Nitrogen and sulfur

1. Lack of Reactivity of Nitrogen (N₂)

• Structure: N₂ is a diatomic molecule with a triple covalent bond (N≡N).
• Bond Strength: The N≡N bond has a very high bond enthalpy (~945 kJ mol⁻¹), making it very stable and inert under normal conditions.
• Polarity: Nitrogen atoms have equal electronegativity, so N₂ is a non-polar molecule, reducing its tendency to react with polar substances.
• Conclusion: These factors lead to the chemical inertness of nitrogen at room temperature.

2. Ammonia and Ammonium Ion

(a) Basicity of Ammonia (NH₃) – Brønsted–Lowry Theory

• Brønsted–Lowry Base: A base is a proton (H⁺) acceptor.
• NH₃ has a lone pair on nitrogen which it uses to accept a proton, forming NH₄⁺. NH₃ + H⁺ → NH₄⁺
• Hence, NH₃ is a weak base (partially ionizes in water).

(b) Structure and Formation of the Ammonium Ion (NH₄⁺)

• Structure: Tetrahedral geometry with bond angles of 109.5°.
• Formation: Formed when NH₃ accepts a proton (H⁺) from an acid. NH₃ + H⁺ → NH₄⁺
  (Coordinate/dative bond forms between nitrogen and H⁺)

(c) Displacement of Ammonia from Ammonium Salts

• Ammonia (NH₃) can be displaced from ammonium salts by a strong base (e.g., NaOH): NH₄Cl + NaOH → NH₃ (gas) + NaCl + H₂O
• This is an acid–base reaction:
  NH₄⁺ (acid) donates a proton to OH⁻ (base).

3. Oxides of Nitrogen: Natural and Man-made Occurrence

⚛️ Main Oxides:

• Nitric oxide (NO)
• Nitrogen dioxide (NO₂)

🔹 Natural Sources:

• Lightning: Provides energy to combine N₂ and O₂ to form NO: N₂ + O₂ → 2NO

🔹 Man-made Sources:

• Internal combustion engines (cars, trucks) operate at high temperatures: N₂ + O₂ → 2NO
  NO then oxidizes in air:
  2NO + O₂ → 2NO₂

🔧 Catalytic Removal:

• Catalytic converters in car exhaust systems use platinum/rhodium catalysts to reduce NO and oxidize CO: 2NO + 2CO → N₂ + 2CO₂
  2NO + 2H₂ → N₂ + 2H₂O

4. Formation of PAN (Peroxyacetyl Nitrate)

• NO and NO₂ from vehicles can react with unburned hydrocarbons (from fuel) in presence of sunlight to form photochemical smog, which includes: PAN (CH₃COOONO₂)
• PAN is:
  • A powerful eye and lung irritant
  • Photochemically stable but decomposes in the body

5. Acid Rain Formation from NO and NO₂

💧 Direct Reaction with Water:

• NO₂ reacts with water to form nitric acid and nitrous acid: 2NO₂ + H₂O → HNO₂ + HNO₃
• These acids fall with rain = acid rain

🧪 Catalytic Role in SO₂ Oxidation:

• NO and NO₂ catalyze the oxidation of SO₂ to SO₃, which forms sulfuric acid:
  1. SO₂ + NO₂ → SO₃ + NO
  2. SO₃ + H₂O → H₂SO₄
  3. NO gets reoxidized to NO₂ by O₂ and re-enters the cycle

Thus, NO and NO₂ act as catalysts in the formation of acid rain from SO₂.

🔚 Summary Table

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