📖 In-Depth Theory
Pollutants from Combustion of Fuels
Burning FOSSIL FUELS releases several POLLUTANTS in addition to CO₂ and water:
CARBON MONOXIDE (CO):
From INCOMPLETE COMBUSTION — insufficient oxygen.
COLOURLESS, ODOURLESS, TOXIC gas.
Binds to haemoglobin in red blood cells — prevents oxygen transport → can be fatal.
Measured by CO detectors in homes.
SULFUR DIOXIDE (SO₂):
Fuels contain sulfur impurities → S + O₂ → SO₂ when burned.
Causes ACID RAIN when SO₂ reacts with water and oxygen in the atmosphere:
SO₂ + H₂O → H₂SO₃ (sulfurous acid) / SO₃ + H₂O → H₂SO₄ (sulfuric acid)
ACID RAIN effects: damages buildings/statues (limestone dissolves), kills trees, acidifies lakes killing fish.
Also causes respiratory problems — irritates airways.
NITROGEN OXIDES (NOₓ — mainly NO and NO₂):
Produced when N₂ and O₂ from air react at the high temperatures inside engines:
N₂ + O₂ → 2NO → oxidised to NO₂
NOₓ also causes ACID RAIN (nitric acid).
Contributes to SMOG and LOW-LEVEL OZONE (O₃) which irritates lungs.
PARTICULATES (soot/carbon particles):
From incomplete combustion of fuels.
Very small particles that penetrate deep into lungs.
Cause respiratory disease, aggravate asthma.
Consciously contribute to GLOBAL DIMMING — reducing sunlight reaching Earth.
Acid Rain — Causes and Effects
ACID RAIN forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) dissolve in rainwater:
FORMATION:
SO₂ + H₂O + ½O₂ → H₂SO₄ (sulfuric acid)
4NO₂ + O₂ + 2H₂O → 4HNO₃ (nitric acid)
Normal rain is slightly acidic (pH ~5.6) due to dissolved CO₂.
Acid rain: pH typically 4–5 — 10× to 100× more acidic than normal rain.
EFFECTS:
BUILDINGS AND STATUES: dissolves limestone and marble:
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
Irreplaceable historical buildings damaged.
ECOSYSTEMS:
Lakes and rivers become too acidic for aquatic life — fish die.
Soil acidification → releases toxic aluminium ions → kills trees.
Forest die-back in areas downwind of industrial zones.
HUMAN HEALTH:
SO₂ and NO₂ irritate respiratory system — worsen asthma, bronchitis.
SOLUTIONS:
REMOVE SULFUR from fuels before burning (desulfurisation).
CATALYTIC CONVERTERS in cars — convert CO, NOₓ and unburned hydrocarbons to CO₂, N₂ and H₂O.
SCRUBBERS in power station chimneys — spray calcium hydroxide to neutralise SO₂.
Catalytic Converters and Reducing Pollution
CATALYTIC CONVERTERS convert toxic exhaust gases to less harmful ones:
PLATINUM AND RHODIUM catalysts:
2CO + 2NO → 2CO₂ + N₂
(toxic carbon monoxide + toxic nitrogen oxide → CO₂ + harmless nitrogen)
Also convert unburned hydrocarbons:
Hydrocarbon + O₂ → CO₂ + H₂O
Reduces: CO (toxic), NOₓ (acid rain, smog) and particulates.
Does NOT reduce CO₂ (a greenhouse gas) — actually increases CO₂ by converting CO to CO₂.
OTHER SOLUTIONS:
LOW-SULFUR FUELS — reduce SO₂ at source.
PARTICULATE FILTERS in diesel vehicles.
ELECTRIC VEHICLES — no exhaust combustion pollutants (though power station emissions are a separate issue).
IMPROVED ENGINE DESIGN — more complete combustion, less CO and particulates.
INTERNATIONAL AGREEMENTS — e.g. limits on SO₂ emissions from power stations.
⚠️ Common Mistake
CO (carbon monoxide) is produced by INCOMPLETE combustion. CO₂ is produced by COMPLETE combustion. Both are colourless gases — but CO is toxic (binds to haemoglobin) while CO₂ is not directly toxic at normal concentrations. Catalytic converters reduce CO and NOₓ but actually INCREASE CO₂ (by converting CO to CO₂).