Summary — What This Chapter Covers
- N₂ is unreactive: the N≡N bond is very strong (994 kJ mol⁻¹) and the molecule is non-polar.
- Ammonia is a Brønsted–Lowry base: its lone pair accepts a proton to form NH₄⁺.
- NH₄⁺ is tetrahedral (109.5°), formed by a coordinate bond in an acid–base reaction.
- Warming an ammonium salt with a base displaces ammonia.
- Oxides of nitrogen form naturally (lightning) and in engines; catalytic converters remove them.
- NO and NO₂ react with unburned hydrocarbons to form PAN, a component of photochemical smog.
- NO₂ causes acid rain directly and catalyses the oxidation of SO₂ to SO₃.
1. Nitrogen and Sulfur
The lack of reactivity of nitrogen
A very strong, non-polar bond
Nitrogen makes up about 78% of the air but is very unreactive because:
- the N≡N triple bond is very strong (bond energy 994 kJ mol⁻¹), so a very large activation energy is needed to break it;
- the molecule is non-polar (both atoms have the same electronegativity), so it has no δ+ or δ− regions to attract electrophiles or nucleophiles.
Ammonia as a base
Brønsted–Lowry proton acceptor
The lone pair on the nitrogen atom of NH₃ can accept a proton (H⁺), so ammonia is a Brønsted–Lowry base.
Ammonia accepting a proton
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) NH₃(g) + HCl(g) → NH₄Cl(s)
Ammonia is a weak base: the first equilibrium lies well to the left, so the solution is only moderately alkaline (pH about 11 for 0.1 mol dm⁻³). The reaction with HCl gas gives a white smoke of ammonium chloride.
The ammonium ion
Structure and formation
When NH₃ accepts H⁺, the nitrogen lone pair forms a coordinate (dative covalent) bond to the proton. In NH₄⁺ there are four bonding pairs and no lone pairs around N, so the ion is tetrahedral with bond angles of 109.5°. All four N–H bonds are identical once formed. This is an acid–base reaction: the acid donates H⁺, ammonia accepts it.
Examples: NH₃ + HNO₃ → NH₄NO₃; 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ (both fertilisers).
Displacing ammonia
Ammonium salt + base
Warming an ammonium salt with a base releases ammonia gas. The OH⁻ ion is a stronger base than NH₃, so it removes a proton from NH₄⁺ (an acid–base reaction):
Displacement
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Full equations: NH₄Cl + NaOH → NaCl + NH₃ + H₂O; 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O. Ammonia turns damp red litmus paper blue. Farmers should not add lime (a base) at the same time as ammonium fertilisers, or nitrogen is lost as NH₃.
Worked Example
2.14 g of ammonium chloride (Mr 53.5) is warmed with excess sodium hydroxide. What volume of ammonia is produced at room conditions?
Practice Question
Explain, in terms of the Brønsted–Lowry theory, the reaction between ammonium sulfate and calcium hydroxide.
Oxides of nitrogen
Where they come from and how they're removed