Question

Binary liquid mixture will show negative deviation from Raoult’s law. Explain.

Answer 1

Raoult's law:

1. Raoult's law states that each component's partial vapour pressure in a mixed solution is proportionate to the pure component's vapour pressure times the mole fraction.
2. Therefore, given homogenous contact forces between the solute - solute, solvent - solvent, and solute-solvent molecules, the decrease in the vapour pressure of an ideal solution containing non-volatile solute is proportional to the mole fraction of non-volatile solute.

Conditions:

1. When the molecules in the solute and the solvent are almost the same size, this may be the case.
2. However, this is typically not possible, because as the nature of interactions between solute and solvent molecules changes, the vapour pressure in a non-ideal solution changes.
3. Raoult's law is broken as a result.

Result:

1. The negative deviation appears when the solute-solvent interaction is smaller than the interaction between solvent and solute molecules, as well as vice versa.
2. When chloroform and acetone are combined, hydrogen bonds develop in the solution, increasing the contact force and making it harder for the molecules to leave.
3. As a result, both the volume of the solution and its vapour pressure is decreased.