Important Nitrogen Compounds Revision Guide

Introduction to Major Compounds of Nitrogen

Nitrogen, a Group 15 element, exhibits a wide range of oxidation states from -3 to +5, leading to a diverse array of compounds with varied chemical properties. Key compounds include ammonia, nitric acid, and various oxides, all of which are crucial for industrial applications and biological systems.

Ammonia (NH₃)

Ammonia is a pungent, colourless gas, widely used in fertilizers and chemical synthesis.

Chemical Formula & Common Name

• Formula: NH₃
• Common Name: Ammonia

Laboratory Preparation

Ammonia is prepared in the laboratory by heating an ammonium salt with a strong base. Equation: 2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2NH₃(g) + 2H₂O(l)

Properties and Exam-Relevant Reactions

Physical Properties

• Colourless gas with a characteristic pungent odour.
• Lighter than air.
• Highly soluble in water, forming ammonium hydroxide solution which is alkaline.
• Easily liquefiable under pressure, used as a refrigerant.

Chemical Properties

1. Basic Nature: Dissolves in water to form a weak base, ammonium hydroxide. Reacts with acids to form ammonium salts.
   • NH₃(g) + H₂O(l) ⇌ NH₄OH(aq)
   • NH₄OH(aq) + HCl(aq) → NH₄Cl(aq) + H₂O(l)
   • 2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)
2. Reducing Agent: Can reduce metal oxides to metals when heated.
   • 2NH₃(g) + 3CuO(s) → 3Cu(s) + N₂(g) + 3H₂O(g)
3. Reaction with Halogens:
   • With excess ammonia, nitrogen gas is formed: 8NH₃(g) + 3Cl₂(g) → 6NH₄Cl(s) + N₂(g)
   • With excess chlorine, nitrogen trichloride is formed (explosive): NH₃(g) + 3Cl₂(g) → NCl₃(l) + 3HCl(g)
4. Formation of Ammines (Complexes): Ammonia acts as a ligand to form complex compounds, especially with transition metals.
   • CuSO₄(aq) + 4NH₄OH(aq) → [Cu(NH₃)₄]SO₄(aq) + 4H₂O(l) (Deep blue solution)

Nitric Acid (HNO₃)

Nitric acid is a strong mineral acid and a powerful oxidizing agent.

Chemical Formula & Common Name

• Formula: HNO₃
• Common Name: Nitric Acid

Laboratory Preparation

Nitric acid is prepared in the laboratory by heating potassium nitrate (or sodium nitrate) with concentrated sulfuric acid. Equation: NaNO₃(s) + H₂SO₄(conc.) → NaHSO₄(s) + HNO₃(g) (The HNO₃ vapour is condensed to liquid nitric acid.)

Properties and Exam-Relevant Reactions

Physical Properties

• Pure nitric acid is a colourless liquid. Commercial nitric acid is often yellowish due to dissolved nitrogen dioxide (NO₂).
• Fuming nitric acid emits reddish-brown fumes of NO₂.
• Strongly corrosive.

