GCSE Chemistry revision

Properties of hydrocarbons

Carbon compounds as fuels and feedstock

AQA 4.7.1.3 Foundation & Higher

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Alkanes are a homologous series of hydrocarbons containing only carbon and hydrogen atoms with no double bonds.
The first four alkanes are methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀).

The boiling point of alkanes increases as the length of the carbon chain increases.
Shorter alkanes (1–4 carbon atoms) have low boiling points and exist as gases at room temperature.
Longer alkanes (more than 4 carbons) are liquids at room temperature, and very long-chain alkanes can be solids.

2. Boiling Point and Physical State AQA

The boiling point of alkanes increases as the length of the carbon chain increases.
Shorter alkanes (1–4 carbon atoms) have low boiling points and exist as gases at room temperature.
Longer alkanes (more than 4 carbons) are liquids at room temperature, and very long-chain alkanes can be solids.

Shorter alkanes are more volatile, meaning they evaporate more easily due to their low boiling points.
Longer alkanes are more viscous, meaning they are thick and sticky, similar to honey.

Shorter alkanes are more flammable, meaning they ignite and burn more easily than longerchain alkanes.
This makes short-chain alkanes particularly useful as fuels, as they release large amounts of energy when burned.

Complete combustion occurs when a hydrocarbon reacts with sufficient oxygen to produce carbon dioxide and water, releasing energy.
Complete combustion is an exothermic reaction, meaning energy is released to the surroundings.
During combustion, the carbon and hydrogen in the hydrocarbon are oxidised, forming CO₂ and H₂O respectively.

5. Complete Combustion of Alkanes AQA

Complete combustion occurs when a hydrocarbon reacts with sufficient oxygen to produce carbon dioxide and water, releasing energy.
Complete combustion is an exothermic reaction, meaning energy is released to the surroundings.
During combustion, the carbon and hydrogen in the hydrocarbon are oxidised, forming CO₂ and H₂O respectively.

To balance a combustion equation, first write the unbalanced equation with the alkane reacting with O₂ to form CO₂ and H₂O.
Balance carbon atoms first by adjusting the number of CO₂ molecules on the right-hand side.
Balance hydrogen atoms next by adjusting the number of H₂O molecules, remembering each H₂O contains two hydrogen atoms.
Finally, balance oxygen atoms by adjusting the number of O₂ molecules on the left-hand side.

6. Balancing Combustion Equations AQA

To balance a combustion equation, first write the unbalanced equation with the alkane reacting with O₂ to form CO₂ and H₂O.
Balance carbon atoms first by adjusting the number of CO₂ molecules on the right-hand side.
Balance hydrogen atoms next by adjusting the number of H₂O molecules, remembering each H₂O contains two hydrogen atoms.
Finally, balance oxygen atoms by adjusting the number of O₂ molecules on the left-hand side.

The balanced equation for the complete combustion of propane is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Three CO₂ molecules are needed to account for the three carbon atoms, and four H₂O molecules account for the eight hydrogen atoms.
The five O₂ molecules on the left provide the 10 oxygen atoms needed to balance the right-hand side (6 from CO₂ + 4 from H₂O).

7. Worked Example: Combustion of Propane AQA

The balanced equation for the complete combustion of propane is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Three CO₂ molecules are needed to account for the three carbon atoms, and four H₂O molecules account for the eight hydrogen atoms.
The five O₂ molecules on the left provide the 10 oxygen atoms needed to balance the right-hand side (6 from CO₂ + 4 from H₂O).

The balanced equation for the complete combustion of nonane is C₉H₂₀ + 14O₂ → 9CO₂ + 10H₂O.
Nine CO₂ molecules balance the nine carbon atoms, and ten H₂O molecules balance the twenty hydrogen atoms in nonane.
There are 28 oxygen atoms on the right-hand side (18 from CO₂ + 10 from H₂O), requiring 14 O₂ molecules on the left.

8. Worked Example: Combustion of Nonane AQA

The balanced equation for the complete combustion of nonane is C₉H₂₀ + 14O₂ → 9CO₂ + 10H₂O.
Nine CO₂ molecules balance the nine carbon atoms, and ten H₂O molecules balance the twenty hydrogen atoms in nonane.
There are 28 oxygen atoms on the right-hand side (18 from CO₂ + 10 from H₂O), requiring 14 O₂ molecules on the left.