Question

Which of the following solutions shows positive deviation from Raoult's law?

• A. Acetone + Aniline
• B. Acetone + Ethanol
• C. Water + Nitric acid
• D. Chloroform + Benzene

Hint:

Positive deviation occurs when mixing causes weakening of intermolecular attractions. \[ \text{Weaker attractions} \Rightarrow \text{Higher vapor pressure} \] Common examples:
• Ethanol + Acetone
• Ethanol + Cyclohexane

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Real solutions often deviate from the ideal behavior predicted by Raoult's Law. These deviations are categorized into positive and negative deviations.
Positive Deviation: This occurs when the intermolecular attractive forces between the different components in the mixture (A-B interactions) are weaker than the forces between similar molecules in pure form (A-A and B-B interactions).
Because the molecules are less attracted to each other in the mixture, they can escape into the vapor phase more easily.
This results in a total vapor pressure higher than expected and an endothermic mixing process (\(\Delta H_{mix}>0\)).
Step 2: Detailed Explanation:
Let's evaluate each option based on molecular interactions:
1. Acetone + Aniline: Aniline molecules have \(–NH_{2}\) groups and acetone has \(>C=O\) groups. These two groups interact strongly via intermolecular hydrogen bonding.
This new interaction is stronger than the original acetone-acetone or aniline-aniline interactions.
Stronger attraction means fewer molecules escape to the vapor phase. This is negative deviation.
2. Acetone + Ethanol: Pure ethanol exhibits strong intermolecular hydrogen bonding. When acetone is added to ethanol, its molecules place themselves between ethanol molecules, disrupting the original hydrogen bond network.
The new acetone-ethanol interactions are dipole-dipole interactions, which are significantly weaker than the hydrogen bonds they replaced.
Weaker attraction leads to higher vapor pressure. This is positive deviation.
3. Water + Nitric acid: Mixing a strong acid with water involves significant hydration and strong ion-dipole attractions.
The resulting A-B interactions are much stronger than the pure components. This is negative deviation.
4. Chloroform + Benzene: Chloroform can form weak hydrogen bonds with the pi-electron cloud of the benzene ring.
This added stability results in lower vapor pressure than ideal. This is negative deviation.
Step 3: Final Answer:
Acetone + Ethanol shows positive deviation because the mixing process breaks the strong hydrogen bonds of ethanol, weakening the overall intermolecular attractions.