Nitrogen and Sulfur
Quick Notes
- Nitrogen gas is unreactive due to its strong N≡N triple bond and non-polar nature.
- Ammonia (NH3) is a Brønsted–Lowry base (it accepts protons).
- Ammonium ion (NH4+) forms when NH3 accepts a H+ ion in acid-base reactions and has a dative covalent bond.
- Ammonia can be displaced from its salts by stronger bases.
- NO and NO2 are produced in engines and removed by catalytic converters.
- NO and NO2 form acid rain and contribute to photochemical smog (PAN).
Full Notes
The Unreactivity of Nitrogen (N₂)
Nitrogen exists as a diatomic molecule (N≡N).
The triple bond between nitrogen atoms is very strong and requires a large amount of energy to break (bond enthalpy ≈ 945 kJ mol⁻¹).
The N₂ molecule is non-polar, so it doesn't easily attract or donate electrons.
Because of this, nitrogen is inert under normal conditions and does not react easily at room temperature.
Ammonia and Ammonium Ions
Basicity of Ammonia
According to Brønsted–Lowry theory a base is a proton acceptor.
NH₃ is able to act as a base (due to nitrogens lone pair of electrons) and accept a proton (H⁺) to form NH₄⁺.
Equation: NH₃ + H⁺ → NH₄⁺
Structure of the Ammonium Ion
The ammonium ion (NH₄⁺) forms when NH₃ donates its lone pair to bond with a H⁺ ion.
This is a dative covalent bond (also called coordinate bond).
NH₄⁺ has a tetrahedral shape with bond angles of 109.5°.
Displacement of Ammonia from Ammonium Salts
Ammonia can be displaced from its salts by stronger bases:
Example: NH₄Cl + NaOH → NH₃ (gas) + NaCl + H₂O
Explanation:
OH⁻ from NaOH accepts H⁺ from NH₄⁺ → forming NH₃ and H₂O.
This is a base displacing a weaker base (NH₃).
This reaction is often used to test for ammonium ions (by detecting NH₃ gas produced).
Nitrogen Oxides in Internal Combustion Engines
Oxides of nitrogen (NO and NO₂) form inside car engines.
Equations:
- N₂ + O₂ → 2NO
- 2NO + O₂ → 2NO₂
These gases are pollutants and contribute to acid rain and smog (see below).
Catalytic Removal
Catalytic converters in vehicles convert harmful gases into safe ones:
Overall reaction: 2CO + 2NO → 2CO₂ + N₂
- Platinum/rhodium catalysts are used
- Converts NO to N₂ and CO to CO₂
Formation of PAN and Photochemical Smog
NO and NO₂ can react with unburned hydrocarbons (from fuel) in sunlight to form peroxyacetyl nitrate (PAN).
Problems with PAN:
- An eye and lung irritant.
- A major component of photochemical smog.
Acid Rain from NO and NO₂
NO and NO₂ are involved in both direct and catalytic formation of acid rain.
Direct Reaction:
- NO₂ + H₂O → HNO₃ + HNO₂
- NO + ½O₂ → NO₂
Forms nitric acid (HNO₃) which falls as acid rain.
Catalytic Role in SO₂ Oxidation:
NO and NO₂ catalyse the oxidation of SO₂ to SO₃
- Oxidation of NO to NO₂:
2NO + O₂ → 2NO₂ - NO₂ oxidises SO₂ to SO₃:
SO₂ + NO₂ → SO₃ + NO
NO is regenerated, making this a catalytic cycle.
The SO₃ can then dissolve in water to form sulfuric acid (H₂SO₄) which falls as acid rain:
SO₃ + H₂O → H₂SO₄
Overall: Nitrogen oxides (NO/NO₂) don’t get used up. They speed up the formation of sulfuric acid, enhancing the severity of acid rain.
Summary
- N₂ is very unreactive due to its strong triple bond and non-polar nature.
- Ammonia is a base; forms NH₄⁺ via a dative bond when it accepts H⁺.
- Ammonia displaced from salts by stronger bases (test for NH₄⁺).
- NO and NO₂ form in car engines; removed by catalytic converters.
- NO and NO₂ contribute to PAN (photochemical smog) and acid rain.
- NO/NO₂ catalyse oxidation of SO₂ to SO₃, accelerating acid rain formation.