Chemistry of Ammonia

Highly useful for NEET, JEE (Main & Advanced), JAM, GATE, CSIR-NET, SET and other competitive exams

1. Introduction and Occurrence

• Ammonia is a colorless gas with pungent smell.
• Molecular formula: NH₃
• Molar mass: 17 g/mol
• It is lighter than air (vapour density = 8.5).
• Highly soluble in water (1 volume water dissolves ~700 volumes NH₃ at 20°C).
• Occurs naturally from decomposition of nitrogenous organic matter (proteins, urea).

2. Structure

• Trigonal pyramidal shape (sp³ hybridization).
• Bond angle: 107° (due to lone pair-bond pair repulsion).
• Has a lone pair on nitrogen → acts as Lewis base.
• Polar molecule (dipole moment 1.46 D).

Key Exam Tip (CSIR NET/GATE): Discuss inversion barrier (~24.7 kJ/mol) and its fluxional nature.

3. Laboratory Preparation

Method: Heating ammonium chloride with slaked lime
2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃↑
Method: From Urea
NH₂CONH₂ + 2H₂O → 2NH₃↑ + H₂O + CO₂↑

Drying agent: Quick lime (CaO). Conc. H₂SO₄ or P₂O₅ cannot be used (react with NH₃).

Collection: By downward delivery (lighter than air) or in inverted dry flask.

4. Industrial Preparation – Haber-Bosch Process

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = –92.4 kJ/mol

Conditions (for high yield):

• High pressure: 200–300 atm (favours forward reaction, decreases volume)
• Optimum temperature: 450–500°C (compromise between rate and equilibrium)
• Catalyst: Finely divided iron (Fe) with Mo or K₂O/Al₂O₃ as promoter
• Recycling of unreacted N₂ and H₂

Le Chatelier’s principle is frequently asked in exams.

5. Physical Properties

Property	Value
Boiling point	–33°C
Melting point	–78°C
Density (liquid)	0.68 g/mL at –33°C
Solubility in water	Very high (forms NH₄OH)

Exam Relevance (NEET/JEE): High solubility tested in identification tests; liquefaction under pressure for storage.

6. Chemical Properties

(a) Basic Nature

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (weak base, Kʙ = 1.8 × 10⁻⁵)

Liquor ammonia (25–30% NH₃ solution) turns red litmus blue.

(b) Reaction with Acids (Salt formation)

• NH₃ + HCl → NH₄Cl (white dense fumes)
• 2NH₃ + H₂SO₄ → (NH₄)₂SO₄
• NH₃ + HNO₃ → NH₄NO₃

(c) Complex Formation (Lewis base)

• Ag⁺ + 2NH₃ → [Ag(NH₃)₂]⁺ (Tollens’ reagent)
• Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺ (deep blue)
• Used in qualitative analysis (Group IV cations)

(d) Combustion

4NH₃ + 3O₂ → 2N₂ + 6H₂O (does not burn in air)
4NH₃ + 5O₂ → 4NO + 6H₂O (burns in pure O₂ with greenish-yellow flame, Pt catalyst at 800°C)

(e) Reducing Nature

• Reduces CuO → Cu (heated in dry NH₃)
• Reduces hot metal oxides to metals.

(f) Reaction with Halogens

• 8NH₃ + 3Cl₂ → 6NH₄Cl + N₂ (excess NH₃)
• NH₃ + 3Cl₂ → NCl₃ + 3HCl (excess Cl₂, explosive nitrogen trichloride)

Key Exam Tip (CSIR NET/SET): Liquid NH₃ as solvent for alkali metals (blue solutions due to solvated e⁻).

7. Uses of Ammonia

• Manufacture of fertilizers (urea, ammonium sulphate, ammonium nitrate)
• Manufacture of HNO₃ (Ostwald process)
• Refrigerant (liquid NH₃)
• Laboratory reagent
• In soda lime (NaOH + CaO) preparation for removing CO₂
• Cleaning Agent: Household ammonia (NH₄OH)
• Explosives: NH₄NO₃

Exam Tip (NEET/JEE): Environmental impact (eutrophication from runoff).

