Properties of Hydrocarbons

This lesson explores the properties of hydrocarbons in detail, as required by the AQA GCSE Chemistry specification (5.8.1). You need to understand how the size and structure of hydrocarbon molecules affect their physical properties, and be able to explain these trends using ideas about intermolecular forces.

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Recap: What Are Hydrocarbons?

A hydrocarbon is a compound that contains only carbon and hydrogen atoms. The two families you need to know are:

• Alkanes — saturated hydrocarbons (single bonds only), general formula CnH2n+2
• Alkenes — unsaturated hydrocarbons (contain at least one C=C double bond), general formula CnH2n

This lesson focuses primarily on alkanes, as they make up the bulk of crude oil.

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Physical Properties of Alkanes

The physical properties of alkanes change in a gradual and predictable way as the number of carbon atoms increases. This is because the alkanes form a homologous series.

Boiling Point

Alkane	Formula	Number of Carbons	Boiling Point (°C)
Methane	CH4	1	-162
Ethane	C2H6	2	-89
Propane	C3H8	3	-42
Butane	C4H10	4	-1
Pentane	C5H12	5	36
Hexane	C6H14	6	69
Heptane	C7H16	7	98
Octane	C8H18	8	126

As the number of carbon atoms increases, the boiling point increases. This is because:

1. Longer molecules have more electrons
2. More electrons mean stronger London dispersion forces (intermolecular forces) between molecules
3. More energy is required to overcome these stronger forces
4. Therefore, higher temperatures are needed to boil the substance

Exam Tip: Always refer to "intermolecular forces" (forces between molecules), not "bonds." The covalent bonds within the molecules are NOT broken when a hydrocarbon boils. Boiling only overcomes the weak forces between molecules.

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Intermolecular Forces Explained

Intermolecular forces are the weak attractive forces that exist between molecules. In hydrocarbons (which are non-polar), the main type of intermolecular force is the London dispersion force (also called van der Waals force).

graph LR
    A["Small Molecule<br/>e.g. Methane CH4"] -->|"Fewer electrons"| B["Weaker London<br/>Dispersion Forces"]
    B --> C["Low Boiling Point"]
    D["Large Molecule<br/>e.g. Octane C8H18"] -->|"More electrons"| E["Stronger London<br/>Dispersion Forces"]
    E --> F["High Boiling Point"]

    style A fill:#3498db,color:#fff
    style D fill:#e74c3c,color:#fff
    style C fill:#2ecc71,color:#fff
    style F fill:#e67e22,color:#fff

These forces arise because electrons are constantly moving within molecules. At any given moment, the electrons may be unevenly distributed, creating a temporary dipole (one end is slightly negative, the other slightly positive). This temporary dipole can induce a dipole in a neighbouring molecule, creating a weak attractive force.

• Larger molecules have more electrons, so the temporary dipoles are larger and the attractive forces are stronger
• Smaller molecules have fewer electrons, so the forces are weaker

It is important to distinguish between:

Type	Definition	Strength
Covalent bonds	Shared pairs of electrons between atoms within a molecule	Strong — require a lot of energy to break
Intermolecular forces	Weak attractive forces between neighbouring molecules	Weak — relatively little energy required to overcome

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Viscosity

Viscosity is a measure of how easily a liquid flows. A liquid with high viscosity is thick and flows slowly; a liquid with low viscosity is thin and flows easily.

Hydrocarbon	Chain Length	Viscosity
Petrol (C5-C8)	Short	Low — flows easily
Diesel (C13-C16)	Medium	Medium
Fuel oil (C17-C20)	Long	High — thick, flows slowly
Bitumen (C20+)	Very long	Very high — almost solid

As chain length increases, viscosity increases because:

1. Longer molecules become more tangled with each other
2. There are more intermolecular forces along the length of each molecule
3. It is harder for the molecules to slide past one another

Exam Tip: If asked to explain why longer hydrocarbons are more viscous, always link it to intermolecular forces: "Longer hydrocarbon molecules have stronger intermolecular forces between them, so the molecules cannot slide past each other as easily."

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Flammability

Flammability refers to how easily a substance catches fire and burns.

Chain Length	Flammability	Reason
Short (C1-C4)	Highly flammable	Gaseous or very volatile; mix easily with air
Medium (C5-C16)	Flammable	Volatile liquids that can vaporise and ignite
Long (C17+)	Less flammable	Less volatile; harder to vaporise and mix with air

Short-chain hydrocarbons are the most useful as fuels precisely because they ignite easily and burn cleanly (when there is sufficient oxygen).

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Volatility

Volatility describes how easily a liquid evaporates — that is, how readily it turns into a gas at room temperature.

• Short-chain hydrocarbons are more volatile — they have low boiling points and evaporate easily
• Long-chain hydrocarbons are less volatile — they have high boiling points and do not evaporate easily

Volatility is directly related to boiling point: a low boiling point means high volatility.

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Summary of Property Trends

The following table summarises how the properties of hydrocarbons change as chain length increases:

Property	As Chain Length Increases
Boiling point	Increases
Viscosity	Increases
Flammability	Decreases
Volatility	Decreases
Colour	Gets darker
Ease of ignition	Decreases

All of these trends can be explained by the fact that longer molecules have stronger intermolecular forces between them.

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State at Room Temperature

The boiling point determines the state of a hydrocarbon at room temperature (approximately 25°C):

Number of Carbons	State at Room Temperature	Examples
1 – 4	Gas	Methane, ethane, propane, butane
5 – 16	Liquid	Petrol, kerosene, diesel
17+	Solid or very thick liquid	Wax, bitumen

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Displayed Formulae of the First Four Alkanes