Combustion of Hydrocarbons

This lesson covers the combustion of hydrocarbons as required by the AQA GCSE Chemistry specification (5.8.1). Combustion is the most important chemical reaction of hydrocarbons and is the basis of how we use fossil fuels. You must understand both complete and incomplete combustion, their products, and their environmental impact.

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What Is Combustion?

Combustion is the reaction of a fuel with oxygen to release energy. When hydrocarbons burn, the carbon and hydrogen atoms in the fuel react with oxygen from the air. Combustion is an exothermic reaction — it transfers energy to the surroundings, usually as heat and light.

There are two types of combustion:

Type	Oxygen Supply	Products
Complete combustion	Plentiful (excess oxygen)	Carbon dioxide + water
Incomplete combustion	Limited (insufficient oxygen)	Carbon monoxide and/or carbon (soot) + water

Exam Tip: All combustion reactions are exothermic — they release energy. If asked about the energy change in combustion, always state this. The products have less energy than the reactants.

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Complete Combustion

Complete combustion occurs when there is plenty of oxygen available. The hydrocarbon reacts with oxygen to produce carbon dioxide (CO2) and water (H2O) only.

General Word Equation

hydrocarbon + oxygen → carbon dioxide + water

Balanced Symbol Equations

You must be able to write and balance combustion equations. Here are the equations for the first four alkanes:

Alkane	Word Equation	Balanced Symbol Equation
Methane	methane + oxygen → carbon dioxide + water	CH4 + 2O2 → CO2 + 2H2O
Ethane	ethane + oxygen → carbon dioxide + water	2C2H6 + 7O2 → 4CO2 + 6H2O
Propane	propane + oxygen → carbon dioxide + water	C3H8 + 5O2 → 3CO2 + 4H2O
Butane	butane + oxygen → carbon dioxide + water	2C4H10 + 13O2 → 8CO2 + 10H2O

Complete combustion produces a blue flame and releases the maximum amount of energy from the fuel.

Exam Tip: When balancing combustion equations, balance the carbons first, then the hydrogens, and finally the oxygens. If you end up with a fractional number of O2 molecules, multiply everything through by 2 to get whole numbers.

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Incomplete Combustion

Incomplete combustion occurs when there is not enough oxygen for complete combustion. Instead of carbon dioxide, carbon monoxide (CO) and/or carbon (soot, C) are produced along with water.

General Word Equations

hydrocarbon + oxygen → carbon monoxide + water

or

hydrocarbon + oxygen → carbon + water

In practice, a mixture of CO2, CO, and C (soot) is often produced when combustion is incomplete.

Example: Incomplete Combustion of Methane

With limited oxygen, methane can produce carbon monoxide:

2CH4 + 3O2 → 2CO + 4H2O

With very limited oxygen, methane can produce carbon (soot):

CH4 + O2 → C + 2H2O

Incomplete combustion produces a yellow or orange flame (due to glowing soot particles) and releases less energy than complete combustion.

graph TD
    A["Combustion of Hydrocarbons"] --> B["Complete Combustion"]
    A --> C["Incomplete Combustion"]
    B --> D["Plenty of Oxygen"]
    D --> E["Products: CO2 + H2O"]
    E --> F["Blue Flame"]
    C --> G["Limited Oxygen"]
    G --> H["Products: CO and/or C + H2O"]
    H --> I["Yellow/Orange Flame"]
    B --> J["More Energy Released"]
    C --> K["Less Energy Released"]

    style A fill:#8e44ad,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e74c3c,color:#fff

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Products of Combustion and Their Effects

Carbon Dioxide (CO2)

• Produced by complete combustion
• A greenhouse gas — it absorbs and re-emits infrared radiation, contributing to the greenhouse effect and global warming
• Increasing atmospheric CO2 levels are linked to climate change, including rising sea levels, changing weather patterns, and loss of biodiversity

Carbon Monoxide (CO)

• Produced by incomplete combustion
• A colourless, odourless, toxic gas
• It binds to haemoglobin in red blood cells, preventing them from carrying oxygen around the body
• Can cause headaches, dizziness, unconsciousness, and death
• Particularly dangerous in enclosed spaces (e.g., faulty gas boilers in homes)

Carbon / Soot (C)

• Produced by incomplete combustion
• Causes global dimming — soot particles in the atmosphere reflect sunlight and reduce the amount of light reaching the Earth's surface
• Contributes to respiratory problems (e.g., asthma, bronchitis) when inhaled
• Blackens buildings and surfaces

Water Vapour (H2O)

• Produced by both types of combustion
• Water vapour is also a greenhouse gas, but its effect is part of the natural water cycle

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Other Pollutants from Burning Fuels

In addition to the products of combustion of the hydrocarbon itself, burning fossil fuels produces other pollutants:

Pollutant	Source	Environmental/Health Effect
Sulfur dioxide (SO2)	Combustion of sulfur impurities in the fuel	Causes acid rain, which damages buildings, statues, lakes, and forests. Irritates the respiratory system.
Nitrogen oxides (NOx)	Formed when nitrogen and oxygen in the air react at the high temperatures inside engines	Cause acid rain and photochemical smog. Irritate the respiratory system.
Particulates	Tiny solid particles (soot, carbon) released during incomplete combustion	Cause respiratory problems, global dimming, and can carry carcinogenic compounds

Exam Tip: Nitrogen oxides are NOT formed from nitrogen in the fuel. They form because the nitrogen and oxygen in the air react together at the very high temperatures inside car engines and power station furnaces. This is a common exam question.

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Reducing Pollution from Combustion

Several methods are used to reduce the harmful effects of burning fossil fuels: