Properties of Hydrocarbons | Chemistry

📖 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).

📐 Key Equations

Hydrocarbon + O₂ → CO₂ + H₂O (complete combustion)

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O (octane complete combustion)

📌 Key Note

Longer chains: higher BP, more viscous, less flammable, less volatile. Shorter chains: lower BP, less viscous, more flammable, more volatile. C₁–C₄ = gases. C₅–C₁₇ = liquids. C₁₈+ = solids. Complete combustion: CO₂ + H₂O. Incomplete: CO + soot.

🎯 Matching Activity — Chain Length → Property

Match each property to whether it increases or decreases with chain length. — drag the symbols on the right to match the component names on the left.

Increases with chain length

Drop here

Increases with chain length

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Decreases with chain length

Drop here

Decreases with chain length

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Viscosity — longer chains tangle, flow less easily

Volatility — longer chains have lower vapour pressure

Boiling point — stronger intermolecular forces in longer molecules

Flammability — longer chains less volatile, harder to ignite

⭐ Higher Tier Only

Explain all physical trends using intermolecular forces. Complete combustion equations for any alkane. Bromine water test: explain mechanism — Br₂ adds across C=C double bond forming dibromoalkane (addition reaction). Compare alkane and alkene reactivity.

🎯 Test Yourself

Question 1 of 2

1. Why is petrol (C₅–C₁₀) more flammable than diesel (C₁₅–C₂₅)?

2. Propane (C₃H₈) is a gas at room temperature but hexadecane (C₁₆H₃₄) is a solid. Why?

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🧪 Properties of Hydrocarbons

How Chain Length Affects Physical Properties

Volatility and State at Room Temperature

Burning Hydrocarbons — Combustion

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