Nitrogen and sulfur

· Nitrogen and sulfur focuses on nitrogen’s low reactivity, ammonia as a base, ammonium salts, oxides of nitrogen, photochemical smog, and acid rain.
· Key exam themes: explain using bonding, polarity, Brønsted–Lowry acid–base theory, oxidation numbers, and environmental chemistry.

Lack of reactivity of nitrogen

· Nitrogen, N₂, is very unreactive under normal conditions.
· Reason 1: very strong N≡N triple bond requires a large amount of energy to break.
· Reason 2: N₂ is non-polar, so it is not easily attacked by polar reagents.
· Exam phrase: “nitrogen is unreactive because the N≡N triple bond has high bond energy and the molecule is non-polar.”
· Do not just say “nitrogen is inert”; always link to triple bond strength and lack of polarity.

Nitrogen exists as N₂, with a triple covalent bond between the two nitrogen atoms. This strong bond explains why nitrogen is chemically unreactive under normal conditions. Source

Ammonia as a Brønsted–Lowry base

· Ammonia, NH₃, is a weak base because it accepts a proton, H⁺.
· By Brønsted–Lowry theory, a base = proton acceptor.
· In water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
· Ammonia forms OH⁻ ions, so aqueous ammonia is alkaline.
· The reaction is reversible because ammonia is only a weak base.
· In acids: NH₃ + H⁺ → NH₄⁺.
· Exam phrase: “NH₃ acts as a base because the nitrogen lone pair accepts a proton.”

Ammonium ion formation and structure

· The ammonium ion, NH₄⁺, forms when NH₃ accepts H⁺.
· Equation: NH₃ + H⁺ → NH₄⁺.
· The bond to the added H⁺ is a coordinate / dative covalent bond because both electrons come from the lone pair on nitrogen.
· After formation, all four N–H bonds in NH₄⁺ are equivalent.
· NH₄⁺ has a tetrahedral structure with bond angle approximately 109.5°.
· Formation of ammonium salts is an acid–base reaction, e.g. NH₃ + HCl → NH₄Cl.

The ammonium ion has a tetrahedral shape because nitrogen is bonded to four hydrogen atoms. It forms when ammonia uses its lone pair to accept a proton. Source

Displacement of ammonia from ammonium salts

· Ammonium salts contain NH₄⁺ ions.
· Ammonia can be displaced by reacting an ammonium salt with a stronger base, usually an alkali.
· General ionic equation: NH₄⁺ + OH⁻ → NH₃ + H₂O.
· Example: NH₄Cl + NaOH → NH₃ + NaCl + H₂O.
· This is an acid–base reaction: NH₄⁺ donates H⁺ to OH⁻.
· Practical observation: ammonia gas is produced; it has a pungent smell and turns damp red litmus paper blue.
· Exam phrase: “OH⁻ removes H⁺ from NH₄⁺, producing NH₃.”

Oxides of nitrogen: occurrence and formation

· Important atmospheric oxides of nitrogen: NO and NO₂, often written as NOₓ.
· Natural occurrence: lightning provides enough energy for N₂ and O₂ in air to react.
· Man-made occurrence: internal combustion engines produce high temperatures that allow N₂ and O₂ to react.
· Initial formation: N₂ + O₂ → 2NO.
· Further oxidation: 2NO + O₂ → 2NO₂.
· Key point: nitrogen and oxygen usually do not react easily, but high temperature provides enough energy to break strong bonds.

Catalytic removal of oxides of nitrogen

· Catalytic converters remove NOₓ from car exhaust gases.
· Catalyst metals commonly include platinum, palladium and rhodium.
· The catalyst provides a surface for reactions, lowering activation energy.
· NOₓ is reduced to N₂, which is harmless and naturally abundant in air.
· Carbon monoxide and unburned hydrocarbons are oxidised at the same time.
· Example reduction: 2NO → N₂ + O₂.
· Example reaction with CO: 2NO + 2CO → N₂ + 2CO₂.
· Exam phrase: “oxides of nitrogen are catalytically reduced to nitrogen in a catalytic converter.”

A catalytic converter removes harmful exhaust pollutants. In CIE questions, focus on the reduction of NOₓ to N₂ and the role of the catalyst surface. Source

Photochemical smog and PAN

· NO and NO₂ can react with unburned hydrocarbons in the atmosphere.
· In sunlight, these reactions form photochemical smog.
· A key component of photochemical smog is PAN, which stands for peroxyacetyl nitrate.
· PAN is a secondary pollutant because it forms from reactions of primary pollutants in air.
· Primary pollutants involved: NOₓ and unburned hydrocarbons from vehicles.
· Exam phrase: “atmospheric NO and NO₂ react with unburned hydrocarbons to form PAN, a component of photochemical smog.”

Photochemical smog forms when NOₓ and hydrocarbons react in sunlight. PAN is one of the oxidising pollutants produced in this process. Source

Acid rain: direct role of NO and NO₂

· NO and NO₂ contribute to acid rain by forming acidic substances in the atmosphere.
· NO₂ can react with water and oxygen to form nitric acid, HNO₃.
· Simplified equation: 4NO₂ + 2H₂O + O₂ → 4HNO₃.
· Nitric acid dissolves in rainwater, lowering pH.
· Acid rain damages plants, aquatic ecosystems, soil chemistry, and limestone buildings.
· Exam phrase: “oxides of nitrogen form nitric acid, which contributes to acid rain.”

Acid rain: catalytic role in sulfur dioxide oxidation

· Sulfur dioxide, SO₂, is also a major cause of acid rain.
· SO₂ can be oxidised to SO₃, which reacts with water to form sulfuric acid, H₂SO₄.
· Equations: 2SO₂ + O₂ → 2SO₃ and SO₃ + H₂O → H₂SO₄.
· NO and NO₂ act catalytically in the oxidation of atmospheric SO₂.
· This means NOₓ helps convert SO₂ into sulfuric acid-forming species but is regenerated in the process.
· Key exam point: NOₓ contributes to acid rain directly by forming HNO₃ and indirectly/catalytically by helping oxidise SO₂.

Acid rain forms when SO₂ and NOₓ are transformed into acidic particles in the atmosphere. These pollutants can travel long distances before falling as rain, snow, fog, or dry deposition. Source

High-yield equations to memorise

· N₂ + O₂ → 2NO
· 2NO + O₂ → 2NO₂
· NH₃ + H⁺ → NH₄⁺
· NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
· NH₄⁺ + OH⁻ → NH₃ + H₂O
· 2NO + 2CO → N₂ + 2CO₂
· 4NO₂ + 2H₂O + O₂ → 4HNO₃
· 2SO₂ + O₂ → 2SO₃
· SO₃ + H₂O → H₂SO₄

Common exam traps

· Do not say nitrogen is unreactive only because it is a gas; the reason is strong N≡N triple bond and non-polarity.
· Do not confuse ammonia, NH₃, with ammonium, NH₄⁺.
· NH₃ is a base because it accepts H⁺, not because it “contains OH⁻”.
· NH₄⁺ is acidic in displacement reactions because it can donate H⁺ to OH⁻.
· NOₓ means mainly NO and NO₂ in this topic.
· PAN is linked to photochemical smog, not acid rain.
· NOₓ contributes to acid rain twice: by forming nitric acid and by catalysing oxidation of SO₂.