📖 In-Depth Theory
How Chain Length Affects Physical Properties
The PHYSICAL PROPERTIES of hydrocarbons change systematically with chain length.
BOILING POINT:
Increases with chain length.
Longer chains have greater LONDON DISPERSION FORCES between molecules.
More energy needed to separate molecules → higher boiling point.
Methane (CH₄): −161°C. Hexadecane (C₁₆H₃₄): 287°C.
VISCOSITY:
Increases with chain length.
Longer chains get tangled → flow less easily → more viscous.
Methane: gas (not viscous at all). Bitumen: extremely viscous semi-solid.
FLAMMABILITY (ease of ignition):
Decreases with chain length.
Shorter chains are more VOLATILE (evaporate easily at room temperature).
High vapour pressure → easy to ignite.
Longer chains: lower vapour pressure, harder to ignite.
This explains why petrol (short chains) catches fire easily while fuel oil (long chains) is much harder to ignite.
Volatility and State at Room Temperature
VOLATILITY — the tendency to evaporate — decreases with chain length.
At room temperature (approximately 20–25°C):
C₁–C₄ hydrocarbons are GASES (boiling point below room temperature).
C₅–C₁₇ hydrocarbons are LIQUIDS.
C₁₈+ hydrocarbons are SOLIDS (boiling point above room temperature).
This explains the states of crude oil fractions:
Refinery gases (methane, propane): gases at room temperature.
Petrol, diesel: liquids.
Waxes, bitumen: solids or semi-solids.
VOLATILITY AND SAFETY:
Highly volatile fuels (petrol) present fire hazards because vapour can ignite easily.
More volatile substances also have higher vapour pressures — they smell more strongly.
This is why petrol smells strongly and diesel smells less — petrol is more volatile.
Burning Hydrocarbons — Combustion
All hydrocarbons burn in oxygen — COMBUSTION is their most important chemical reaction.
COMPLETE COMBUSTION (excess oxygen):
All carbon burned to CO₂. All hydrogen burned to H₂O.
Produces only CO₂ and water — maximum energy release.
INCOMPLETE COMBUSTION (limited oxygen):
Produces CO and/or soot (carbon particulates) in addition to or instead of CO₂.
Less energy released.
CO: toxic, causes carbon monoxide poisoning.
Soot/particulates: cause respiratory problems, global dimming.
BALANCING COMBUSTION EQUATIONS:
Octane (C₈H₁₈) + 12.5O₂ → 8CO₂ + 9H₂O (complete combustion)
Or: 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
The LONGER the chain, the more oxygen needed for complete combustion.
⚠️ Common Mistake
Shorter chain hydrocarbons have LOWER boiling points and are MORE flammable/volatile. Longer chains are LESS flammable and MORE viscous. Students often get flammability and chain length confused — think of it this way: petrol (short chains) is much more flammable than diesel (longer chains).
🎯 Test Yourself
Question 1 of 2
1. Why is petrol (C₅–C₁₀) more flammable than diesel (C₁₅–C₂₅)?
Petrol contains more hydrogen — hydrogen is more flammable than carbon
Petrol has shorter chains — more volatile, evaporates easily at room temperature, produces flammable vapour readily
Diesel is more viscous — it can't flow to the flame and doesn't ignite
Petrol has double bonds — alkenes are more reactive and flammable than alkanes
2. Propane (C₃H₈) is a gas at room temperature but hexadecane (C₁₆H₃₄) is a solid. Why?
Hexadecane has much longer chains — stronger intermolecular forces mean its boiling point is far above room temperature
Hexadecane has ionic bonds that hold it as a solid
Hexadecane contains more hydrogen — hydrogen atoms make it solid
Propane is lighter, so it exists as a gas by buoyancy