14.3.2 Combustion

This topic explains combustion — the burning of alkanes in oxygen to produce carbon dioxide, water, and heat.

Definition

Combustion is the rapid reaction of a substance with oxygen, producing heat and light.

• Carbon dioxide (CO₂)
• Water (H₂O)
• Heat energy

Simple explanation: Combustion is burning — alkanes burn in oxygen to release energy.

Main Idea

Alkanes are widely used as fuels because they burn readily in oxygen, releasing large amounts of heat energy.

General combustion reaction:

Alkane + Oxygen → Carbon dioxide + Water + Heat

Simple explanation: Alkanes burn to produce CO₂, H₂O, and heat — this is why they are used as fuels.

Core Concept

General Combustion Equation

For an alkane with the formula CₙH₂ₙ₊₂:

C_nH_2n+2 + (3n+12)O₂ → nCO₂ + (n+1)H₂O + heat

Simple explanation: The equation shows that alkanes burn in oxygen to produce CO₂ and H₂O.

Complete Combustion

Complete combustion occurs when there is enough oxygen.

• Carbon dioxide (CO₂)
• Water (H₂O)
• Heat energy

Examples:

Methane:

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Ethane:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O + heat

Propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + heat

Butane:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O + heat

Simple explanation: With enough oxygen, alkanes burn cleanly to CO₂ and H₂O.

Incomplete Combustion

Incomplete combustion occurs when there is not enough oxygen.

• Carbon monoxide (CO) — a toxic gas
• Carbon (soot)
• Water (H₂O)
• Less heat energy

Example:

2CH₄ + 3O₂ → 2CO + 4H₂O + heat

Simple explanation: With limited oxygen, alkanes produce toxic CO instead of CO₂.

Detailed Explanation

Why Alkanes Are Good Fuels

• They burn readily in oxygen.
• They release large amounts of heat energy.
• They are easily available (natural gas, petrol, diesel, LPG).
• They are easy to store and transport.
• They have high energy density.

Simple explanation: Alkanes release a lot of energy when burned — that's why they are used as fuels.

Complete Combustion — In Detail

• Excess oxygen (enough O₂ for complete burning)
• Good ventilation

• CO₂ — carbon dioxide (greenhouse gas)
• H₂O — water (as vapour)
• Heat — released as energy

• Blue flame (clean burn)
• Maximum heat released
• No soot produced

Simple explanation: Complete combustion = enough oxygen → clean blue flame → maximum heat.

Incomplete Combustion — In Detail

• Limited oxygen (not enough O₂ for complete burning)
• Poor ventilation

• CO — carbon monoxide (toxic gas)
• C — carbon (soot)
• H₂O — water
• Less heat released

• Yellow/orange flame (dirty burn)
• Less heat released
• Soot produced
• CO produced (dangerous)

Simple explanation: Incomplete combustion = not enough oxygen → yellow flame → less heat, soot, and toxic CO.

Carbon Monoxide — The Silent Killer

• Is colourless and odourless.
• Is highly toxic.
• Combines with hemoglobin in blood to form carboxyhemoglobin.
• This prevents blood from carrying oxygen.
• Can cause unconsciousness and death.

• Ensure good ventilation.
• Install CO detectors.
• Maintain appliances properly.

Simple explanation: CO is a dangerous gas that can kill without warning — good ventilation is essential.

Heat Released by Combustion

Simple explanation: Larger alkanes release more heat per molecule — that's why petrol (octane) is a good fuel.

Environmental Impact of Combustion

Simple explanation: Burning alkanes produces CO₂ (greenhouse gas) and, if incomplete, CO and soot (harmful pollutants).

Intermediate Level

Complete Combustion Equations

General formula for alkane combustion:

C_nH_2n+2 + (3n+12)O₂ → nCO₂ + (n+1)H₂O

Examples:

Balancing Combustion Equations

• Write the alkane formula.
• Balance carbon → CO₂.
• Balance hydrogen → H₂O.
• Balance oxygen → O₂.
• Check that all atoms are balanced.

• C₃H₈ + O₂ → CO₂ + H₂O
• Balance C: C₃H₈ + O₂ → 3CO₂ + H₂O
• Balance H: C₃H₈ + O₂ → 3CO₂ + 4H₂O
• Balance O: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Environmental Concerns

Simple explanation: Burning fossil fuels causes pollution — CO₂ causes global warming, CO is toxic, and NOₓ/SO₂ cause acid rain.

Uses of Combustion

Important Points

• - Combustion: rapid reaction with oxygen, producing heat and light.
• - Complete combustion: enough oxygen → CO₂ + H₂O + heat; blue flame.
• - Incomplete combustion: limited oxygen → CO + H₂O + heat; yellow flame; soot.
• - General equation: Alkane + O₂ → CO₂ + H₂O + heat.
• - Examples:
• - CH₄ + 2O₂ → CO₂ + 2H₂O
• - C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• - CO is toxic — colourless, odourless, combines with hemoglobin.
• - Environmental impact: CO₂ → global warming; CO → toxic; soot → pollution; NOₓ/SO₂ → acid rain.
• - Uses: fuels for heating, cooking, transport, and industry.

Remember

• - Combustion = burning in oxygen.
• - Complete: CO₂ + H₂O + heat (blue flame).
• - Incomplete: CO + H₂O + heat (yellow flame).
• - Examples:
• - CH₄ + 2O₂ → CO₂ + 2H₂O
• - C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• - CO is toxic (colourless, odourless).
• - Environmental impact: CO₂ → global warming; CO → toxic; soot → pollution.
• - Uses: fuels for heating, cooking, transport.

Quick Revision

• Combustion: the rapid reaction of a substance with oxygen, producing heat and light.
• Complete combustion: occurs with enough oxygen.
• Products: carbon dioxide (CO₂) + water (H₂O) + heat
• Blue flame; maximum heat; no soot.
• CH₄ + 2O₂ → CO₂ + 2H₂O
• C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• Incomplete combustion: occurs with limited oxygen.
• Products: carbon monoxide (CO) + water (H₂O) + heat (and soot)
• Yellow/orange flame; less heat; soot produced.
• 2CH₄ + 3O₂ → 2CO + 4H₂O
• Carbon monoxide (CO): colourless, odourless, toxic; combines with hemoglobin; can cause death.
• Environmental impact: CO₂ → global warming; CO → toxic; soot → air pollution; NOₓ/SO₂ → acid rain.
• Uses: alkanes are used as fuels (natural gas, LPG, petrol, diesel).

60-Second Revision

• Combustion = burning in oxygen.
• Complete: CO₂ + H₂O + heat (blue flame).
• Incomplete: CO + H₂O + heat (yellow flame).
• Example: CH₄ + 2O₂ → CO₂ + 2H₂O.
• CO is toxic — colourless, odourless, deadly.
• Warning: incomplete combustion produces toxic CO — ensure good ventilation.