Chemical Properties

1. Strong Acid: Dissociates completely in water.
   • HNO₃(aq) + H₂O(l) → H₃O⁺(aq) + NO₃⁻(aq)
2. Powerful Oxidizing Agent: Its oxidizing action depends on concentration and temperature.
   • Reaction with Metals:
     • Copper with dilute HNO₃ (cold): Forms nitric oxide (NO). 3Cu(s) + 8HNO₃(dil.) → 3Cu(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)
     • Copper with concentrated HNO₃ (hot): Forms nitrogen dioxide (NO₂). Cu(s) + 4HNO₃(conc.) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
     • Zinc with very dilute HNO₃: Forms ammonium nitrate. 4Zn(s) + 10HNO₃(very dil.) → 4Zn(NO₃)₂(aq) + NH₄NO₃(aq) + 3H₂O(l)
     • Zinc with dilute HNO₃: Forms nitrous oxide (N₂O). 4Zn(s) + 10HNO₃(dil.) → 4Zn(NO₃)₂(aq) + N₂O(g) + 5H₂O(l)
     • Zinc with concentrated HNO₃: Forms nitrogen dioxide (NO₂). Zn(s) + 4HNO₃(conc.) → Zn(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
     • Passivity: Iron, aluminium, and chromium become passive (form a protective oxide layer) when treated with concentrated nitric acid, preventing further reaction.
   • Reaction with Non-metals: Oxidizes non-metals to their respective oxyacids.
     • Carbon: C(s) + 4HNO₃(conc.) → CO₂(g) + 4NO₂(g) + 2H₂O(l)
     • Sulphur: S₈(s) + 48HNO₃(conc.) → 8H₂SO₄(aq) + 48NO₂(g) + 16H₂O(l)
     • Phosphorus: P₄(s) + 20HNO₃(conc.) → 4H₃PO₄(aq) + 20NO₂(g) + 4H₂O(l)
3. Aqua Regia: A 3:1 mixture of concentrated HCl and concentrated HNO₃, capable of dissolving noble metals like gold and platinum.
   • Au(s) + 4HCl(aq) + HNO₃(aq) → H[AuCl₄](aq) + NO(g) + 2H₂O(l)

Important Oxides of Nitrogen

Nitrogen forms several oxides, exhibiting various oxidation states. The most common and exam-relevant are N₂O, NO, and NO₂.

Nitrous Oxide (N₂O)

• Formula: N₂O
• Oxidation State of N: +1
• Preparation: By gently heating ammonium nitrate. NH₄NO₃(s) → N₂O(g) + 2H₂O(l)
• Properties: Colourless, neutral gas. Known as “laughing gas.” Supports combustion like oxygen.

Nitric Oxide (NO)

• Formula: NO
• Oxidation State of N: +2
• Preparation: Reaction of copper with dilute nitric acid (as shown above). 3Cu(s) + 8HNO₃(dil.) → 3Cu(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)
• Properties: Colourless, neutral gas. Paramagnetic in gaseous state. Reacts instantly with oxygen to form reddish-brown nitrogen dioxide. 2NO(g) + O₂(g) → 2NO₂(g)

Nitrogen Dioxide (NO₂)

• Formula: NO₂
• Oxidation State of N: +4
• Preparation:
  • Reaction of copper with concentrated nitric acid (as shown above). Cu(s) + 4HNO₃(conc.) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
  • Decomposition of heavy metal nitrates: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
• Properties: Reddish-brown, pungent, toxic gas. Acidic oxide (forms a mixture of nitric and nitrous acids with water). Exists in equilibrium with colourless dinitrogen tetroxide (N₂O₄). 2NO₂(g) (brown) ⇌ N₂O₄(g) (colourless) (The equilibrium shifts with temperature.)

Comparative Properties of Key Nitrogen Compounds

Compound	Formula	Oxidation State of N	Nature	Key Preparation Method	Important Reaction/Property
Ammonia	NH₃	-3	Basic	Haber’s Process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)	Forms ammonium salts; forms deep blue solution with Cu²⁺
Nitric Acid	HNO₃	+5	Acidic, Oxidizing	Ostwald Process (industrial): NH₃ → NO → NO₂ → HNO₃	Oxidizes most metals & non-metals; forms aqua regia
Nitrous Oxide	N₂O	+1	Neutral	Thermal decomposition of NH₄NO₃: NH₄NO₃ → N₂O + 2H₂O	”Laughing gas”; supports combustion (like O₂)
Nitric Oxide	NO	+2	Neutral	Reaction of Cu with dil. HNO₃: 3Cu + 8HNO₃(dil) → 3Cu(NO₃)₂ + 2NO + 4H₂O	Reacts with O₂ to form brown NO₂; paramagnetic
Nitrogen Dioxide	NO₂	+4	Acidic	Reaction of Cu with conc. HNO₃: Cu + 4HNO₃(conc) → Cu(NO₃)₂ + 2NO₂ + 2H₂O	Reddish-brown gas; exists in equilibrium with colourless N₂O₄