Q.125 Chemistry ⭐ 360° Option Elimination Active

Vault ↗

NEET 2020 Chemistry Question 125 Solution

Topic: Redox Reactions · Official NTA Booklet Code E1

Question

What is the change in oxidation number of carbon in the following reaction? CH4(g)+4Cl2(g) → CCl4(l)+4HCl(g)

✓

Verified Against NTA Final Answer Key

Option (3) : −4 to +4

📚 Redox Reactions Easy

📋 Given Parameters & Problem Setup

Chemical Equation CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)

Element of Interest Carbon (C)

Question Objective Determine the change in oxidation state of carbon

⚡

The 30-Second Exam Hall Shortcut: ⚡ "In CH4, C is -4 (bonded to less electronegative H); in CCl4, C is +4 (bonded to more electronegative Cl)!"

⏱️ 15 seconds

Hydrogen is less electronegative than carbon, so in $CH_4$ : $x + 4(+1) = 0 \implies x = -4$ . Chlorine is more electronegative than carbon, so in $CCl_4$ : $x + 4(-1) = 0 \implies x = +4$ . The oxidation state of carbon changes from $-4$ to $+4$ (loss of 8 electrons, oxidation)! Mark Option (3) in 5 seconds.

1 In CH4: C bonded to 4 H atoms (+1 each) -> Oxidation state of C = -4.

2 In CCl4: C bonded to 4 Cl atoms (-1 each) -> Oxidation state of C = +4.

3 Change in oxidation number = -4 to +4.

4 Select Option (3).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Redox Reactions (Oxidation Number Rules)

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Rules for Assigning Oxidation Numbers

When assigning oxidation states, bonding electrons are completely assigned to the more electronegative atom: - Electronegativity values on the Pauling scale: $\text{Cl (3.16)} > \text{C (2.55)} > \text{H (2.20)}$ .

Step 2

Calculate Oxidation State of Carbon in CH4

Because carbon is more electronegative than hydrogen, hydrogen has an oxidation number of $+1$ : $x + 4(+1) = 0$ $x = -4$ Thus, carbon is in the $-4$ oxidation state in methane ( $\text{CH}_4$ ).

Step 3

Calculate Oxidation State of Carbon in CCl4

Because chlorine is more electronegative than carbon, each chlorine atom has an oxidation number of $-1$ : $x + 4(-1) = 0$ $x = +4$ Thus, carbon is in the $+4$ oxidation state in carbon tetrachloride ( $\text{CCl}_4$ ).

Step 4

Determine the Overall Redox Change

The oxidation state of carbon increases from $-4$ in $\text{CH}_4$ to $+4$ in $\text{CCl}_4$ , representing an 8-electron oxidation. (Chlorine is concurrently reduced from $0$ in $\text{Cl}_2$ to $-1$ in $\text{CCl}_4$ and $\text{HCl}$ ).

⭐

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 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; erroneously assigns +4 to carbon in methane as if hydrogen were a hydride (-1).

⚠️

Student Fallacy (Why candidates pick this trap): Treating hydrogen as hydride (-1) instead of +1.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; 0 is the oxidation state of elemental carbon (graphite/diamond), not methane.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming covalent carbon in hydrocarbons has zero oxidation state.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

Factually and chemically exact; carbon undergoes an 8-electron oxidation from -4 in CH4 to +4 in CCl4.

💡

Why this is airtight: None; clean electronegativity-based assignment.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; completely inverted sign and erroneous initial state.

⚠️

Student Fallacy (Why candidates pick this trap): Reversing signs and initial states.

⚠️

NTA Trap Alert: Electronegativity Comparison for C-H vs C-Cl

- In C-H: Carbon is MORE electronegative than H -> Carbon is NEGATIVE ( $-4$ ). - In C-Cl: Carbon is LESS electronegative than Cl -> Carbon is POSITIVE ( $+4$ ).

🛡️ How to avoid the -1 mark: Remember:

CH_4

= -4;

CCl_4

= +4! Change is -4 to +4!

📖

Core NEET Syllabus Standard

Chemistry · Redox Reactions Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Oxidation Number Calculation of Carbon: Conversion of CH4 to CCl4

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 180-minute timed NTA simulator.

Launch NEET 2020 CBT Test →

← Previous (Q.124) All Chemistry Questions Grid Next (Q.126) →

Spotted a mistake in this solution? Report an error (support@mycompetitiveexams.com)

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2020 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2020 Chemistry Question 125 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "In CH4, C is -4 (bonded to less electronegative H); in CCl4, C is +4 (bonded to more electronegative Cl)!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Rules for Assigning Oxidation Numbers

Calculate Oxidation State of Carbon in CH4

Calculate Oxidation State of Carbon in CCl4

Determine the Overall Redox Change

360° Option Elimination Engine Key Feature

NTA Trap Alert: Electronegativity Comparison for C-H vs C-Cl

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Redox Reactions

Equivalence Point & n-factor in Permanganate-Oxalate Titration

Decreasing Order of Oxidation State of Central Atoms

Oxidation States of Underlined Elements in KO₂, H₂O₂ and H₂SO₄

Identification of Non-Redox (Double Displacement) Reactions

Solve 4 NEET Questions a Day — One per Subject