Combustion of Hydrocarbons

This lesson covers complete and incomplete combustion of hydrocarbons as required by the Edexcel GCSE Combined Science specification (1SC0). You need to be able to write equations, describe the products of each type of combustion, and explain the dangers of incomplete combustion.

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What Is Combustion?

Combustion means burning. When a hydrocarbon burns in air (or oxygen), it is an exothermic reaction — energy is released to the surroundings as heat and light.

Hydrocarbons are used as fuels because they release a large amount of energy when they combust.

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Complete Combustion

Complete combustion occurs when there is plenty of oxygen available. The hydrocarbon reacts fully with oxygen to produce only two products:

hydrocarbon + oxygen → carbon dioxide + water

Word and Symbol Equations

Fuel	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

Observations

• A blue flame (e.g. a Bunsen burner with the air hole open).
• No soot or smoke.
• The flame is clean and hot.

Exam Tip: In complete combustion, every carbon atom ends up as CO₂ and every hydrogen atom ends up as H₂O. There are only two products.

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Incomplete Combustion

Incomplete combustion occurs when there is a limited (insufficient) supply of oxygen. The hydrocarbon does not burn completely, and different products are formed:

hydrocarbon + limited oxygen → carbon monoxide + water

or

hydrocarbon + very limited oxygen → carbon (soot) + water

In practice, incomplete combustion often produces a mixture of CO₂, CO, C (soot) and H₂O depending on how much oxygen is available.

Example Equations

Condition	Example Equation
Limited oxygen (CO produced)	2CH₄ + 3O₂ → 2CO + 4H₂O
Very limited oxygen (C produced)	CH₄ + O₂ → C + 2H₂O

Observations

• A yellow/orange flickering flame (e.g. a Bunsen burner with the air hole closed).
• Black soot (carbon particles) may be deposited on surfaces.
• Less energy is released compared with complete combustion.

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Products of Combustion — Summary Table

Product	Complete Combustion	Incomplete Combustion	Notes
Carbon dioxide (CO₂)	Yes	Sometimes (in mixture)	Greenhouse gas
Water (H₂O)	Yes	Yes	Always produced
Carbon monoxide (CO)	No	Yes	Toxic, colourless, odourless gas
Carbon / soot (C)	No	Yes (very limited O₂)	Causes health and environmental problems

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The Danger of Carbon Monoxide

Carbon monoxide (CO) is produced by incomplete combustion. It is extremely dangerous because:

• It is a colourless, odourless gas — you cannot see, smell or taste it.
• It is toxic — it binds to haemoglobin in red blood cells more strongly than oxygen does.
• This prevents the blood from carrying oxygen around the body, leading to oxygen starvation.
• Symptoms include headache, dizziness, nausea, confusion, and — in severe cases — death.

Preventing Carbon Monoxide Poisoning

• Ensure gas appliances (boilers, heaters, cookers) are properly maintained and serviced regularly.
• Make sure rooms have adequate ventilation so there is enough oxygen for complete combustion.
• Install a carbon monoxide alarm in your home.

graph LR
    A["Hydrocarbon + O₂"] --> B{"Enough oxygen?"}
    B -- "Yes (plenty)" --> C["CO₂ + H₂O<br/>Complete combustion<br/>Blue flame"]
    B -- "No (limited)" --> D["CO + H₂O<br/>Incomplete combustion<br/>Yellow flame"]
    B -- "Very little" --> E["C + H₂O<br/>Soot produced<br/>Smoky flame"]

    style A fill:#2c3e50,color:#fff
    style B fill:#f39c12,color:#000
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#c0392b,color:#fff

Exam Tip: Carbon monoxide is dangerous because it is colourless and odourless — you cannot detect it without an alarm. It binds to haemoglobin, reducing the blood's ability to carry oxygen.

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Environmental Impact of Combustion

Burning fossil fuels releases several substances into the atmosphere:

Substance	Environmental Problem
CO₂	Greenhouse gas → global warming and climate change
CO	Toxic to humans and animals
Soot / particulates	Causes respiratory problems; contributes to global dimming
SO₂	Acid rain (from sulfur impurities in the fuel)
NOₓ	Acid rain and smog (formed at high temperatures in engines)

Exam Tip: CO₂ is a product of complete combustion, not just incomplete combustion. It is released whenever any carbon-containing fuel burns. It is a greenhouse gas even when combustion is complete.

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Summary

• Complete combustion (plenty of O₂) produces CO₂ and H₂O only — gives a blue flame and releases maximum energy.
• Incomplete combustion (limited O₂) produces CO and/or C (soot) alongside H₂O — gives a yellow/smoky flame and releases less energy.
• Carbon monoxide is toxic, colourless and odourless — it binds to haemoglobin and reduces oxygen transport in the blood.
• Gas appliances must be maintained and ventilated; carbon monoxide alarms should be fitted.
• Burning hydrocarbons releases CO₂ (greenhouse gas), and can release soot, CO, SO₂ and NOₓ.

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Worked Examples — Balancing Combustion Equations

Worked Example 1 — Complete combustion of methane

Question: Write a balanced symbol equation for the complete combustion of methane (CH₄).

Step 1 — Skeleton: CH₄ + O₂ → CO₂ + H₂O.

Step 2 — Balance carbon: 1 C each side. OK.

Step 3 — Balance hydrogen: 4 H on the left (in CH₄). Need 2 H₂O on the right. CH₄ + O₂ → CO₂ + 2H₂O.

Step 4 — Balance oxygen: Right side has (2 × 1) + (2 × 1) = 4 O atoms. Need 2 O₂ on the left.

Balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O.

Worked Example 2 — Complete combustion of propane

Question: Balance: C₃H₈ + O₂ → CO₂ + H₂O.

Step 1 — Carbon: 3 C left, so need 3 CO₂. Step 2 — Hydrogen: 8 H left, so need 4 H₂O. Step 3 — Oxygen: Right has (3 × 2) + (4 × 1) = 10 O atoms, so need 5 O₂.

Balanced: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Worked Example 3 — Incomplete combustion

Question: Write an equation for the incomplete combustion of methane producing carbon monoxide.

Answer: 2CH₄ + 3O₂ → 2CO + 4H₂O.

Common Mistake: Learners sometimes write CO₂ and CO in the same equation. In an exam, stick to one product — either CO₂ (complete), CO (incomplete) or C (very incomplete) — unless told otherwise.

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Comparison Tables

Table 1 — Complete vs incomplete combustion