Properties of Hydrocarbons

This lesson covers how the physical properties of hydrocarbons change with chain length, and why there is a mismatch between supply and demand for different fractions. This is key content for the Edexcel GCSE Chemistry specification (1CH0) and builds directly on what you learned about fractional distillation.

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How Properties Change with Chain Length

As the number of carbon atoms in a hydrocarbon chain increases, several physical properties change in a predictable way. These trends apply to alkanes and other homologous series.

The Key Trends

Property	As chain length increases…	Reason
Boiling point	Increases	Longer chains have stronger intermolecular forces — more energy is needed to separate the molecules
Viscosity	Increases (thicker, flows less easily)	Longer molecules become more tangled and experience greater intermolecular forces, making the liquid thicker
Flammability	Decreases (harder to ignite)	Longer molecules need more energy to vaporise before they can burn; shorter chains ignite more easily
Colour	Becomes darker	Short-chain fractions are colourless or pale yellow; long-chain fractions are dark brown to black

Exam Tip: The three trends you must know are boiling point, viscosity and flammability. Always explain trends in terms of intermolecular forces — this is what earns the marks.

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Explaining the Trends — Intermolecular Forces

The physical properties of hydrocarbons depend on the intermolecular forces (forces of attraction between molecules), not on the covalent bonds within the molecules.

What Are Intermolecular Forces?

Intermolecular forces are weak electrostatic attractions between neighbouring molecules. In hydrocarbons, these are London dispersion forces (also called van der Waals forces). Their strength depends on the surface area of the molecule:

• Longer chains → greater surface area → more points of contact between molecules → stronger intermolecular forces.
• Shorter chains → smaller surface area → fewer contact points → weaker intermolecular forces.

Applying This to Each Property

Boiling point: To boil a liquid, you must give the molecules enough energy to overcome the intermolecular forces and escape into the gas phase. Longer chains have stronger intermolecular forces, so more energy (a higher temperature) is required → higher boiling point.

Viscosity: Viscosity is a measure of how easily a liquid flows. Longer molecules become tangled together and have stronger intermolecular forces, making it harder for the molecules to slide past each other → higher viscosity (thicker).

Flammability: For a hydrocarbon to burn, it must first vaporise. Shorter chains have low boiling points and vaporise easily, so they mix with air and ignite readily → high flammability. Longer chains need much more energy to vaporise and are harder to ignite → low flammability.

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Summary Table

Fraction	Chain length	Boiling point	Viscosity	Flammability	Ease of ignition
Refinery gases (LPG)	Very short (C₁–C₄)	Very low	Very runny (gas)	Very high	Very easy
Petrol	Short (C₅–C₁₀)	Low	Runny	High	Easy
Kerosene	Medium (C₁₀–C₁₆)	Medium	Medium	Medium	Moderate
Diesel	Medium–long (C₁₄–C₂₀)	High	Thick	Low	Harder
Fuel oil	Long (C₂₀–C₅₀)	Very high	Very thick	Very low	Difficult
Bitumen	Very long (C₅₀+)	Extremely high	Semi-solid	Extremely low	Very difficult

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Which Fractions Are Most Useful as Fuels?

Short-chain fractions are the most useful as fuels because they:

• Ignite easily (highly flammable).
• Burn with a cleaner flame (less soot and smoke).
• Are liquids or gases at room temperature, making them easy to transport and use.

Long-chain fractions are less useful as fuels because they are viscous, difficult to ignite, and burn with a smoky flame.

Exam Tip: In a comparison question, always state the property for both short-chain and long-chain hydrocarbons. For example: "Short-chain hydrocarbons have low viscosity whereas long-chain hydrocarbons have high viscosity."

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Worked Example — Explaining a Trend

Question: Explain why bitumen has a much higher boiling point than petrol. [3 marks]

Model answer:

1. Bitumen contains much longer hydrocarbon chains than petrol.
2. Longer chains have a greater surface area, so there are stronger intermolecular forces (London forces) between the molecules.
3. More energy is needed to overcome these stronger intermolecular forces, so bitumen has a higher boiling point.

Question: Explain why petrol is more flammable than diesel. [2 marks]

Model answer:

1. Petrol has shorter hydrocarbon chains than diesel, so it has weaker intermolecular forces and a lower boiling point.
2. Petrol vaporises more easily, allowing its vapour to mix with air and ignite more readily.

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Combustion Behaviour and Chain Length

The way a hydrocarbon burns is also related to chain length:

• Short-chain hydrocarbons tend to burn with a clean, blue flame — complete combustion occurs readily because the molecules vaporise easily and mix well with air.
• Long-chain hydrocarbons tend to burn with a smoky, yellow flame — incomplete combustion is more likely because the molecules do not vaporise or mix with air as well.

This is another reason why short-chain fractions are preferred as fuels — they produce less soot and less carbon monoxide when burned.

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Supply and Demand Mismatch

Fractional distillation of crude oil produces large quantities of every fraction. However, the demand for each fraction is not the same:

• There is high demand for short-chain fractions (petrol, kerosene, LPG) because they are used as fuels for cars, planes and heating.
• There is low demand for long-chain fractions (fuel oil, bitumen) — far more of these are produced than are needed.

This creates a supply and demand mismatch: the refinery produces too much of the heavier fractions and not enough of the lighter ones.