Q.89 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 89 Solution

Topic: Redox Reactions · Official NTA Booklet Code G5

Question

On balancing the given redox reaction, a Cr₂O₇²⁻ + b SO₃²⁻(aq) + c H⁺(aq) → 2a Cr³⁺(aq) + b SO₄²⁻(aq) + (c/2) H₂O(l), the coefficients a, b and c are found to be, respectively -

✓

Verified Against NTA Final Answer Key

Option (2) : 1, 3, 8

📚 Redox Reactions Easy

📋 Given Parameters & Problem Setup

Unbalanced equation a Cr₂O₇²⁻ + b SO₃²⁻(aq) + c H⁺(aq) → 2a Cr³⁺(aq) + b SO₄²⁻(aq) + (c/2) H₂O(l)

Medium Acidic

⚡

The 30-Second Exam Hall Shortcut: ⚡ Electron Transfer Matching: Cr₂O₇²⁻ (6 e⁻) : SO₃²⁻ (2 e⁻) = 1 : 3

⏱️ Save ~40 seconds

Dichromate accepts 6 electrons (2 Cr⁺⁶ → 2 Cr⁺³). Sulphite donates 2 electrons (S⁺⁴ → S⁺⁶). To balance electrons, ratio a : b must be 1 : 3! Immediately eliminates options (1), (3), and (4) where a:b is not 1:3! Answer is (1, 3, 8) in 5 seconds!

1 Oxidation half-reaction: S⁺⁴ (in SO₃²⁻) ⟶ S⁺⁶ (in SO₄²⁻) + 2 e⁻.

2 Reduction half-reaction: Cr₂⁺⁶ (in Cr₂O₇²⁻) + 6 e⁻ ⟶ 2 Cr³⁺.

3 Equalize electrons: Multiply sulphite half-reaction by 3. Thus, a = 1, b = 3.

4 Balance charge: Left charge = -2(1) - 2(3) + c = -8 + c. Right charge = +3(2) - 2(3) = 0. -8 + c = 0 ⟹ c = 8.

5 Water molecules: c/2 = 4 H₂O.

6 Coefficients: a = 1, b = 3, c = 8 (Option 2).

🛡️

Why You Can 100% Trust This Shortcut: Ion-electron balancing method based on conservation of mass and charge.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Balance the Reduction Half-Reaction

Dichromate ion Cr₂O₇²⁻ is reduced to Cr³⁺ in acidic medium. Balance Cr atoms, then oxygen with H₂O, and hydrogen with H⁺:

\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \longrightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}

Step 2

Balance the Oxidation Half-Reaction

Sulphite ion SO₃²⁻ is oxidized to sulphate ion SO₄²⁻. Add H₂O to the left and H⁺ to the right:

\text{SO}_3^{2-} + \text{H}_2\text{O} \longrightarrow \text{SO}_4^{2-} + 2\text{H}^+ + 2e^-

Step 3

Equalize Electron Transfer

Dichromate accepts 6 electrons while sulphite releases 2 electrons. Multiply the oxidation half-reaction by 3:

3\text{SO}_3^{2-} + 3\text{H}_2\text{O} \longrightarrow 3\text{SO}_4^{2-} + 6\text{H}^+ + 6e^-

Step 4

Combine Half-Reactions and Cancel Common Terms

Add both half-reactions and cancel net H₂O (7 - 3 = 4) and net H⁺ (14 - 6 = 8):

\text{Cr}_2\text{O}_7^{2-} + 3\text{SO}_3^{2-} + 8\text{H}^+ \longrightarrow 2\text{Cr}^{3+} + 3\text{SO}_4^{2-} + 4\text{H}_2\text{O}

⭐

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:

Inverts proton and dichromate stoichiometric coefficients.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate confused the stoichiometric order of coefficients a, b, and c.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate: a = 1, b = 3, c = 8 balances both atoms and total charge in acidic medium.

💡

Why this is airtight: None. Candidate correctly balanced electron gain and loss.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Scrambles coefficients, violating electron conservation.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate failed to balance oxidation numbers of chromium and sulphur.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Assigns 8 to sulphite instead of protons, leaving an enormous charge and electron mismatch.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate swapped the coefficients of sulphite and hydrogen ions.

⚠️

NTA Trap Alert: Charge Imbalance Mistake

Students often balance atoms without checking the algebraic sum of electrical charges on both sides. Left side: -2 + 3(-2) + 8(+1) = 0. Right side: 2(+3) + 3(-2) = 0. Verifying charges guarantees 100% accuracy.

🛡️ How to avoid the -1 mark: Always verify net charge equality: Net left charge must equal net right charge.

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Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Redox Reactions Standard Benchmark

Core Scientific Principle Tested:

"In balanced redox reactions, total electrons lost by reducing agent equal total electrons gained by oxidizing agent."

Syllabus Focus: Balancing of Redox Reactions – Ion-Electron Method

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 89 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Electron Transfer Matching: Cr₂O₇²⁻ (6 e⁻) : SO₃²⁻ (2 e⁻) = 1 : 3

Step-by-Step Rigorous Workflow & Mathematical Proof

Balance the Reduction Half-Reaction

Balance the Oxidation Half-Reaction

Equalize Electron Transfer

Combine Half-Reactions and Cancel Common Terms

360° Option Elimination Engine Key Feature

NTA Trap Alert: Charge Imbalance Mistake

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Redox Reactions

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Identification of Non-Redox (Double Displacement) Reactions

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