Question

What is a positive and negative deviation from Raoult's law?

Answer 1

Part 1: Raoults' law

1. According to Raoult's law the partial vapor pressure of the solvent in a solution is equal to the vapour pressure of pure solvent multiplied by the mole fraction of the solution.
2. This implies that the depression in freezing point and elevation in the boiling point of the pure amount of solvent is equal to the mole fraction of the solute.
3. It can be understood with the help of a mathematical representation of Raoult's law:
   ${\mathrm{P}}_{\mathrm{solution}}={\mathrm{X}}_{\mathrm{solvent}}.{\mathrm{P}}_{\mathrm{solvent}}^0$
4. Where, ${\mathrm{P}}_{\mathrm{solution}}$ is the vapor pressure of the solution, ${\mathrm{X}}_{\mathrm{solvent}}$ is the mole fraction of the solvent, and ${\mathrm{P}}_{\mathrm{solvent}}^0$ is the vapour pressure of pure solvent.

Part 2: Deviation of Raoult's law

1. Raoult's law can show two types of deviation: Positive deviation and negative deviation
2. Positive deviation: Positive deviation is encountered when the adhesion between the unlike molecule is reduced by the cohesion between the like molecules.
3. This means that the vapour pressure exceeded the expected vapour pressure.
4. Example: The positive behavior is shown by the Benzene$\left(C_6{H}_6\right)$ and Methanol$\left(C_2{H}_5\mathrm{OH}\right)$ mixture.
5. Negative deviation: Negative deviation is encountered when the vapor pressure is lowered than expected.
6. The force between the particles is weaker in pure liquids compared with the force between the particles in the solution.
7. Example: The negative behavior is shown by the solution of Hydrochloric acid$\mathbf{\left(}\mathbf{HCl}\mathbf{\right)}$ and water.