Limitations of Raoult’s Law

Ideal solutions are those that follow Raoult’s law throughout the whole concentration range.

The optimum solutions have two key characteristics. 

The mixing enthalpy of the pure components to produce the solution is zero, as is the mixing volume, i.e. . 

It indicates that when the components are blended, no heat is absorbed or released. In addition, the volume of the solution would be equal to the sum of the two components’ volumes.

Non-ideal Solutions

The volume of a non-ideal solution is not necessarily the simple sum of the volumes of the component pure liquids, and solubility is not guaranteed across the whole composition range, unlike ideal solutions, whose volumes are strictly additive and mixing is always complete. Component densities can be used to determine their thermodynamic activity.

Non-ideal Solution: Characteristics

The following are characteristics of non-ideal solutions:

• Interactions between solutes and solutes and solvents and solvents are different from solute and solvent interactions

• If the enthalpy of mixing is negative ( ), heat may have been released; if the enthalpy of mixing is positive ( ), heat may have been seen

• The mixing volume, i.e., indicates that there will be some expansion or contraction in the solution

What is Raoult’s Law?

According to Raoult’s law, the partial vapour pressure of a component in an ideal mixture is equal to the vapour pressure of the pure component multiplied by its mole fraction.

An ideal mixture is one in which all molecules have equal intermolecular interactions.

Mathematical Expression

According to Raoult’s law

Where,

• is the vapour pressure of the solution

• is the mole fraction of the solvent

• is the vapour pressure of the pure solvent

Why do we observe this Effect?

The molecules on the surface cause the drop in vapour pressure as the solute concentration rises. All the molecules on the surface of a pure solvent are solvent molecules. These are the surface molecules that will dissolve into the vapour phase. There are fewer solvent molecules on the surface to escape to the vapour phase as the solute concentration rises. Solute molecules have taken up some of those surface patches. The entire vapour pressure is reduced as a result.

The Law’s Limitations

• Raoult’s law is particularly useful because it explains ideal solutions, which are those in which the gas phase has thermodynamic properties similar to that of a mixture of ideal gases

• The main issue is that they are uncommon and difficult to obtain

• Due to the lack of chemical equivalency between different chemical components, many solutions depart from Raoult’s law

• As a result, you won’t be able to follow it well

Deviation from Raoult’s Law

Raoult’s law predicts that the vapour pressure of such a solution will be either higher or lower than expected. The solution shows a positive deviation from Raoult’s law if it is higher, and a negative deviation if it is lower.

• Positive Deviation with an Example

When the partial pressure of component ‘A’ in the mixture of ‘A’ and ‘B’ is found to be greater than that calculated from Raoult’s law in non-ideal solutions. Similar to Raoult’s law, the partial vapour pressure of component ‘B’ can be higher than calculated.

Example: Water and ethanol.

• Negative Deviation with an Example

On adding the second component ‘B’, the partial vapour pressure of component ‘A’ is found to be smaller than calculated from Raoult’s law. When A is mixed with B, the partial vapour pressure of the solution falls below that of an ideal solution with the same composition. 

Examples include chloroform and acetone.

Conclusion

An ideal mixture assumes that all molecules in the mixture have equal intermolecular interactions.

According to Raoult’s law, the partial vapour pressure of a component in an ideal mixture is equal to the vapour pressure of the pure component multiplied by its mole fraction.

A solution that follows Raoult’s law is referred to as an “ideal solution.” This rule only applies to dilute solutions, however, some liquid combinations adhere to it across a concentration range.