9701 Chemistry · Topic 12 · AS Level
Nitrogen and Sulfur Cheat Sheet — A Level Chemistry 9701
Topic 12 is applied chemistry: the same equilibrium and kinetics ideas from Topics 7 and 8, put to work in industry and the atmosphere. This sheet covers the unreactivity of nitrogen, ammonia as a base and a ligand, the Haber and Contact processes with their compromise conditions, how NOₓ forms in an engine and is removed by a catalytic converter, and the chemistry of acid rain.
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Nitrogen and Sulfur
01 · Why N₂ is unreactive
The N≡N triple bond has a very high bond enthalpy (994 kJ mol⁻¹), so the activation energy for any reaction is large.
N₂ is also non‑polar with no permanent dipole, so nothing attacks it readily. This is why 78 % of the air is chemically inert nitrogen.
02 · Ammonia — shape and basicity
NH₃ is trigonal pyramidal, bond angle 107°, because the lone pair repels the three bonding pairs more strongly.
That lone pair accepts a proton, so ammonia is a Brønsted base: NH₃ + H⁺ → NH₄⁺. The resulting bond is dative covalent, and NH₄⁺ is tetrahedral at 109.5°.
03 · The Haber process
Conditions ~450 °C, ~200 atm, iron catalyst. High pressure favours the side with fewer gas moles; the temperature is a compromise between yield and rate.
Ammonia is condensed out and unreacted gases recycled, so the overall conversion is far higher than the single‑pass yield.
04 · Ammonium salts and fertilisers
NH₃ + acid → ammonium salt: NH₃ + HNO₃ → NH₄NO₃, used as fertiliser because it supplies soluble nitrogen for protein synthesis.
Never mix an ammonium fertiliser with lime: NH₄⁺ + OH⁻ → NH₃ + H₂O releases ammonia and wastes the nitrogen. Warming with NaOH and testing with damp red litmus is the test for NH₄⁺.
Excess fertiliser leaches into rivers and causes eutrophication.
05 · Oxides of nitrogen
Formed in engines where the temperature is high enough to overcome the N≡N bond: N₂ + O₂ → 2NO, then 2NO + O₂ → 2NO₂.
NO₂ dissolves to give acid rain and, with unburnt hydrocarbons and sunlight, photochemical smog. NO also catalyses the oxidation of SO₂ to SO₃ in the atmosphere.
06 · Catalytic converters
hydrocarbons + NO → CO₂ + H₂O + N₂
Pt/Rh on a honeycomb ceramic gives a huge surface area. The gases adsorb onto active sites, react and desorb.
07 · The Contact process
Conditions ~450 °C, ~2 atm, V₂O₅ catalyst. The yield is already about 96 %, so high pressure is not worth its cost.
SO₃ is absorbed into concentrated H₂SO₄ to give oleum, then diluted — adding SO₃ directly to water is dangerously violent.
08 · The V₂O₅ catalyst
2V₂O₄ + O₂ → 2V₂O₅
Vanadium cycles between +5 and +4, which is why a transition metal makes a good catalyst: it can lend and take back electrons.
09 · Sulfur dioxide and acid rain
SO₂ comes from burning sulfur‑containing fossil fuels. SO₂ + H₂O → H₂SO₃, and oxidation gives H₂SO₄, both far stronger than the natural acidity of rain from dissolved CO₂.
Effects: limestone buildings eroded, lakes acidified, trees damaged. Control: flue‑gas desulfurisation with CaO or CaCO₃, giving CaSO₃ which is oxidised to gypsum.
10 · Uses of sulfuric acid and SO₂
H₂SO₄ — fertilisers, detergents, paints, and as the electrolyte in lead–acid batteries. It is the most produced industrial chemical by mass.
SO₂ — food preservative and bleach for paper, in both cases acting as a reducing agent that kills bacteria or destroys coloured compounds.
11 · Worked example — Haber yield
N₂ + 3H₂ → 2NH₃
n(N₂) = 28.0 ÷ 28.0 = 1.00 t‑mol
theoretical n(NH₃) = 2.00 t‑mol → 34.0 t
actual = 34.0 × 0.15 = 5.1 t
The unreacted gas is recycled, so the process yield is far higher than 15 % overall.
12 · Tests to know
| Test | Result |
|---|---|
| NH₄⁺ + NaOH, warm | NH₃ turns damp red litmus blue |
| SO₄²⁻ + BaCl₂ / HCl | white precipitate |
| SO₂ + acidified Cr₂O₇²⁻ | orange → green |
13 · Equations to know
NH₃ + HNO₃ → NH₄NO₃
NH₄⁺ + OH⁻ → NH₃ + H₂O
N₂ + O₂ → 2NO
2NO + O₂ → 2NO₂
2CO + 2NO → 2CO₂ + N₂
S + O₂ → SO₂
2SO₂ + O₂ ⇌ 2SO₃
SO₃ + H₂SO₄ → H₂S₂O₇
CaO + SO₂ → CaSO₃
14 · Worked example — ammonia in a fertiliser
2NH₃ + H₂SO₄ → (NH₄)₂SO₄
n(NH₃) = 34.0 ÷ 17.0 = 2.00 kg‑mol
n((NH₄)₂SO₄) = 1.00 kg‑mol
m = 1.00 × 132.1 = 132 kg
Nitrogen content = 28.0 ÷ 132.1 × 100 = 21.2 %.
Marks lost here
— Saying N₂ is unreactive “because it is a gas” instead of quoting the strong triple bond and high Ea.
— Giving the Contact process a high pressure; it runs at about 2 atm.
— Writing SO₃ + H₂O as the industrial step; SO₃ is absorbed in H₂SO₄ first.
— Explaining ammonia’s basicity without mentioning the lone pair.
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Nitrogen and Sulfur — Frequently Asked Questions
Why are the Haber process conditions a compromise?
The forward reaction is exothermic, so a low temperature would give a higher yield — but the rate would be uneconomically slow. About 450 °C gives an acceptable yield at an acceptable rate. High pressure favours the side with fewer gas molecules but is expensive and hazardous, so around 200 atm is used.
Why is nitrogen gas so unreactive?
The N≡N triple bond has a bond enthalpy of about 994 kJ mol⁻¹ and the molecule is non-polar, so there is a very high activation energy for any reaction that breaks it.
How does a catalytic converter remove nitrogen oxides?
Over a platinum–rhodium catalyst, nitrogen monoxide is reduced by carbon monoxide: 2CO + 2NO → 2CO₂ + N₂. One pollutant is used to remove another.
