Pollutants from Fuels

This lesson examines the pollutants produced by the combustion of hydrocarbon fuels in greater detail, as required by AQA GCSE Chemistry specification (5.9.3). You need to be able to write balanced equations for the complete and incomplete combustion of hydrocarbons, explain why different pollutants form under different conditions, and describe the catalytic converter as a key technology for reducing emissions from vehicles.

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Hydrocarbon Fuels

Hydrocarbons are compounds that contain only hydrogen and carbon atoms. The most common hydrocarbon fuels are:

Fuel	Main Hydrocarbon	Formula	State at Room Temperature
Natural gas	Methane	CH₄	Gas
LPG (Liquefied Petroleum Gas)	Propane / Butane	C₃H₈ / C₄H₁₀	Gas (stored as liquid under pressure)
Petrol	Octane (approximately)	C₈H₁₈	Liquid
Diesel	Longer-chain hydrocarbons	~C₁₂H₂₆ to C₂₀H₄₂	Liquid

All of these fuels are derived from crude oil (petroleum) or natural gas, which are fossil fuels formed from the remains of ancient organisms buried and compressed over millions of years.

Exam Tip: When writing combustion equations, always check that the formula of the hydrocarbon is correct. Methane is CH₄ (not CH₃), propane is C₃H₈, and octane is C₈H₁₈. Errors in the formula will make the entire equation wrong.

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Complete Combustion of Hydrocarbons

When a hydrocarbon burns in a plentiful supply of oxygen, the products are carbon dioxide (CO₂) and water (H₂O). The flame is usually blue.

General Equation

hydrocarbon + oxygen → carbon dioxide + water

Balanced Equations

Hydrocarbon	Balanced Equation
Methane	CH₄ + 2O₂ → CO₂ + 2H₂O
Propane	C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Butane	2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O
Octane	2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Steps for Balancing Combustion Equations

1. Write the correct formula for the hydrocarbon, O₂, CO₂, and H₂O.
2. Balance the carbon atoms first (number of C on the left = number of CO₂ on the right).
3. Balance the hydrogen atoms next (number of H on the left = number of H₂O on the right).
4. Balance the oxygen atoms last (count total O needed on the right, then adjust O₂ on the left).
5. If you get a fraction for O₂, multiply the entire equation by 2 to eliminate it.

Exam Tip: Practise balancing combustion equations until you can do them quickly and accurately. They appear in almost every GCSE Chemistry exam paper. Always check your answer by counting atoms on both sides.

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Incomplete Combustion of Hydrocarbons

When a hydrocarbon burns in a limited supply of oxygen, the products may include carbon monoxide (CO) and/or carbon (soot/particulates) instead of carbon dioxide. The flame is often yellow or orange and may be smoky.

Products of Incomplete Combustion

Oxygen Supply	Products
Plentiful	CO₂ + H₂O (complete combustion)
Limited	CO + H₂O (some carbon monoxide formed)
Very limited	C + H₂O (carbon/soot formed)
Mixed	A mixture of CO₂, CO, C, and H₂O

Example Equations for Incomplete Combustion

Methane — producing carbon monoxide:

2CH₄ + 3O₂ → 2CO + 4H₂O

Methane — producing carbon (soot):

CH₄ + O₂ → C + 2H₂O

Propane — producing carbon monoxide:

2C₃H₈ + 7O₂ → 6CO + 8H₂O

Why Incomplete Combustion Occurs

Incomplete combustion typically occurs in:

• Car engines — the fuel-air mixture may not be perfectly balanced, especially during acceleration or when the engine is cold.
• Gas boilers and heaters — if the air supply is restricted or the appliance is faulty.
• Candles and open fires — the fuel is not well-mixed with air.
• Diesel engines — diesel fuel has longer hydrocarbon chains and is more prone to incomplete combustion, producing more particulates.

flowchart TD
    A["Hydrocarbon Fuel"] --> B["Burns in Air"]
    B --> C{"Oxygen Supply"}
    C -->|"Plentiful O₂"| D["Complete Combustion<br/>CO₂ + H₂O<br/>Blue flame"]
    C -->|"Limited O₂"| E["Incomplete Combustion<br/>CO + H₂O<br/>Yellow/smoky flame"]
    C -->|"Very limited O₂"| F["Incomplete Combustion<br/>C (soot) + H₂O<br/>Smoky yellow flame"]

    D --> G["Less harmful<br/>(but CO₂ is a greenhouse gas)"]
    E --> H["Dangerous<br/>(CO is toxic)"]
    F --> I["Harmful<br/>(particulates cause<br/>respiratory problems)"]

    style D fill:#27ae60,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#c0392b,color:#fff

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Sulfur Dioxide from Fuels

Many fossil fuels contain sulfur impurities. When these fuels burn, the sulfur reacts with oxygen to form sulfur dioxide (SO₂):

S + O₂ → SO₂

This is not a combustion product of the hydrocarbon itself, but of the sulfur impurities within the fuel. Coal typically contains more sulfur than oil or natural gas, which is why coal-fired power stations are a major source of SO₂.

Removing Sulfur

Sulfur can be removed from fuels before they are burned (a process called desulfurisation). Crude oil is treated in refineries to remove sulfur compounds. Coal can be cleaned, but this is more difficult and expensive.

After combustion, sulfur dioxide can be removed from the flue gases of power stations using a process called flue gas desulfurisation (FGD). In this process, SO₂ reacts with calcium carbonate (limestone) or calcium oxide (quicklime):

CaCO₃ + SO₂ → CaSO₃ + CO₂

Or with calcium oxide:

CaO + SO₂ → CaSO₃

The calcium sulfite produced can be further oxidised to calcium sulfate (gypsum), which is used to make plasterboard for the construction industry.

Exam Tip: Flue gas desulfurisation is a common exam topic. Remember: limestone (CaCO₃) or quicklime (CaO) is added to remove SO₂ from power station exhaust gases. The product (gypsum/calcium sulfate) has a useful application in construction.

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Nitrogen Oxides from Fuels

Nitrogen oxides (NOₓ) — principally NO and NO₂ — are formed when nitrogen and oxygen from the air react at the very high temperatures inside vehicle engines and power station furnaces. The nitrogen does NOT come from the fuel; it comes from the air.

N₂ + O₂ → 2NO (at high temperatures)

The nitrogen monoxide (NO) produced then reacts with more oxygen in the air to form nitrogen dioxide (NO₂):

2NO + O₂ → 2NO₂

Nitrogen dioxide is a brown, toxic gas that contributes to: