Q.58 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 58 Solution

Topic: Redox Reactions · Official NTA Booklet Code M1

Question

Which of the following reactions is the metal displacement reaction? Choose the right option.

✓

Verified Against NTA Final Answer Key

Option (2) : Cr2O3 + 2Al -> Al2O3 + 2Cr

📚 Redox Reactions Easy

📋 Given Parameters & Problem Setup

Category Metal displacement reaction

⚡

The 30-Second Exam Hall Shortcut: ⚡ Metal Replaces Metal = Metal Displacement

⏱️ Save ~20 seconds

A metal displacement reaction occurs when a more electropositive free metal displaces a less electropositive metal from its compound. In $Cr_2O_3 + 2Al \xrightarrow{\Delta} Al_2O_3 + 2Cr$ , metallic aluminum ( $Al$ ) displaces chromium ( $Cr$ ) from chromium oxide! Match Option (2)!

1 Check reaction (1):

2KClO_3 \to 2KCl + 3O_2

\implies

Decomposition reaction.

2 Check reaction (2):

Cr_2O_3 + 2Al \to Al_2O_3 + 2Cr

\implies Al

(metal) displaces

Cr

(metal) from its oxide

\implies

Metal displacement reaction.

3 Check reaction (3):

Fe + 2HCl \to FeCl_2 + H_2\uparrow

\implies

Iron displaces hydrogen (non-metal)

\implies

Hydrogen/non-metal displacement reaction.

4 Check reaction (4):

2Pb(NO_3)_2 \to 2PbO + 4NO_2 + O_2\uparrow

\implies

Decomposition reaction.

5 Match Option (2).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Redox Reactions classification of redox reaction types.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Definition of Metal Displacement Reaction

In NCERT Redox Reactions, a metal displacement reaction is defined as a redox process in which a metal in a compound is displaced by another metal in the uncombined elementary state. The displacing metal is more reducing (has a more negative standard reduction potential) than the displaced metal.

Step 2

Analyzing Option (2): Thermite Reduction of Chromium

In the aluminothermic (Goldschmidt) reduction process: - Aluminum is oxidized from oxidation state $0$ to $+3$ in $Al_2O_3$ . - Chromium is reduced from oxidation state $+3$ in $Cr_2O_3$ to $0$ in elemental $Cr$ metal. Because elemental aluminum (a metal) displaces chromium (another metal) from its compound:

Cr_2O_3 + 2Al \xrightarrow{\Delta} Al_2O_3 + 2Cr \quad (\mathbf{\text{Metal Displacement Reaction}})

Step 3

Classification of Alternative Reactions

- Reaction 1: Decomposition reaction (decomposition of $KClO_3$ ). - Reaction 3: Hydrogen displacement (non-metal displacement) reaction where iron displaces hydrogen gas from an acid. - Reaction 4: Thermal decomposition of lead nitrate.

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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 1

✗ Incorrect

Verdict & Scientific Reason:

Thermal decomposition redox reaction, not a displacement reaction.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing decomposition with displacement.

2 2

✓ 100% Correct

Verdict & Scientific Reason:

Exact textbook example of a metal displacement reaction (Al displaces Cr).

💡

Why this is airtight: None; canonical NCERT classification.

3 3

✗ Incorrect

Verdict & Scientific Reason:

This is a NON-METAL (hydrogen) displacement reaction, not a metal displacement reaction.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing hydrogen displacement with metal displacement.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Thermal decomposition reaction of lead(II) nitrate.

⚠️

Student Fallacy (Why candidates pick this trap): Misclassifying nitrate decomposition.

⚠️

NTA Trap Alert: Hydrogen Displacement vs. Metal Displacement Trap

Reaction (3) $Fe + 2HCl \to FeCl_2 + H_2$ is commonly picked by students who think "a metal reacted, so it's metal displacement". In Reaction (3), the displaced entity is HYDROGEN (a non-metal), making it a non-metal displacement. In Reaction (2), aluminum displaces CHROMIUM (a metal).

🛡️ How to avoid the -1 mark: Check the DISPLACED entity: If a metal is pushed out, it is metal displacement. If

H_2

gas is pushed out, it is non-metal displacement.

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Core NEET Syllabus Standard

NCERT Chemistry · Redox Reactions Standard Benchmark

Core Scientific Principle Tested:

"A metal displacement reaction involves displacement of a metal from its compound by another metal in uncombined state."

Syllabus Focus: Types of Redox Reactions (Displacement Reactions)

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NEET 2021 Chemistry Question 58 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Metal Replaces Metal = Metal Displacement

Step-by-Step Rigorous Workflow & Mathematical Proof

Definition of Metal Displacement Reaction

Analyzing Option (2): Thermite Reduction of Chromium

Classification of Alternative Reactions

360° Option Elimination Engine Key Feature

NTA Trap Alert: Hydrogen Displacement vs. Metal Displacement Trap

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

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