What is meant by positive and negative deviations from Raoult's law, and how is the sign of $\Delta_{mix}H$ related to positive and negative deviations from Raoult's law?

CBSE Class 12 · 2009 · All India Compartment · SolutionsShort Answer

What is meant by positive and negative deviations from Raoult's law, and how is the sign of

Δ_{mi x} H

related to positive and negative deviations from Raoult's law?

Answer: Positive: p observed > ideal, A–B weaker, ΔH_mix > 0. Negative: p observed < ideal, A–B stronger, ΔH_mix < 0.

Solution

Positive deviation: the vapour pressure of the solution is higher than Raoult's law predicts, because the A–B attractions are weaker than A–A and B–B. Energy is absorbed on mixing, so

Δ_{mi x} H > 0

.
Negative deviation: the vapour pressure is lower than predicted, because the A–B attractions are stronger. Energy is released on mixing, so

Δ_{mi x} H < 0

CBSE Class 12 Board · 2009 · All India Compartment · Subjective

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