Properties of hydrocarbons and their combustion

The properties of hydrocarbons depend on the chain length. Longer chains have stronger intermolecular forces (more electrons → more London dispersion forces).

Property trends with chain length

As chain length increases:

• Boiling point increases (more energy needed to overcome forces between chains).
• Viscosity increases (longer chains tangle more — thicker liquid).
• Flammability decreases (harder to vaporise).

This is why short chains make good fuels and long chains make bitumen/lubricants.

Combustion

Hydrocarbons release energy when they react with oxygen. There are two types:

Complete combustion (plenty of O₂)

hydrocarbon + O₂ → CO₂ + H₂O (releases lots of energy)

Methane: CH₄ + 2O₂ → CO₂ + 2H₂O

Propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Incomplete combustion (limited O₂)

Produces carbon monoxide CO (toxic) and/or soot (carbon particles).

2CH₄ + 3O₂ → 2CO + 4H₂O

Or even further: CH₄ + O₂ → C + 2H₂O.

Why incomplete combustion is dangerous

• Carbon monoxide CO is toxic. It binds to haemoglobin 300× more strongly than O₂, preventing oxygen transport. Symptoms: headache, dizziness, unconsciousness, death.
• Soot triggers asthma and respiratory disease, blackens buildings.
• Less energy released compared to complete combustion.

This is why gas appliances must be ventilated and have CO detectors.

Balancing combustion equations

For CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O:

1. Balance C atoms (n × CO₂).
2. Balance H atoms ((2n+2)/2 = (n+1) × H₂O).
3. Balance O atoms (sum on right ÷ 2 = O₂ on left).

Worked example: butane.

• C₄H₁₀ + O₂ → 4CO₂ + 5H₂O
• Right side: 4×2 + 5×1 = 13 O atoms → 6.5 O₂.
• Multiply through by 2: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.

Test for products

• CO₂: bubble through limewater → turns cloudy.
• H₂O: anhydrous CuSO₄ goes blue (or anhydrous CoCl₂ goes blue → pink).

Links

Builds on C7.1, C7.2. Sets up C9.5 (atmospheric pollutants from fuels) and connects to C5.3 (bond energies in combustion).