Combustion Reactions

Combustion is the reaction of a substance with oxygen, releasing energy as heat and light. It is examined in organic chemistry (burning fuels), environmental chemistry (pollution), and chemical energetics.

Definition and requirements

Combustion requires:

1. A fuel (the substance that burns)
2. Oxygen (from the air)
3. Heat (to reach the ignition temperature)

All combustion reactions are exothermic.

Complete combustion

Occurs when there is an excess (plentiful supply) of oxygen.

General pattern for hydrocarbons

hydrocarbon + oxygen -> carbon dioxide + water

Examples

Fuel	Equation
Methane	CH4 + 2O2 -> CO2 + 2H2O
Ethane	2C2H6 + 7O2 -> 4CO2 + 6H2O
Propane	C3H8 + 5O2 -> 3CO2 + 4H2O
Butane	2C4H10 + 13O2 -> 8CO2 + 10H2O
Ethanol	C2H5OH + 3O2 -> 2CO2 + 3H2O
Hydrogen	2H2 + O2 -> 2H2O

Observations: blue flame (for clean-burning fuels like methane or ethanol), heat produced, water condenses on cool surfaces, CO2 turns limewater milky.

Incomplete combustion

Occurs when the oxygen supply is limited.

Products

Instead of all carbon forming CO2:

• Some carbon forms CO (carbon monoxide) — toxic gas
• Some carbon forms C (soot/carbon particles) — black solid
• Water is still produced

Examples

2CH4 + 3O2 -> 2CO + 4H2O (carbon monoxide produced)

CH4 + O2 -> C + 2H2O (soot produced)

Observations

• Yellow/orange smoky flame (not blue)
• Black soot deposited on surfaces
• Less heat produced than complete combustion

The danger of carbon monoxide

CO is:

• Colourless and odourless — cannot be detected without a CO alarm
• Toxic — binds to haemoglobin in red blood cells more strongly than oxygen
• Reduces oxygen transport in the blood
• Can cause headaches, dizziness, unconsciousness, and death

Faulty gas heaters and boilers in poorly ventilated rooms are the most common source of CO poisoning.

Combustion of metals

Metals also undergo combustion:

Metal	Equation	Observation
Magnesium	2Mg + O2 -> 2MgO	Brilliant white flame, white ash
Iron	3Fe + 2O2 -> Fe3O4	Sparkles, orange sparks (iron wool)
Sodium	4Na + O2 -> 2Na2O	Yellow flame

Environmental impact

Greenhouse effect

CO2 from combustion of fossil fuels contributes to the greenhouse effect and climate change. See air quality and climate change.

Other pollutants from combustion

Pollutant	Source	Problem
CO	Incomplete combustion	Toxic
CO2	Complete combustion	Greenhouse gas
SO2	Burning sulfur-containing fuels	Acid rain
NOx	N2 + O2 at high temp in engines	Acid rain, smog
Soot (C)	Incomplete combustion	Respiratory problems

Fuels compared

Fuel	Advantages	Disadvantages
Methane (natural gas)	Clean-burning, piped to homes	Fossil fuel, CO2 produced
Hydrogen	Only product is water	Difficult to store, explosive
Ethanol	Renewable (from fermentation), cleaner	Less energy per gram than petrol
Petrol/diesel	High energy density, easy to transport	Fossil fuel, pollutants

See fuels.

How to recognise in exams

Key words: “burns”, “flame”, “combustion”, “excess air”, “limited air”. Questions may ask you to: write balanced equations, compare complete and incomplete combustion, explain environmental effects, or discuss fuel choices.

Write equations for the complete and incomplete combustion of ethane (C2H6). [3 marks]

• Complete: 2C2H6 + 7O2 -> 4CO2 + 6H2O [1]
• Incomplete (CO): 2C2H6 + 5O2 -> 4CO + 6H2O [1]
• Incomplete (C): 2C2H6 + 3O2 -> 4C + 6H2O [1]

Explain why a yellow flame on a Bunsen burner indicates incomplete combustion. [2 marks]

• The air hole is closed or partially closed, so there is a limited supply of oxygen [1]
• Carbon particles (soot) are produced and glow yellow in the flame, and less energy is released [1]