Nitrogen Oxides & Sulfur Dioxide

Fossil fuels — petrol, diesel, kerosene, natural gas, coal — are mostly hydrocarbons (compounds of hydrogen and carbon only). When they burn (react with oxygen) they release energy, but what gases come out depends on how much oxygen is available.

Complete combustion — plenty of oxygen.

When there is excess oxygen, every carbon atom is fully oxidised to carbon dioxide (CO₂) and every hydrogen atom to water (H₂O):

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$

• Releases the most energy (highly exothermic).
• Clean blue flame — no soot.

Incomplete combustion — limited oxygen.

When there is not enough oxygen, the carbon is only partly oxidised. Instead of CO₂ you also get:

• Carbon monoxide (CO) — a toxic gas (carbon partly oxidised).
• Carbon (C) — soot / particulates (carbon not oxidised at all).

The hydrogen still becomes water. For example:

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$

• Releases less energy than complete combustion.
• Yellow / orange sooty flame and deposits black soot on cold surfaces.

Incomplete combustion happens in poorly-ventilated gas heaters and in old or badly-tuned engines.

Fossil fuels are not pure hydrocarbons — they contain small amounts of sulfur as an impurity. When the fuel burns, this sulfur burns too, forming sulfur dioxide (SO₂):

$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$

What SO₂ does — acid rain.

Sulfur dioxide gas rises into the atmosphere and dissolves in rainwater, forming an acidic solution (sulfurous acid, then sulfuric acid). This falls as acid rain, which:

• Acidifies lakes and rivers, killing fish and other aquatic life.
• Damages plants and trees (leaves are damaged and nutrients are washed out of the soil).
• Erodes buildings and statues made of limestone (calcium carbonate), and corrodes metal structures.

How SO₂ is reduced. Sulfur is removed from fuels before they are sold (low-sulfur petrol and diesel), and in power stations the waste gases are passed through an alkali (such as calcium oxide / calcium hydroxide) to neutralise the SO₂ before it escapes.

Nitrogen oxides (NOₓ — mainly NO and NO₂) are different from SO₂: the nitrogen does not come from the fuel — it comes from the air.

Where they come from.

Air is about 78% nitrogen and 21% oxygen. Normally these gases do not react. But inside a hot engine or furnace, the temperature is so high that the nitrogen and oxygen react together:

$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$

The NO then reacts with more oxygen in the air to form nitrogen dioxide (NO₂). Together these are called nitrogen oxides (NOₓ).

Key exam phrase: nitrogen oxides form "at high temperatures, when nitrogen and oxygen from the air react together" — this only happens because of the heat inside the engine, not because of impurities in the fuel.

What NOₓ does.

• Acid rain — NOₓ dissolves in rainwater to form nitric acid, with the same damage as SO₂ (kills lakes, damages trees, erodes limestone buildings).
• Breathing / respiratory problems — NOₓ irritates the lungs and can trigger asthma. It also helps form photochemical smog (a brown haze over busy cities).

How NOₓ is reduced. Modern cars use a catalytic converter in the exhaust, which converts the harmful NOₓ and CO into harmless nitrogen and carbon dioxide.

Carbon monoxide (CO) — toxic.

CO is made by incomplete combustion (not enough oxygen). It is a toxic, colourless, odourless gas — you cannot see or smell it, which is why it is called the "silent killer".

• CO is breathed into the lungs and binds to haemoglobin in red blood cells.
• This stops the haemoglobin carrying oxygen, so less oxygen reaches the body's cells (oxygen starvation).
• Symptoms: headache, dizziness, drowsiness, unconsciousness, and at high levels death.

Carbon dioxide (CO₂) — greenhouse gas.

CO₂ is made by complete combustion of any fuel. It is not toxic, but it is a greenhouse gas: it traps heat radiated from the Earth's surface, adding to the greenhouse effect and causing climate change (rising temperatures, melting ice, sea-level rise).

Particulates (soot) — breathing problems.

Particulates are tiny solid particles of unburned carbon (soot) from incomplete combustion. They:

• Are breathed deep into the lungs and cause respiratory problems (bronchitis, worsened asthma, lung disease).
• Cause global dimming — they reflect sunlight back into space, blocking some light from reaching the Earth's surface.
• Blacken buildings.

Summary table — the pollutants from burning fossil fuels.

Pollutant	Where it comes from	Main effect
CO₂	Complete combustion	Greenhouse gas → climate change
CO	Incomplete combustion	Toxic — binds to haemoglobin
SO₂	Sulfur impurities in fuel	Acid rain
NOₓ	High temperature in engines (N₂ + O₂ from air)	Acid rain + breathing problems
Particulates (soot)	Incomplete combustion	Breathing problems + global dimming