Air

Clean dry air composition.

Gas	Percentage
Nitrogen (N₂)	$\sim 78\%$
Oxygen (O₂)	$\sim 21\%$
Argon (Ar)	$\sim 1\%$
Carbon dioxide (CO₂)	$\sim 0.04\%$
Other noble gases (Ne, Kr, Xe)	trace

(Real air also contains water vapour, but composition tables usually quote DRY air.)

Memorise. N₂ at $78\%$, O₂ at $21\%$ — these two together are $99\%$ of clean dry air. Ar is the next $1\%$. CO₂ a tiny fraction.

Worked qualitative. Why is nitrogen the major gas? It's relatively inert and doesn't react readily with most things. Atmospheric N₂ is mostly unreactive — that's why oxygen, despite being highly reactive, doesn't get used up.

Cambridge tip. Pollutants are EXTRA gases — CO, SO₂, NOₓ, etc. — added to clean air by human activity (or natural sources like volcanoes).

Carbon monoxide (CO).

• Source: incomplete combustion of fuels (insufficient oxygen).
  • $2\text{C} + \text{O}_{2} \to 2\text{CO}$ (limited O₂).
• Effect: poisonous. Binds to haemoglobin $\sim 250\times$ more strongly than O₂ → blood can't carry oxygen → suffocation.
• Reduction: ensure complete combustion (good ventilation); catalytic converters.

Sulfur dioxide (SO₂).

• Source: burning fossil fuels containing sulfur (especially coal, some oils).
  • $\text{S} + \text{O}_{2} \to \text{SO}_{2}$.
• Effect: causes ACID RAIN (forms H₂SO₃ / H₂SO₄ in atmospheric water). Damages buildings (limestone), kills aquatic life, harms forests. Also causes respiratory irritation.
• Reduction: low-sulfur fuels; flue-gas desulfurisation in power stations (CaCO₃ scrubbers).

Nitrogen oxides (NOₓ — chiefly NO and NO₂).

• Source: high temperatures in car engines and power stations cause N₂ + O₂ to react.
  • $\text{N}_{2} + \text{O}_{2} \to 2\text{NO}$ (then $2\text{NO} + \text{O}_{2} \to 2\text{NO}_{2}$).
• Effect: contribute to ACID RAIN, photochemical smog, breathing difficulties.
• Reduction: catalytic converters; lower combustion temperatures.

Particulates / soot.

• Source: incomplete combustion (especially diesel).
• Effect: respiratory problems, lung damage, premature deaths.
• Reduction: particulate filters in diesel exhausts; cleaner combustion technology.

Greenhouse gases (CO₂, CH₄, water vapour).

• Effect: trap heat in the atmosphere → global warming, climate change.
• Reduction: switch to renewables, reduce fossil fuel burning, plant trees.

Catalytic converter. A device fitted in car exhaust systems containing a ceramic honeycomb coated with platinum, palladium, and rhodium catalysts. Hot exhaust gases pass through; the catalysts speed up reactions that convert harmful pollutants to less harmful gases.

Reactions catalysed.

1. CO oxidation. $2\text{CO} + \text{O}_{2} \to 2\text{CO}_{2}$. 2. Hydrocarbon combustion. Unburnt $\text{C}_{x}\text{H}_{y} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}$. 3. NOₓ reduction. $2\text{CO} + 2\text{NO} \to 2\text{CO}_{2} + \text{N}_{2}$ — CO reduces NO to N₂.

Net effect. Toxic outputs (CO, NO, hydrocarbons) → less harmful (CO₂, N₂, H₂O).

Why honeycomb shape? Maximises surface area for catalyst contact with gases. More surface = more effective.

Why precious metals? Pt/Pd/Rh catalyse the reactions effectively even at relatively low exhaust temperatures.

Limitations.

• Doesn't work until warm (cold-start emissions are higher).
• Catalysts can be poisoned by lead → why leaded fuel was phased out.
• Doesn't remove CO₂ — only changes which carbon-containing molecule is emitted.
• Doesn't remove SO₂ (driver: switch to low-sulfur fuels).

Worked qualitative. Why doesn't a catalytic converter solve the global warming problem? It STILL produces CO₂ — just from previously-CO-or-hydrocarbon carbon. Total carbon emitted unchanged. Only fuel-source change (e.g. electric cars) reduces total CO₂.