AS Chemistry: Nitrogen and Sulfur — Revision Notes

Condensed for the final weeks. For the full explanation, use the Nitrogen and Sulfur study guide.

Why nitrogen is so unreactive

Two reasons, and both are needed for full marks:

1. The N≡N triple bond has a very high bond enthalpy (944 kJ mol⁻¹), so a large activation energy is required.
2. The molecule is non-polar, so it is not attacked by nucleophiles or electrophiles.

Ammonia

Basic because the nitrogen lone pair accepts a proton:

NH3 + H+  ->  NH4+        (dative covalent bond formed)
NH3 + H2O <=> NH4+ + OH-  (weak base, partially ionised)

Shape: pyramidal, bond angle 107° — four electron pairs, one of which is a lone pair repelling more strongly.

The ammonium ion NH₄⁺ is tetrahedral, 109.5°, because the lone pair has been used to form the dative bond. All four N–H bonds in NH₄⁺ are identical once formed, even though one was made differently (by the lone pair donating to H⁺) from the other three.

Test for ammonium ions: warm with NaOH(aq); ammonia gas turns damp red litmus blue. This is the same acid-base logic run in reverse: NH₄Cl + NaOH → NaCl + NH₃ + H₂O.

Oxides of nitrogen

Formation: at the high temperatures inside a car engine, nitrogen and oxygen from the air combine. NO also forms naturally — the enormous energy of a lightning strike is enough to overcome nitrogen’s high activation energy, driving the same reaction directly in the atmosphere.

N2 + O2  ->  2NO           (in the engine, or from lightning)
2NO + O2 ->  2NO2          (in the atmosphere)

NO acts as a catalyst in the oxidation of sulfur dioxide — it is regenerated:

NO + 1/2 O2  ->  NO2
NO2 + SO2    ->  SO3 + NO      <- NO regenerated

Showing that NO reappears unchanged is the mark-earning point.

Catalytic converter (platinum/rhodium/palladium catalyst):

2CO + 2NO  ->  2CO2 + N2

One reaction removes a toxic gas and an acid-rain precursor.

Photochemical smog: atmospheric NO and NO₂ also react with unburned hydrocarbons from vehicle exhaust, driven by sunlight, to form peroxyacetyl nitrate (PAN) — an eye and lung irritant characteristic of sunny, traffic-heavy cities.

Acid rain

Pollutant	Source	Product
SO₂	Sulfur impurities in fossil fuels	H₂SO₃ then H₂SO₄
NOₓ	High-temperature combustion in engines	HNO₃

Effects: acidifies lakes killing fish; leaches nutrients and releases toxic Al³⁺ from soils; damages foliage; erodes limestone and marble buildings; corrodes metals.

NOₓ contributes in two distinct ways. Directly: nitrogen oxides dissolve in atmospheric water and are oxidised to nitric acid, which falls as acid rain. Indirectly, as a catalyst: NO₂ oxidises SO₂ to SO₃ (NO₂ + SO₂ → NO + SO₃), regenerating NO₂ from the NO produced — NO₂ is not consumed overall, it simply speeds up sulfur dioxide’s contribution.

Control: flue-gas desulfurisation with calcium oxide or carbonate, catalytic converters, low-sulfur fuels.

The Contact process

2SO2 + O2  <=>  2SO3        exothermic
450 C, 1-2 atm, V2O5 catalyst

V₂O₅ is a heterogeneous catalyst working by a redox cycle: it oxidises SO₂ and is then re-oxidised by O₂, returning to its original state.

Low pressure is used because the yield is already high — high pressure would add cost for little gain.

Exam traps

• Giving only the triple bond as the reason for nitrogen’s inertness; the non-polarity matters too.
• Saying ammonia is basic “because it has nitrogen” rather than because the lone pair accepts a proton.
• Forgetting NH₄⁺ is 109.5°, not 107° — the lone pair is gone.
• Describing NO as a reactant in SO₂ oxidation rather than a catalyst.
• Attributing acid rain to CO₂ — it is SO₂ and NOₓ.
• Confusing photochemical smog (PAN, formed with hydrocarbons in sunlight) with acid rain (formed via nitric or sulfuric acid) — they are distinct pollution mechanisms even though NOₓ is involved in both.

Self-test

1. Give two reasons nitrogen is unreactive.
2. Explain why ammonia acts as a base.
3. Why is the bond angle in NH₄⁺ larger than in NH₃?
4. Show, with equations, why NO is a catalyst in the oxidation of SO₂.
5. Name the two pollutants that cause acid rain and their sources.
6. What is formed when atmospheric NOₓ reacts with unburned hydrocarbons in sunlight, and why does it matter?
7. Explain the two distinct ways that NOₓ contributes to acid rain.

Answers: 1. The N≡N triple bond has a very high bond enthalpy, giving a large activation energy; and the molecule is non-polar, so it does not attract nucleophiles or electrophiles. 2. The nitrogen lone pair accepts a proton, forming a dative covalent bond — it is a proton acceptor. 3. In NH₄⁺ the lone pair has been used to form the fourth bond, so there are four bonding pairs and no lone-pair repulsion; the angle opens from 107° to 109.5°. 4. NO + ½O₂ → NO₂, then NO₂ + SO₂ → SO₃ + NO — NO is regenerated unchanged. 5. Sulfur dioxide, from sulfur impurities in fossil fuels; and nitrogen oxides, from high-temperature combustion in vehicle engines. 6. Peroxyacetyl nitrate (PAN), a key component of photochemical smog and an eye and lung irritant in sunny, traffic-heavy cities. 7. Directly, by dissolving in atmospheric water and oxidising to nitric acid; indirectly, by acting as a catalyst that oxidises SO₂ to SO₃ (which then forms sulfuric acid), without NO₂ itself being consumed overall.