What type of deviation from Raoult's law is shown by mixture of ethanol and acetone? Give reason. What will happen to the boiling point of the solution on mixing ethanol and acetone?
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Raoult's law deviations arise from differences between A-A, B-B and A-B intermolecular forces. Pure ethanol has strong hydrogen bonding between its molecules.
Step 1: What decides the deviation. Deviation from Raoult's law depends on how the new A-B forces compare with the original A-A and B-B forces.
Step 2: Look at pure ethanol. Pure ethanol molecules are held together by strong hydrogen bonds.
Step 3: Add acetone. Acetone molecules slip in between the ethanol molecules and break some of these hydrogen bonds, so the ethanol-acetone attraction is weaker than the original ethanol-ethanol attraction.
Step 4: Effect on vapour pressure. Weaker forces let molecules escape more easily, so the vapour pressure rises above the value expected from Raoult's law. This is positive deviation.
Step 5: Effect on boiling point. Higher vapour pressure means the mixture boils more easily, so the boiling point of the solution becomes lower than that of the pure components.
Answer: The mixture of ethanol and acetone shows positive deviation from Raoult's law, because acetone breaks the hydrogen bonding of ethanol and weakens the intermolecular forces. As a result the boiling point of the solution decreases (it boils at a lower temperature).