A plot of vapour pressure (y-axis) against mole fraction (x-axis, from pure component 1 at the left to pure component 2 at the right) for a binary liquid mixture shows the total vapour pressure as a curve that bulges above the straight dashed line joining the vapour pressures of the two pure liquids. Which pair of liquids, and which sign of ΔmixH, fit this plot?
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A total vapour pressure above the Raoult's law line is a positive deviation: A–B attractions are weaker than A–A and B–B. Ethanol and acetone behave this way because acetone breaks some hydrogen bonds of ethanol, and mixing is endothermic (ΔmixH > 0).
- A.
Chloroform and acetone form hydrogen bonds with each other and show a negative deviation, with the curve below the line.
- B.
n-Hexane and n-heptane form a nearly ideal solution, whose total vapour pressure lies on the straight line.
- C.
In a positive deviation the solute–solvent attractions are weaker, so mixing absorbs heat and ΔmixH is positive.
NCERT: Class 12 Chemistry, Chapter 1