Nitrogen and Sulfur

0. What Examiners Want

Where this topic appears. Recent AS papers have asked for: the reasons nitrogen is unreactive, the equation forming a mixture of NO and NO₂ in an engine, the products of NO₂ with water, the consequence of PAN formation, the catalytic removal of NO in a converter, the role of NO₂ in making SO₃, and the Haber process (equilibrium and rate). The topic is assumed in Paper 4.

1. Nitrogen Gas: Why So Unreactive

Nitrogen makes up about 78% of the air and exists as diatomic molecules, N₂. Each N atom (1s² 2s² 2p³) needs three more electrons for a noble-gas configuration, so the two atoms share three pairs: a triple covalent bond (one σ and two π bonds).

Why nitrogen is unreactive

• The N≡N triple bond is very strong: bond energy 944 kJ mol⁻¹ (Data Booklet), so a high activation energy is needed to break it.
• The molecule is non-polar, so it is not attacked by nucleophiles or electrophiles, and the electrons are not easily polarised.

N₂ reacts only under extreme conditions, such as lightning or the high temperature and pressure in a car engine, and in the Haber process (high pressure, an iron catalyst).

Formation of the oxides (three steps in the nitrogen cycle)

1. N₂(g) + O₂(g) → 2NO(g)   (lightning or a hot engine; 0 to +2)
2. 2NO(g) + O₂(g) → 2NO₂(g)   (+2 to +4; NO₂ is a brown gas)
3. 4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃(aq)   (+4 to +5; dilute nitric acid falls in rain)

An engine produces a mixture of the two oxides, for example N₂(g) + 1½O₂(g) → NO(g) + NO₂(g).

NO₂ reacting with water alone is a disproportionation: 2NO₂ + H₂O → HNO₂ + HNO₃ (nitrogen +4 goes to +3 and +5).

2. Ammonia and Ammonium Compounds

Ammonia, NH₃, has three N–H bonds and a lone pair on nitrogen, so the molecule is pyramidal (bond angle about 107°). It is the only common alkaline gas. It is made on a large scale in the Haber process:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)   ΔH = −92 kJ mol⁻¹ (iron catalyst; Δ_fH of NH₃ = −46 kJ mol⁻¹)

Ammonia as a base. The lone pair on nitrogen accepts H⁺ (a Brønsted–Lowry base, a proton acceptor) and forms a dative (co-ordinate) bond:

• NH₃ + H⁺ → NH₄⁺
• NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq): ammonia is a weak base, the equilibrium lies well to the left, so the solution is only weakly alkaline.
• NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)

The ammonium ion, NH₄⁺: four bond pairs and no lone pair, so it is tetrahedral (109.5°). All four N–H bonds are the same length, which shows that the dative bond is equivalent to the other three and the charge is spread over the whole ion.

Ammonium salts (fertilisers): ammonium chloride NH₄Cl, ammonium nitrate NH₄NO₃, ammonium sulfate (NH₄)₂SO₄, ammonium phosphate (NH₄)₃PO₄. Fertilisers must be soluble for plants to absorb them, so excess can be washed into rivers and cause eutrophication (algal bloom, then bacteria use up the dissolved oxygen and fish die).

Displacement of ammonia from ammonium salts. Heating an ammonium salt with a base releases ammonia gas. The reaction is acid–base: NH₄⁺ is the acid (proton donor) and OH⁻ is the base.

• 2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2H₂O(l) + 2NH₃(g)   (solids, heated: the laboratory preparation)
• (NH₄)₂SO₄(s) + 2NaOH(s) → Na₂SO₄(s) + 2H₂O(l) + 2NH₃(g)
• Ionic: NH₄⁺ + OH⁻ → NH₃ + H₂O

Test for ammonium ions / ammonia gas: warm the compound with sodium hydroxide solution; the ammonia given off turns damp red litmus paper blue.

3. Oxides of Nitrogen and Pollution

Sources. Natural: lightning. Human-made: internal combustion engines, where air is heated to a high temperature and pressure and nitrogen reacts with oxygen. NO and NO₂ (together NO_x) are primary pollutants, because they are released directly.

Effects

• Acid rain. NO is oxidised to NO₂, which dissolves in water with oxygen to form dilute nitric acid (the equations in section 1).
• Photochemical smog. NO_x react with unburnt hydrocarbons and other volatile organic compounds (VOCs) in sunlight to form PAN (peroxyacetyl nitrate, CH₃CO₃NO₂). PAN is a secondary pollutant, because it is formed in the atmosphere and not emitted directly. It irritates the lungs and eyes and damages plants.
• Catalysts for SO₂ oxidation (section 4).

4. Sulfur Dioxide and Acid Rain

Sulfur dioxide, SO₂, forms when sulfur-containing fossil fuels burn (S + O₂ → SO₂) and from volcanoes. It dissolves in water to give acidic solutions and is the main cause of acid rain. In the atmosphere it is oxidised to SO₃, and the oxidation is catalysed by oxides of nitrogen:

1. SO₂(g) + NO₂(g) → SO₃(g) + NO(g)
2. NO(g) + ½O₂(g) → NO₂(g)
3. SO₃(g) + H₂O(l) → H₂SO₄(aq)

Adding steps 1 and 2 (and doubling) gives 2SO₂ + O₂ → 2SO₃; NO₂ does not appear in the overall equation because it is regenerated, so it is a catalyst. (NO can also catalyse the reaction, forming NO₂ first.)

5. Catalytic Converters

A catalytic converter in the exhaust is a honeycomb coated with platinum, palladium or rhodium. The gases adsorb on the hot metal surface (a heterogeneous catalyst), react and then leave.

• NO is reduced to N₂: 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g). Nitrogen goes +2 to 0 (reduced); carbon goes +2 to +4 (oxidised).
• CO is oxidised: 2CO + O₂ → 2CO₂.
• Unburnt hydrocarbons are oxidised: for example 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O.

6. Quick Sheet and Checklist

Before you leave the question, check:

• Two reasons are given for the unreactivity of N₂ (strong bond and non-polar) when two marks are available.
• Shapes, bond angles and bond types are named for NH₃ and NH₄⁺.
• Equations are balanced and have state symbols; the displacement of NH₃ uses a base and heat.
• Pollutants are labelled primary (NO, NO₂) or secondary (PAN), and sunlight is stated for smog.
• The catalytic cycle shows NO₂ being regenerated, and the converter equation shows NO reduced and CO oxidised.