Chemistry of Dinitrogen

NEET │ JEE Main & Advanced │ IIT-JAM │ GATE │ CSIR-NET │ SET │ BARC

1. Occurrence & Physical Properties

• Most abundant element in Earth’s atmosphere (~78% by volume)
• Colourless, odourless, tasteless, diamagnetic gas
• Molecular formula: N₂  Molar mass: 28 g/mol
• Boiling point: –195.8°C  Melting point: –210°C
• Extremely low solubility in water (23.2 mL per litre at 0°C)
• Very high bond energy of N≡N → 946 kJ/mol (strongest among diatomic molecules)
• Breaking even the first bond (N≡N → N=N) requires ~410 kJ/mol, and complete dissociation to 2N atoms needs ~945 kJ/mol

Reason for inertness: Very high bond dissociation energy + Very large (~10.8 eV) HOMO–LUMO energy gap + non-polar nature

2. Preparation of Dinitrogen

Method	Reaction	Purity
Laboratory (Pure)	NH₄Cl(aq) + NaNO₂(aq) → N₂↑ + NaCl + 2H₂O (heated gently)	Pure, used in labs
Commercial	Liquefaction & fractional distillation of liquid air	Very pure
From air (chemical)	Pass air over heated Cu → 2Cu + O₂ → 2CuO; N₂ remains	Reasonably pure
From urea	(NH₂)₂CO + 2HNO₂ → 2N₂ + CO₂ + 3H₂O	Lab method

Most important lab method asked in exams: (NH₄)₂Cr₂O₇ → N₂↑ + Cr₂O₃ + 4H₂O (on heating)

NEET, JEE, SLET: Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide
2NaN₃ → 2Na + 3N₂↑
Ba(N₃)₂ → Ba + 3N₂↑

3. Chemical Properties (Inert but Reacts under Specific Conditions)

(a) With Metals (Forms Nitrides)

3Mg + N₂ → Mg₃N₂ (magnesium nitride)
2Al + N₂ → 2AlN
6Li + N₂ → 2Li₃N (highly reactive)

(b) With Non-metals

N₂ + O₂ ⇌ 2NO (electric arc, 3000°C)
N₂ + 3H₂ ⇌ 2NH₃ (Haber process, Fe/Mo, 200 atm, 450°C)
N₂ + C → C₃N₄ (cyanogen at high temp)

(c) With Compounds

N₂ + 3CaC₂ → CaNCN + C (calcium cyanamide, 1000°C)
N₂ + 3Ca → Ca₃N₂ (at high temp)

4. Uses of Dinitrogen

• Manufacture of ammonia (Haber process)
• Inert atmosphere in metallurgy & chemical industries
• Liquid N₂ as coolant (cryogenics, preservation of biological samples)
• Filling electric bulbs (prevents filament oxidation)
• High-pressure cylinders for welding (mixed with argon)

Key Exam Points – Must Remember

• N≡N bond energy = 946 kJ/mol → highest among homonuclear diatomic
• Li reacts with N₂ at room temp; Mg burns in air to form nitride
• Only Li among alkali metals forms stable nitride
• N₂ is inert at room temp but reacts at high temp/electric arc/catalyst
• Best lab method: Heating ammonium dichromate or NH₄Cl + NaNO₂
• Liquid nitrogen boiling point: –196°C

Practice Questions (High-Yield MCQs + Assertion-Reason)

1. The bond order of N₂ molecule is

(A) 1 (B) 2 (C) 3 (D) 4

Answer: (C) 3

2. Pure nitrogen gas is obtained in laboratory by

(A) Heating (NH₄)₂Cr₂O₇ (B) Thermal decomposition of NaN₃
(C) Heating NH₄NO₂ (D) All of the above

Answer: (D) All of the above

3. Which metal reacts with N₂ at room temperature?

(A) Ca (B) Li (C) Mg (D) Al

Answer: (B) Li (6Li + N₂ → 2Li₃N)

4. Nitrogen is chemically inert due to

(A) Low bond energy (B) Absence of d-orbitals
(C) High bond energy of N≡N (D) High electronegativity

Answer: (C) High bond energy of N≡N (946 kJ/mol)

5. Liquid nitrogen is used for

(A) Preserving biological specimens (B) Coolant in superconductors
(C) Cryosurgery (D) All of the above

Answer: (D) All of the above

6. The incorrect statement about nitrogen is

(A) It is more electronegative than phosphorus
(B) It does not form pentahalides
(C) It shows maximum covalency of 4
(D) N₂ molecule is diamagnetic

Answer: (B) It does not form pentahalides → Correct statement!
Trick option – actually all are correct

7. Assertion: Magnesium continues to burn in atmosphere of nitrogen.
Reason: Magnesium reacts with N₂ to form Mg₃N₂.

Answer: Both A and R are true and R is correct explanation of A

8. When sodium azide is heated, the gas evolved is

(A) N₂O (B) NO (C) N₂ (D) NH₃

Answer: (C) N₂ (2NaN₃ → 2Na + 3N₂)

9. The catalyst used in Birkeland-Eyde process is

(A) Fe/Mo (B) Pt (C) Electric arc (D) V₂O₅

Answer: (C) Electric arc (N₂ + O₂ → 2NO)

10. The volume percentage of nitrogen in air is

(A) 21% (B) 78% (C) 0.03% (D) 0.93%

Answer: (B) 78%