Q.52 Chemistry · Section A ⭐ 360° Option Elimination Active

NEET 2024 Chemistry Question 52 Solution

Topic: Hydrocarbons · Official NTA Booklet Code T1

Question

Given below are two statements: Statement I: The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane Statement II: When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point. In the light of the above statements, choose the most appropriate answer from the options given below:

✓

Verified Against NTA Final Answer Key

Option (3) : Both Statement I and Statement II are correct.

📚 Hydrocarbons Easy

📋 Given Parameters & Problem Setup

Isomers n-pentane, isopentane (2-methylbutane), neopentane (2,2-dimethylpropane)

Statement I Boiling point order: n-pentane > isopentane > neopentane

Statement II Branching gives spherical shape, smaller surface area, weaker intermolecular forces, lowering boiling point

⚡

The 30-Second Exam Hall Shortcut: ⚡ Branching vs Boiling Point Rule

⏱️ Save ~35 seconds

More Branches -> More Spherical -> Minimum Surface Area -> Weakest van der Waals forces -> Lowest Boiling Point!

1 n-pentane has a continuous straight chain with maximal surface area of contact (b.p. = 309.1 K).

2 isopentane has one methyl branch (b.p. = 301 K).

3 neopentane is highly branched and symmetrical, adopting a nearly spherical shape with minimal surface area (b.p. = 282.5 K).

4 Boiling point sequence: n-pentane > isopentane > neopentane -> Statement I is TRUE.

5 Statement II provides the exact theoretical rationale -> Statement II is TRUE.

6 Both statements are correct -> Option (3)!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Hydrocarbons (Intermolecular forces and physical properties of isomeric alkanes).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Analysis of Statement I (Boiling Point Data)

For constitutional isomers of formula C5H12, the experimental boiling points decrease monotonically with increasing branching: n-pentane (309.1 K) > isopentane / 2-methylbutane (301.0 K) > neopentane / 2,2-dimethylpropane (282.5 K). Hence, Statement I is correct.

T_b(\text{n-pentane}) > T_b(\text{isopentane}) > T_b(\text{neopentane})

Step 2

Analysis of Statement II (Structural and Intermolecular Mechanism)

Van der Waals dispersion forces act across the contact surface between neighboring molecules. As branching increases, molecular geometry becomes compact and approaches a sphere. Because a sphere possesses the minimum surface area for a given volume, intermolecular contact area diminishes, weakening London dispersion forces and lowering the boiling point. Hence, Statement II is correct.

\text{Branching} \uparrow \implies \text{Surface Area} \downarrow \implies \text{van der Waals forces} \downarrow \implies T_b \downarrow

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

Statement II is the precise NCERT scientific explanation.

⚠️

Student Fallacy (Why candidates pick this trap): Believing spherical shape increases intermolecular contact.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Statement I is an established empirical truth.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking branching raises boiling point.

3 (3)

✓ 100% Correct

Verdict & Scientific Reason:

Both empirical trend and mechanistic explanation are 100% sound.

💡

Why this is airtight: None.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Both statements are completely valid.

⚠️

Student Fallacy (Why candidates pick this trap): Total rejection of organic structure-property relationship.

⚠️

NTA Trap Alert: Confusing Boiling Point with Melting Point in Neopentane

Neopentane has a high melting point (256.5 K) due to crystal packing symmetry, but its boiling point is the lowest (282.5 K) due to low surface area.

🛡️ How to avoid the -1 mark: Distinguish liquid vaporization (surface contact area governs boiling point) from crystal lattice packing (symmetry governs melting point).

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Core NEET Syllabus Standard

Chemistry Core Organic Chemistry · Hydrocarbons Standard Benchmark

Core Scientific Principle Tested:

"Among isomeric alkanes, straight-chain isomers have higher boiling points than branched-chain isomers due to greater van der Waals contact area."

Syllabus Focus: Alkanes: Physical Properties and Intermolecular Forces

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Branches make a ball, balls touch the least, boiling drops fast when branches increase!

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NEET 2024 Chemistry Question 52 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Branching vs Boiling Point Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Analysis of Statement I (Boiling Point Data)

Analysis of Statement II (Structural and Intermolecular Mechanism)

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Boiling Point with Melting Point in Neopentane

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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