Question

A binary solution containing two volatile solutes A and B shows positive deviation from Raoult’s law. Which of the following statement is true?

• A. The molecular interactions between A-B are greater than those of A-A or B-B.
• B. The molecular interactions between A-B are weaker than those of A-A or B-B.
• C. There are equal molecular interactions between A-A or B-B.
• D. There are no molecular interactions between A-B compared to those of A-A and B-B.

Hint:

Mnemonic: "Positive = More pressure \(\rightarrow\) Easier to evaporate \(\rightarrow\) Weaker grip (A-B)".
"Negative = Less pressure \(\rightarrow\) Harder to evaporate \(\rightarrow\) Stronger grip (A-B)".

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Raoult's law for ideal solutions assumes that the intermolecular forces between the components in the mixture (A-B interactions) are similar to those in the pure components (A-A and B-B interactions). Deviations occur when these forces differ.

Step 2: Detailed Explanation:
1. Positive Deviation: This occurs when the escaping tendency of molecules from the liquid to the vapor phase increases. This happens because the A-B attractive forces are weaker than the A-A or B-B forces in the pure liquids.
2. Because of weaker A-B interactions, the molecules find it easier to leave the liquid surface, leading to a higher vapor pressure than predicted by Raoult's law.
3. Characteristic properties: \(\Delta H_{mix} > 0\) (endothermic) and \(\Delta V_{mix} > 0\).

Step 3: Final Answer:
The true statement is that A-B interactions are weaker than A-A or B-B interactions.