Question

The mixture which shows positive deviation from Raoult's law is:

• A. Ethanol + acetone
• B. Benzene + toluene
• C. Acetone + chloroform
• D. Chloroethane + bromoethane

Hint:

To spot positive deviation, look for a component that breaks strong existing bonds (like an alcohol having its H-bonds disrupted by a non-polar or less polar solvent). To spot negative deviation, look for new bonds forming (like H-bonding between two different species).

Answer 1

The Correct Option is A

Concept: Raoult's law states that the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction. Real solutions often deviate from this ideal behavior:
• Ideal Solution: Interactions between A-B are similar to A-A and B-B.
• Positive Deviation: A-B interactions are weaker than A-A or B-B. Molecules escape more easily into the vapor phase, increasing total vapor pressure.
• Negative Deviation: A-B interactions are stronger than A-A or B-B. Molecules are held more tightly, decreasing total vapor pressure.

Step 1: Analyzing Ethanol + acetone (Option A).
In pure ethanol, molecules are held together by strong hydrogen bonding. When acetone is added, its molecules get between the ethanol molecules and break some of the hydrogen bonds. This weakens the attractive forces in the mixture (A-B < A-A). Therefore, the molecules escape more easily, showing a positive deviation.

Step 2: Analyzing Benzene + toluene and Chloroethane + bromoethane (Options B and D).
These are pairs of molecules with very similar structures and polarities. The intermolecular forces in the mixture are almost identical to those in the pure components. These mixtures behave nearly as ideal solutions.

Step 3: Analyzing Acetone + chloroform (Option C).
When acetone and chloroform are mixed, a new, strong hydrogen bond forms between the oxygen of acetone and the hydrogen of chloroform. This makes the A-B interaction stronger than the individual A-A or B-B interactions, leading to a negative deviation.