Q.125 Chemistry ⭐ 360° Option Elimination Active

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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)

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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

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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).

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

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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 $\text{HCl}$ ).

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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.

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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!

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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 ↗

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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?

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