Properties of hydrocarbons Revision Notes for AQA GCSE Chemistry

Properties of hydrocarbons

How molecular size affects properties

The properties of alkane molecules depend on how big the molecules are. Alkanes are hydrocarbon molecules made of carbon and hydrogen atoms only.

As alkane molecules get bigger (more carbon atoms), their properties change in predictable ways:

Boiling points get higher
Viscosity increases (they become thicker and flow less easily)
Flammability decreases (they become harder to ignite)

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These predictable patterns occur because as molecules get larger, the intermolecular forces between them become stronger. This fundamental relationship helps chemists predict how different hydrocarbons will behave.

Boiling point trends

Looking at the first four alkanes shows a clear pattern:

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Pattern Analysis: Boiling Points of Small Alkanes

Methane: -162°C
Ethane: -89°C
Propane: -42°C
Butane: 0°C

Notice how each additional carbon atom increases the boiling point significantly!

Each time we add another carbon atom, the boiling point goes up. This happens because larger molecules have stronger forces between them.

Viscosity and flammability patterns

Viscosity means how thick a liquid is and how easily it flows.

Small hydrocarbon molecules = runny liquids (low viscosity)
Large hydrocarbon molecules = thick liquids (high viscosity)

Flammability means how easily something catches fire.

Small hydrocarbon molecules = very flammable
Large hydrocarbon molecules = less flammable

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These contrasting properties make different sized hydrocarbons suitable for different applications. The size of the molecule directly determines its practical uses.

Why these properties matter for fuels

These different properties make hydrocarbons useful for different jobs:

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Real-World Applications:

Petrol (small molecules) is runny and flammable - perfect for cars
Heavy fuel oil (large molecules) is thick and less flammable - used in big boilers and factories

The properties help us separate crude oil into useful fractions through fractional distillation.

Complete combustion of hydrocarbons

When hydrocarbons burn completely in plenty of air, they always make the same products:

Carbon atoms become carbon dioxide ( $CO_2$ )
Hydrogen atoms become water ( $H_2O$ )
Lots of heat is released

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Example: Butane Burning Completely

$C_4H_{10} + O_2 \rightarrow CO_2 + H_2O$

(This equation needs to be balanced - see the next section!)

Balancing combustion equations

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Worked Example: Balancing Pentane Combustion

To balance these equations, follow this systematic approach:

Step 1: Balance carbon first - count carbon atoms on left, put that number before $CO_2$

Pentane ( $C_5H_{12}$ ) has 5 carbons → $5CO_2$

Step 2: Balance hydrogen next - count hydrogen atoms on left, divide by 2 for $H_2O$

Pentane has 12 hydrogens → $6H_2O$

Step 3: Balance oxygen last - count all oxygen atoms needed on right, put that number before $O_2$

Total oxygen needed: $(5 \times 2) + (6 \times 1) = 16$ , so $8O_2$

Final balanced equation: $C_5H_{12} + 8O_2 \rightarrow 5CO_2 + 6H_2O$

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Key Points to Remember:

Bigger hydrocarbon molecules have higher boiling points, are more viscous, and less flammable
Small molecules make good fuels for vehicles, large molecules are used in industry
Complete combustion always makes $CO_2$ and $H_2O$ plus heat
Balance combustion equations by doing carbon first, then hydrogen, then oxygen
These property differences allow crude oil to be separated into useful fractions

Properties of hydrocarbons (AQA GCSE Chemistry): Revision Notes