Nitrogen and Sulphur Chemistry & Environmental Effects

Vocabulary flashcards covering key terms, reactions and environmental aspects of nitrogen and sulphur chemistry presented in the lecture notes.

Nitrogen (N₂)

Diatomic, inert gas composing about 78 % of Earth’s atmosphere; has a very high triple-bond energy (944 kJ mol⁻¹).

Triple-bond energy of N₂

The 944 kJ mol⁻¹ required to break the N≡N bond, making N₂ reluctant to react under normal conditions.

Natural nitrogen fixation

Lightning converts N₂ and O₂ into NO/NO₂, which further form nitrous acid that plants can absorb.

Anthropogenic nitrogen oxides

NO and NO₂ produced in car engines at high temperatures; contribute to smog and acid rain.

Photochemical smog

Brownish haze formed when NO/NO₂ react with hydrocarbons (e.g., CH₄) in sunlight.

Acid rain (from NOx)

Rainwater acidified by HNO₂ and HNO₃ formed from atmospheric NO and NO₂.

Haber Process

Industrial synthesis of NH₃ from N₂ + 3 H₂ ⇌ 2 NH₃, ΔH = −92 kJ mol⁻¹.

Haber conditions

≈150–250 atm, ~450 °C, iron catalyst; compromise between rate and yield (~20 % NH₃).

Source of industrial N₂

Fractional distillation of liquid air based on differing boiling points.

Steam reforming

CH₄ + H₂O → CO + 3 H₂; major source of H₂ for the Haber process.

Cracking (ethane)

C₂H₆ → C₂H₄ + H₂; an alternative hydrogen source.

Ammonia (NH₃)

Pungent, weakly basic gas that forms hydrogen bonds with water and turns red litmus blue.

Hydrogen bonding in NH₃

Intermolecular attraction between N–H and O of water, giving high solubility.

Ammonium ion (NH₄⁺)

Tetrahedral cation formed by NH₃ accepting H⁺ through a dative (coordinate) bond.

Coordinate (dative) bond

Covalent bond in which both electrons come from the same atom, as in NH₄⁺.

Ammonium salt

Compound containing NH₄⁺ paired with an anion (e.g., NH₄Cl, (NH₄)₂SO₄).

Ammonium + base reaction

NH₄⁺ + OH⁻ → NH₃ + H₂O; liberates ammonia on heating.

Ammonium test

Warm sample with NaOH; released NH₃ turns damp red litmus blue.

Nitrate ion (NO₃⁻)

Oxidation state +5 nitrogen oxyanion; component of many fertilisers.

Nitrate reduction test

Heat sample with NaOH and Al foil; Al reduces NO₃⁻ to NH₃, detected by litmus.

Nitrite ion (NO₂⁻)

Nitrogen oxyanion in oxidation state +3; also turns to NH₃ with NaOH/Al but gives brown NO₂ gas with acid.

Eutrophication

Excess nutrients (nitrates, ammonium) in water cause algal bloom, depleting oxygen and killing fish.

Fertiliser N-P-K

Major plant nutrients: nitrogen, phosphorus, potassium; examples include KNO₃ and (NH₄)₃PO₄.

Elemental sulphur (S₈)

Yellow solid cyclo-octasulphur obtained chiefly from volcanic deposits.

Sulphur dioxide (SO₂)

Pungent, bent-shaped gas; acidic oxide causing eye irritation, acid rain, and used as a food preservative.

Sulphurous acid (H₂SO₃)

Weak acid formed when SO₂ dissolves in water.

Sulphur trioxide (SO₃)

Planar-trigonal, highly reactive acidic oxide; reacts with water to form H₂SO₄.

Sulphuric acid (H₂SO₄)

Strong diprotic acid that ionises completely; pH ≈ 1–2 for 1 mol dm⁻³ solution.

Contact Process

Industrial route to H₂SO₄: 1) S or SO₂ combustion, 2) 2 SO₂ + O₂ ⇌ 2 SO₃ (V₂O₅ catalyst, 450 °C, 1–2 atm), 3) SO₃ absorbed in H₂SO₄ to form oleum then diluted.

Vanadium(V) oxide (V₂O₅)

Heterogeneous catalyst used in the reversible oxidation of SO₂ to SO₃.

Oleum (H₂S₂O₇)

Fuming sulphuric acid made by dissolving SO₃ in concentrated H₂SO₄; later diluted to form H₂SO₄.

Ba²⁺ sulphite test

SO₃²⁻ + Ba²⁺ → BaSO₃ white ppt that dissolves in HCl.

Ba²⁺ sulphate test

SO₄²⁻ + Ba²⁺ → BaSO₄ white ppt insoluble in HCl, confirming sulfate.

Acid rain (from SOx)

Rain containing H₂SO₃/H₂SO₄ formed when atmospheric SO₂/SO₃ react with water.

Effects of acid rain

Corrodes metals, damages limestone buildings (CaCO₃), harms aquatic life and vegetation.

Catalytic converter

Pt/Rh/Ir device in car exhaust that reduces NOx to N₂ and oxidises CO, thus lowering acid-rain precursors.

Converter reactions

2 CO + 2 NO → N₂ + 2 CO₂ (plus oxidation of hydrocarbons).

Flue-gas desulfurisation

SO₂ scrubbed by CaO/CaCO₃ in chimneys forming CaSO₄ (gypsum).

Gypsum (CaSO₄·2H₂O)

Harmless by-product of SO₂ scrubbing; used in construction materials.

Smog

Mixture of smoke and fog containing pollutants like NOx and unburnt hydrocarbons.

Homogeneous catalyst

Catalyst in the same phase as reactants; e.g., NO₂ catalysing SO₂ oxidation in acid-rain chemistry.

Fractional distillation of liquid air

Separation of N₂, O₂, Ar based on boiling-point differences.

Steam cracking of hydrocarbons

Thermal decomposition producing smaller alkenes and H₂, aiding ammonia production.

Trigonal pyramidal geometry

Shape of NH₃ with 107° H–N–H bond angle due to one lone pair.

Tetrahedral geometry

Shape of NH₄⁺ with ~109° bond angles around the central nitrogen.