8. Important Compounds

Compound	Preparation/Reaction	Key Point
Liquid ammonia	Auto-ionization: 2NH₃ ⇌ NH₄⁺ + NH₂⁻	Acidic solutions with metals (e.g., Na → sodamide)
Ammonium hydroxide	NH₃ + H₂O → NH₄OH	Weak base, partially ionized
Nessler’s reagent	K₂[HgI₄] + KOH	Test for NH₄⁺ (brown ppt/colour)

9. Test for Ammonia

• Pungent smell
• Turns moist red litmus blue
• White fumes with HCl
• Brown ppt with Nessler’s reagent
  Nessler's reagent (K₂[HgI₄] + KOH): Test for NH₄⁺ (brown ppt/colour)
• Deep blue with CuSO₄ solution.

Key Exam Points

• Haber process conditions and catalyst/promoter
• Basic nature and complex formation
• Reaction with halogens (excess vs limited)
• Drying agent for NH₃ (CaO, not H₂SO₄)
• Ammonia fountain experiment (high solubility demonstration)
• NH₃ as reducing agent

Practice Questions (MCQs + Assertion-Reason)

Multiple Choice Questions (MCQs)

1. In Haber-Bosch process, the catalyst and promoter respectively are

(A) Fe, Mo  (B) V₂O₅, K₂O  (C) Pt, Al₂O₃  (D) Ni, Cr₂O₃

Answer: (A) Fe, Mo

2. Ammonia cannot be dried by passing over

(A) CaO  (B) P₂O₅  (C) Conc. H₂SO₄  (D) Anhydrous CaCl₂

Answer: (C) Conc. H₂SO₄

Conc. H₂SO₄ reacts with NH₃ to form (NH₄)₂SO₄.

3. The brown precipitate or colour with Nessler’s reagent is due to

(A) Hg₂I₂  (B) Hg(NH₂)I  (C) Millon’s base  (D) HgI₂

Answer: (B) Hg(NH₂)I (Iodide of Millon’s base)

4. When excess chlorine is passed through cold dilute ammonia, the product formed is

(A) NH₄Cl  (B) NCl₃  (C) N₂  (D) HCl

Answer: (B) NCl₃

5. Ammonia acts as a ligand because nitrogen has

(A) Lone pair  (B) Vacant d-orbital  (C) Positive charge  (D) Negative charge

Answer: (A) Lone pair

6. The geometry of ammonia molecule is

(A) Tetrahedral  (B) Trigonal pyramidal  (C) Trigonal planar  (D) Square planar

Answer: (B) Trigonal pyramidal

7. Which of the following metals dissolves in liquid ammonia giving deep blue solution?

(A) Fe  (B) Cu  (C) Na  (D) Ag

Answer: (C) Na (alkali & alkaline earth metals)

8. In the reaction 2NH₃ + NaOCl → N₂H₄ + NaCl + H₂O, ammonia acts as

(A) Oxidising agent  (B) Reducing agent  (C) Both  (D) None

Answer: (B) Reducing agent

9. Ammonia fountain works because

(A) Ammonia is lighter than air
(B) Ammonia is highly soluble in water
(C) Ammonia has high boiling point
(D) Ammonia is basic in nature

Answer: (B) Ammonia is highly soluble in water

10. The hybridization of nitrogen in NH₃ molecule is

(A) sp  (B) sp²  (C) sp³  (D) dsp²

Answer: (C) sp³

Assertion–Reasoning Questions (For JEE Advanced/CSIR-NET)

1. Assertion (A): Ammonia is dried by quick lime and not by conc. H₂SO₄.
Reason (R): Conc. H₂SO₄ reacts with ammonia to form ammonium sulphate.

(A) Both A and R are correct and R is correct explanation of A
(B) Both A and R are correct but R is not correct explanation
(C) A is correct, R is incorrect
(D) A is incorrect, R is correct

Answer: (A)

2. Assertion (A): Bond angle in NH₃ is 107° while in NF₃ it is 102°.
Reason (R): F is more electronegative than H, causing greater lone pair-bond pair repulsion in NF₃.

(A) (B) (C) (D) same as above

Answer: (C) A is correct, R is incorrect

Actual reason: Greater bp-bp repulsion in NF₃ due to highly electronegative F atoms.

3. Assertion (A): In Haber process high pressure is used.
Reason (R): The forward reaction has decrease in number of moles.

Answer: (A)

4. Assertion (A): Liquid ammonia is used as a refrigerant.
Reason (R): Liquid ammonia has very high latent heat of vaporization.

Answer: (A)

5. Assertion (A): Ammonia turns moist red litmus blue.
Reason (R): Aqueous solution of ammonia is basic due to formation of OH⁻ ions.

Answer: (A)