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Question
Explain with reasons and diagrams, the positive and negative deviations from Raoult’s law shown by nonideal solutions.
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Solution
- Positive deviation from Raoult’s law:
The solutions in which solute-solvent intermolecular attractions are weaker than those between solute-solute molecules and solvent-solvent molecules, exhibit positive deviations. The vapour pressures of such solutions are higher than those of pure components as shown in the diagram.
e.g. The solutions of ethanol + acetone, carbon disulphide + acetone show positive deviations from Raoult’s law.
Positive derivations from Raoult's law - Negative deviation from Raoult’s law:
The solutions in which the interactions between solvents and solute molecules are stronger than solute-solute or solvent-solvent interactions, exhibit negative deviations. The vapour pressures of such solutions are lower than those of pure components as shown in the diagram.
e.g. Solutions of phenol + aniline, chloroform + acetone exhibit negative deviations from Raoult’s law.
Negative derivations from Raoult's law
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