Q.129 Chemistry ⭐ 360° Option Elimination Active

NEET 2020 Chemistry Question 129 Solution

Topic: The p-Block Elements (Group 15) · Official NTA Booklet Code E1

Question

Urea reacts with water to form A which will decompose to form B. B when passed through Cu2+ (aq), deep blue colour solution C is formed. What is the formula of C from the following?

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Verified Against NTA Final Answer Key

Option (2) : [Cu(NH3)4]2+

📚 The p-Block Elements (Group 15) Easy

📋 Given Parameters & Problem Setup

Step 1 Reactant Urea (NH2CONH2) + H2O -> A

Step 2 Decomposition A decomposes to form B (Ammonia gas)

Step 3 Complexation B passed through Cu2+(aq) -> Deep blue solution C

Question Objective Identify the chemical formula of compound C

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The 30-Second Exam Hall Shortcut: ⚡ "Urea -> (NH4)2CO3 -> NH3; Cu2+ + 4NH3 -> [Cu(NH3)4]2+ (Deep Blue Solution)!"

⏱️ 20 seconds

Urea on hydrolysis forms ammonium carbonate ( $A = (NH_4)_2CO_3$ ), which decomposes to release ammonia gas ( $B = NH_3$ ). When $NH_3$ is passed through aqueous $Cu^{2+}$ , it forms the classic deep blue coordination complex ion $[Cu(NH_3)_4]^{2+}$ (tetraamminecopper(II) cation)! Mark [Cu(NH3)4]2+ (Option 2) in 5 seconds.

1 Hydrolysis of urea: NH2CONH2 + 2H2O -> (NH4)2CO3 (Compound A).

2 Decomposition of A: (NH4)2CO3 -> 2NH3 + H2O + CO2 (Gas B is NH3).

3 Complexation: Cu2+ (aq) + 4NH3 (aq) -> [Cu(NH3)4]2+ (aq) (Deep blue solution C).

4 Match with Option (2) [Cu(NH3)4]2+.

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · The p-Block Elements (Group 15: Ammonia - Preparation and Complex Formation)

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Hydrolysis of Urea to Ammonium Carbonate (A)

On a small scale, ammonia is prepared from urea. Urea reacts with water to form ammonium carbonate ( $A$ ): $\text{NH}_2\text{CONH}_2 + 2\text{H}_2\text{O} \to (\text{NH}_4)_2\text{CO}_3 \text{ (Compound A)}$

Step 2

Thermal Decomposition of Compound A to Gas B

Ammonium carbonate is unstable and readily decomposes into ammonia, water, and carbon dioxide: $(\text{NH}_4)_2\text{CO}_3 \rightleftharpoons 2\text{NH}_3\text{ (Gas B)} + \text{H}_2\text{O} + \text{CO}_2$ Thus, component $B$ is ammonia ( $\text{NH}_3$ ).

Step 3

Complex Formation with Aqueous Cu2+

Due to the presence of a lone pair of electrons on the nitrogen atom, ammonia acts as a Lewis base and donates electron pairs to transition metal cations to form coordination complexes. When ammonia solution is added to pale blue aqueous copper sulphate, it initially precipitates pale blue $\text{Cu(OH)}_2$ , which dissolves in excess ammonia to form an intense DEEP BLUE solution ( $C$ ): $\text{Cu}^{2+} \text{(aq, pale blue)} + 4\text{NH}_3 \text{(aq)} \rightleftharpoons [\text{Cu}(\text{NH}_3)_4]^{2+} \text{(aq, deep blue)}$

Step 4

Conclude Chemical Formula

The deep blue species $C$ is the tetraamminecopper(II) complex ion, $[\text{Cu}(\text{NH}_3)_4]^{2+}$ .

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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; copper sulphate solution is pale blue, not the intense deep blue ammine coordination complex.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking of starting copper salt.

2 B

✓ 100% Correct

Verdict & Scientific Reason:

Factually and chemically exact; ammonia coordinates to copper(II) to form the deep blue square planar tetraamminecopper(II) complex ion.

💡

Why this is airtight: None; classic inorganic qualitative and p-block test.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; $Cu(OH)_2$ is an insoluble pale blue precipitate, which dissolves in excess ammonia to form the soluble deep blue complex.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing transient precipitate with the final deep blue soluble complex.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; basic copper carbonate (malachite) is a green insoluble solid, not a deep blue solution.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming carbonate ion from urea forms a precipitate with copper.

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NTA Trap Alert: Transient Precipitate vs Soluble Complex

Pale blue precipitate is $Cu(OH)_2$ . DEEP BLUE solution in excess ammonia is the complex ion $[Cu(NH_3)_4]^{2+}$ !

🛡️ How to avoid the -1 mark: Deep blue solution with

Cu^{2+}

and

NH_3

[Cu(NH_3)_4]^{2+}

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Core NEET Syllabus Standard

Chemistry · The p-Block Elements Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Hydrolysis of Urea, Ammonia Formation and Deep Blue Tetraamminecopper(II) Complex

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Chemistry Question 129 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Urea -> (NH4)2CO3 -> NH3; Cu2+ + 4NH3 -> [Cu(NH3)4]2+ (Deep Blue Solution)!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Hydrolysis of Urea to Ammonium Carbonate (A)

Thermal Decomposition of Compound A to Gas B

Complex Formation with Aqueous Cu2+

Conclude Chemical Formula

360° Option Elimination Engine Key Feature

NTA Trap Alert: Transient Precipitate vs Soluble Complex

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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