12. Nitrogen and sulfur

Nitrogen gas exists as non-polar N₂ molecules containing a very strong N≡N triple bond. Breaking this bond requires substantial energy, so many reactions have a high activation energy.

The absence of a permanent dipole also limits attraction to polar reagents. Reactivity can still be increased by high temperature, pressure, catalysts or an appropriate reaction partner; “unreactive” is not “inert in every condition”.

The Haber process makes ammonia from nitrogen and hydrogen only with elevated pressure, temperature and an iron catalyst. The catalyst changes rate, not the equilibrium position.

Do not explain nitrogen’s behaviour using only “it is a gas”, and do not confuse a strong N≡N bond with zero possible chemistry.

Ammonia acts as a base because the lone pair on nitrogen accepts a proton: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The ammonium ion has four N–H bonds and a tetrahedral arrangement.

Ammonium salts can release ammonia when a stronger base supplies OH⁻: NH₄⁺ + OH⁻ → NH₃ + H₂O. The same proton-transfer framework explains both ammonia’s basicity and its displacement from salts.

Warming ammonium chloride with sodium hydroxide produces NH₃, which can be identified by its alkaline effect on damp red litmus. The gas test follows the acid–base equation.

Ammonia is not a hydroxide-containing compound. Its basicity comes from proton acceptance by the nitrogen lone pair.

Nitrogen oxides form naturally in high-temperature combustion and are also produced by human activities. In a catalytic converter, NOx is reduced while carbon monoxide and hydrocarbons are oxidised.

A simplified reaction is 2CO + 2NO → 2CO₂ + N₂. The catalyst provides a lower-energy pathway; it does not remove the underlying production of pollutants or make every exhaust gas harmless.

A three-way converter uses a precious-metal surface so CO can reduce NO to N₂ while CO becomes CO₂. Oxygen availability and engine conditions affect which reactions dominate.

Do not say the catalyst “destroys” nitrogen or shifts an equilibrium. It changes reaction rate and the products depend on the exhaust composition.

Nitrogen oxides in air can react with unburned hydrocarbons under sunlight to produce secondary pollutants, including peroxyacetyl nitrate (PAN), a component of photochemical smog.

NO₂ absorbs light and generates reactive species that help oxidise hydrocarbons. Smog formation therefore depends on sunlight, NOx and volatile organic compounds; it is not simply “smoke from engines”.

A sunny urban basin with heavy traffic can accumulate ozone and PAN when weak winds limit dispersion. Reducing either NOx or hydrocarbon emissions can alter the reaction chain.

Photochemical smog is not the same as sulfurous smog or acid rain. Keep the reactants, sunlight requirement and secondary products distinct.

Sulfur dioxide can be oxidised in air to sulfur trioxide and then hydrated to sulfuric acid. NO and NO₂ participate in the catalytic oxidation pathway, so nitrogen oxides can accelerate acid formation without being consumed overall.

Separate direct acid formation from catalytic regeneration: NO₂ can oxidise SO₂ while being reduced to NO, and atmospheric oxidants then regenerate NO₂. The net effect links combustion emissions to acid deposition.

A simplified sequence is SO₂ + NO₂ → SO₃ + NO followed by SO₃ + H₂O → H₂SO₄. The first equation alone is not the whole atmospheric mechanism.

NOx are not the only source of acid rain, and “catalytic” does not mean nitrogen oxides disappear. Track the cycle and the sulfur-containing product.