Complete and Incomplete Combustion

This lesson covers the combustion of hydrocarbons — both complete and incomplete — as required by the Edexcel GCSE Chemistry specification (1CH0). You need to be able to write balanced equations for complete combustion, describe the products and dangers of incomplete combustion, and explain the environmental problems caused by burning fossil fuels.

---

What Is Combustion?

Combustion is the reaction of a fuel with oxygen. It is an exothermic reaction — energy is transferred to the surroundings (released as heat and light).

Hydrocarbons are widely used as fuels because they release a large amount of energy when they burn. The products of combustion depend on how much oxygen is available.

---

Complete Combustion

Complete combustion occurs when a hydrocarbon burns in a plentiful supply of oxygen. The only products are:

• Carbon dioxide (CO₂)
• Water (H₂O)

Word Equation

hydrocarbon + oxygen → carbon dioxide + water

Balanced Symbol Equations

You need to be able to write and balance symbol equations for the complete combustion of hydrocarbons.

Methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

Ethane:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Butane:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

Exam Tip: To balance a combustion equation: (1) balance the carbon atoms first, (2) balance the hydrogen atoms next, (3) balance the oxygen atoms last. If you end up with a half O₂, multiply everything by 2 to get whole numbers.

Recognising Complete Combustion

Complete combustion produces a blue flame and no soot or smoke. A Bunsen burner with the air hole fully open demonstrates complete combustion.

---

Incomplete Combustion

Incomplete combustion occurs when a hydrocarbon burns in a limited (insufficient) supply of oxygen. Not all of the carbon is fully oxidised to CO₂.

Products of Incomplete Combustion

Instead of carbon dioxide, incomplete combustion produces one or more of the following alongside water:

Product	Formula	Description
Carbon monoxide	CO	A toxic, colourless, odourless gas
Carbon (soot)	C	Black solid particles (particulates)
Water	H₂O	Same as complete combustion

The exact mix of products depends on how limited the oxygen supply is. With a very restricted supply, more soot (carbon) is produced; with a moderate restriction, more carbon monoxide is produced.

Example Word Equation

hydrocarbon + oxygen (limited) → carbon monoxide + carbon + water

Example Symbol Equation (Methane — Limited Oxygen)

2CH₄ + 3O₂ → 2CO + 4H₂O

Or with even less oxygen, producing soot:

CH₄ + O₂ → C + 2H₂O

Recognising Incomplete Combustion

Incomplete combustion produces a yellow/orange flame, and often black smoke (soot). A Bunsen burner with the air hole closed demonstrates incomplete combustion.

---

Combustion Flow — Oxygen Supply Determines the Products

graph TB
    A[Hydrocarbon fuel + O₂] --> B{Oxygen supply}
    B -->|Plentiful O₂| C[Complete combustion]
    B -->|Limited O₂| D[Incomplete combustion]
    C --> E[CO₂ carbon dioxide]
    C --> F[H₂O water]
    C --> G["Clean blue flame<br/>Maximum energy"]
    D --> H["CO carbon monoxide<br/>toxic"]
    D --> I[C soot / particulates]
    D --> J[H₂O water]
    D --> K["Yellow / sooty flame<br/>Less energy"]

    style C fill:#4a90e2,color:#fff
    style D fill:#e67e22,color:#fff
    style H fill:#c0392b,color:#fff
    style I fill:#2c3e50,color:#fff

The same hydrocarbon can undergo either type of combustion — the only thing that changes is how much oxygen is available. This is why a Bunsen burner with the air hole open burns cleanly (blue flame, complete combustion) while one with the air hole closed produces a sooty yellow flame (incomplete combustion).

---

Dangers of Carbon Monoxide

Carbon monoxide (CO) is extremely dangerous because it is:

• Colourless — you cannot see it.
• Odourless — you cannot smell it.
• Toxic — it binds to haemoglobin in red blood cells more strongly than oxygen does.

When CO binds to haemoglobin, it forms carboxyhaemoglobin, which cannot carry oxygen. This reduces the amount of oxygen transported around the body. In high concentrations, carbon monoxide causes death by oxygen starvation.

This is why:

• Gas appliances (boilers, cookers, heaters) must be properly maintained and well ventilated.
• Carbon monoxide detectors are recommended in homes with gas appliances.
• Faulty or badly maintained gas boilers are a major source of indoor CO poisoning.

Exam Tip: When asked about the dangers of CO, give three key points: (1) it is colourless and odourless so it cannot be detected without an alarm, (2) it binds to haemoglobin, (3) it reduces the blood's ability to carry oxygen, which can be fatal.

---

Particulates (Soot)

Particulates are tiny solid particles released during incomplete combustion. They include carbon (soot) particles.

Problems Caused by Particulates

Problem	Explanation
Respiratory disease	Particulates are small enough to enter the lungs and can cause or worsen conditions such as asthma, bronchitis and lung cancer
Global dimming	Particulates in the atmosphere reflect sunlight back into space, reducing the amount of light reaching the Earth's surface
Blackening of buildings	Soot deposits on surfaces cause discolouration

---

Sulfur Dioxide and Acid Rain

Many fossil fuels contain sulfur impurities. When the fuel burns, the sulfur is oxidised to sulfur dioxide (SO₂):

sulfur + oxygen → sulfur dioxide

S + O₂ → SO₂

Sulfur dioxide dissolves in rainwater and reacts with water and oxygen in the atmosphere to form sulfuric acid (H₂SO₄), which falls as acid rain.

Effects of Acid Rain

• Damages limestone and marble buildings and statues (these are made of calcium carbonate, which reacts with acid).
• Kills trees and plants by damaging leaves and leaching nutrients from the soil.
• Acidifies lakes and rivers, harming aquatic life (fish and other organisms).

---

Nitrogen Oxides and Smog

At the high temperatures inside car engines and power stations, nitrogen and oxygen from the air react to form nitrogen oxides (NOₓ — a mixture of NO and NO₂):