Q.Considering the formation, breaking and strength of hydrogen bond, predict which of the following mixtures will show a positive deviation from Raoult's law?

The key idea is that positive deviation from Raoult’s law occurs when the A–B interactions in the mixture are weaker than the A–A and B–B interactions in the pure components. This makes the vapour pressure higher than ideal.

Step 1 – Analyse pure components:

Methanol (CH₃OH) forms strong hydrogen bonds with itself. Acetone (CH₃COCH₃) has a polar carbonyl group but no –OH group; its self-interactions are weaker dipole–dipole forces.

Step 2 – Compare mixture interactions: …

Why This Approach Works

Raoult’s law states that the partial vapour pressure of each component in an ideal solution is proportional to its mole fraction. Real solutions deviate from this depending on how A-B interactions compare with A-A and B-B interactions.

Positive deviation: A-B interactions weaker than A-A/B-B - higher vapour pressure than ideal.

Negative deviation: A-B interactions stronger - lower vapour pressure than ideal.

Step-by-Step Reasoning

1. (i) Methanol and acetone - Pure methanol has strong O-H...O self-association. The cross hydrogen bond with acetone's carbonyl oxygen is weaker than methanol-methanol bonds. A-B < A-A -> positive deviation. …

Concept: Deviation from Raoult’s Law and Hydrogen Bonding

Method: Intermolecular Force Comparison Method

Why this method works

Raoult’s law assumes ideal behaviour — the intermolecular forces between A–A, B–B, and A–B are all equal.

• Positive deviation occurs when A–B interactions are weaker than A–A and B–B.
• This makes the vapour pressure higher than predicted, because molecules escape more easily.

Steps to apply

1. Identify the dominant intermolecular force in each pure component (usually hydrogen bonding here).
2. Compare the strength of A–B hydrogen bonds with the pure component hydrogen bonds.
3. If A–B bonds are weaker → positive deviation. If A–B bonds are stronger → negative deviation.

Applying to the options

(i) Methanol and acetone ✓

• Pure methanol: strong O–H···O hydrogen bonding.
• Pure acetone: only weak dipole-dipole (C=O is polar, but no –OH or –NH).
• In mixture: Methanol’s –OH can form H-bonds with acetone’s C=O, but acetone disrupts the strong methanol–methanol network.
• Result: A–B bonds are weaker than A–A bonds → positive deviation.

(ii) Chloroform and acetone ✗

• Chloroform (CHCl₃) has a slightly acidic H (C–H···O type H-bond possible).
• Acetone’s C=O accepts H-bonds.
• Result: A–B H-bonds are stronger than pure component interactions → negative deviation.

(iii) Nitric acid and water ✗ …

Common Mistakes & How to Avoid Them

This question tests your understanding of Raoult's law deviations in liquid-liquid solutions, specifically linked to hydrogen bonding between components.

✗ Mistake 1: Confusing "positive deviation" with "stronger intermolecular forces"

The error:

Students think that if a mixture forms strong hydrogen bonds, it will show positive deviation.

Actually, positive deviation occurs when A–B interactions are weaker than A–A and B–B interactions. This makes the mixture easier to vaporise (higher vapour pressure than ideal).

How to avoid:

Remember the rule:

Positive deviation → weaker A–B forces → vapour pressure higher than ideal

Negative deviation → stronger A–B forces → vapour pressure lower than ideal

So for positive deviation, the mixture must break strong self-association (like in pure methanol or water) and form weaker cross hydrogen bonds.

✗ Mistake 2: Assuming all hydrogen-bonding mixtures show negative deviation

The error:

Many students think "hydrogen bond = negative deviation". But that’s only true if the cross hydrogen bond is stronger than the self-hydrogen bonds in the pure components.

Example:

• Methanol + acetone: Methanol has strong self-association. Acetone can only accept H-bonds (C=O). The cross bond is weaker than methanol–methanol bonds → positive deviation.
• Chloroform + acetone: Chloroform’s H is acidic, acetone’s O is basic → stronger cross bond → negative deviation.

How to avoid:

Compare the strength of the cross bond vs. the self bonds. If the pure component has strong self-association (like alcohols, water, carboxylic acids), and the other component is a weak H-bond acceptor (like acetone, ether), expect positive deviation.

✗ Mistake 3: Not recognising when both components form strong self-association

The error:

Students sometimes pick options like (iii) Nitric acid + water or (iv) Phenol + aniline, thinking they show positive deviation because both have H-bonding.

Correction:

• Nitric acid + water: Both form very strong self-association, but the cross H-bond (acid–water) is even stronger → negative deviation.
• Phenol + aniline: Both have strong self-association, but phenol (acidic H) and aniline (basic N) form a very strong cross H-bond → negative deviation.

How to avoid: …