Nitrogen, Sulfur and Pollution

Nitrogen is unreactive because of its strong N≡N triple bond (945 kJ mol⁻¹) and lack of polarity. Ammonia is a weak base: $\mathrm{NH_3+H^+\rightarrow NH_4^+}$ (dative bond, tetrahedral ion). Warming an ammonium salt with alkali releases NH₃. Oxides of nitrogen form in lightning and hot engines ( $\mathrm{N_2+O_2\rightarrow2NO}$ ); catalytic converters remove them ( $\mathrm{2CO+2NO\rightarrow2CO_2+N_2}$ ). NO₂ catalyses the oxidation of SO₂ in the atmosphere: $\mathrm{SO_2+NO_2\rightarrow SO_3+NO}$ , $\mathrm{NO+\frac12O_2\rightarrow NO_2}$ , contributing to acid rain. NOₓ and hydrocarbons in sunlight form photochemical smog (including PAN).

Cambridge 9701 Paper 1Cambridge 9701 Paper 2≈ 40 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Explain the lack of reactivity of nitrogen.
Describe the basicity of ammonia and the formation of the ammonium ion.
Describe sources and removal of NOₓ and SO₂.
Explain the catalytic role of NO₂ in acid rain formation.

Pause and think

Air is 78% nitrogen and car engines burn fuel in air. Why does NO only form in significant amounts inside hot engines?

Section 2 of 14

Core idea

Start hereInvisible pollutants

Brown haze over cities, acid rain on forests: both come from oxides of nitrogen and sulfur produced when we burn fuels.

The big idea

N₂ is unreactive because of its triple bond, but high temperatures in engines form NOₓ. NO₂ catalyses the oxidation of SO₂, leading to acid rain.

1. Catalytic role of NO₂

NO₂ catalytic cycle

Drawing diagram…

NO₂ is regenerated.

Worked exampleCatalytic converter

Write an equation for the removal of NO and CO in a converter.

Goal: Balanced equation.
Strategy: Redox: CO oxidised, NO reduced.

$\mathrm{CO\rightarrow CO_2}$
C: +2 → +4.
$\mathrm{2CO+2NO\rightarrow2CO_2+N_2}$
N: +2 → 0.

Answer

\mathrm{2CO+2NO\rightarrow2CO_2+N_2}

✓ CheckAtoms and electrons balance ✓.

Key takeawayAmmonium test Warm with NaOH(aq): NH₃ released turns damp red litmus blue.

Pause and think

Why does photochemical smog peak in the afternoon rather than at rush hour?

Summary

N≡N very strong.
NH₃ weak base.
NOₓ, SO₂, acid rain, smog.

NOₓ: Oxides of nitrogen, NO and NO₂.
Acid rain: Rain acidified by dissolved SO₂, SO₃ and NOₓ.
PAN: Peroxyacetyl nitrate, a component of photochemical smog.

HistoryLos Angeles smog. In the 1940s Los Angeles was hit by eye-stinging brown smog. Arie Haagen-Smit showed it formed from car exhaust reacting in sunlight, leading to catalytic converters.

Fun factLightning fertiliser. Lightning makes NO, which forms nitric acid in rain, delivering naturally fixed nitrogen to soils.

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