling pointThe temperature at which a liquid boils, with the vapor pressure equal to the given external pressure.
  • boiling point elevationThe temperature at which a substance’s vapor pressure equals the external pressure increases when another compound is added.
  • colligative propertyA property of solutions that is directed by the ratio of solute to solvent, regardless of the identity of the solute.

Colligative Properties and Boiling Point Elevation

There is one category of properties that can only be applied to solutions; these are known as colligative properties. Properties can be considered colligative only if they are dependent on the amount of solute present in the solution, disregarding the identity of the solute itself.

The boiling point of a solvent will increase when a solute is dissolved in it. This is referred to as boiling point elevation. The elevation of the boiling point is directly dependent on the amount of solute present in the solution, but it is not based on the identity of the solute, so it is considered a colligative property.

The Relationship Between Boiling Point Elevation and Vapor Pressure

Boiling point elevation can be explained in terms of vapor pressure. Vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. In layman’s terms, it is simply a measure of the tendency of the solution molecules to escape by entering the gas phase. A liquid boils when its vapor pressure is equal to the air pressure.

Boiling pointThe boiling point of a pure liquid. When the vapor pressure of the liquid matches the atmospheric pressure, the liquid will boil.

Boiling Point Elevation

A solvent’s vapor pressure will lower when a solute is added. This happens because of the displacement of solvent molecules by the solute. This means that some of the of solvent molecules at the surface of the liquid are replaced by the solute; it can occur in both electrolytic and non-electrolytic solutions. The lower number of solvent molecules at the surface means that fewer will evaporate, and thus the vapor pressure is lowered. For the vapor pressure to equal the atmospheric pressure, a higher temperature is required, and a higher boiling point is observed.

Calculating Boiling Point Elevation

The extent of the boiling point elevation can be calculated. It is directly proportional to the molal concentration of the solution. The amount the boiling point is elevated is determined using the equation:

[latex]\Delta T_b = i \times K_b \times m[/latex]

In this equation, [latex]\Delta T_b[/latex] is the boiling point elevation, [latex]K_b[/latex] is the boiling point elevation constant, and m is the molality of the solution. The “i” factor of the equation also factors in any dissociation that the solute may undergo; since boiling point elevation is a colligative property, the number of ions present in an electrolyte affects the elevation.

Example

Calculate the boiling point of an aqueous solution where enough NaCl is added to make a 0.37 molal solution. The Kb for water is 0.512 [latex]\frac {^oC}{m}[/latex].

[latex]\Delta T_b = i \times K_b \times m[/latex]

[latex]\Delta T_b = 2 \times 0.512 \frac{^oC}{m} \times 0.37\ m[/latex]

[latex]\Delta T_b = 0.38^oC[/latex]

Water normally boils at 100 oC, so the new boiling point of the solution would be 100.38 oC.