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This lesson covers hydrocarbons, alkanes, and crude oil as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what hydrocarbons are, be able to name and draw the first four alkanes, and explain why crude oil is such an important resource.
A hydrocarbon is a compound that contains only hydrogen and carbon atoms. There are no other elements present. Hydrocarbons are the main compounds found in crude oil and natural gas.
Exam Tip: If a compound contains any element other than carbon and hydrogen — for example oxygen or nitrogen — it is not a hydrocarbon.
Alkanes are a homologous series of hydrocarbons. A homologous series is a family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties.
The general formula for alkanes is:
CₙH₂ₙ₊₂
where n is the number of carbon atoms.
| Name | Molecular Formula | Number of C Atoms | Number of H Atoms | State at Room Temperature |
|---|---|---|---|---|
| Methane | CH₄ | 1 | 4 | Gas |
| Ethane | C₂H₆ | 2 | 6 | Gas |
| Propane | C₃H₈ | 3 | 8 | Gas |
| Butane | C₄H₁₀ | 4 | 10 | Gas |
In a displayed formula you show every bond between every atom:
graph LR
subgraph Methane CH₄
C1["C"] --- H1["H"]
C1 --- H2["H"]
C1 --- H3["H"]
C1 --- H4["H"]
end
subgraph Ethane C₂H₆
C2a["C"] --- C2b["C"]
C2a --- H5["H"]
C2a --- H6["H"]
C2a --- H7["H"]
C2b --- H8["H"]
C2b --- H9["H"]
C2b --- H10["H"]
end
Exam Tip: "Saturated" means all carbon–carbon bonds are single bonds. If a hydrocarbon has a C=C double bond it is unsaturated (an alkene, not an alkane).
Crude oil is a fossil fuel formed over millions of years from the remains of ancient marine organisms (plankton and algae) that were buried under layers of sediment. Heat and pressure converted the remains into a complex mixture of hydrocarbons.
Crude oil is a mixture of many different hydrocarbons — mostly alkanes — with different chain lengths. Because it is a mixture, the components are not chemically bonded to each other and can be separated by physical methods.
| Property | Short-Chain Hydrocarbons | Long-Chain Hydrocarbons |
|---|---|---|
| Boiling point | Low | High |
| Viscosity | Runny (low) | Thick / sticky (high) |
| Flammability | Highly flammable | Less flammable |
| Colour | Light / pale | Dark |
| Volatility | High (evaporate easily) | Low |
Crude oil is one of the most important natural resources because:
graph TD
A["Increasing chain length"] --> B["Boiling point INCREASES"]
A --> C["Viscosity INCREASES"]
A --> D["Flammability DECREASES"]
A --> E["Volatility DECREASES"]
A --> F["Colour becomes DARKER"]
style A fill:#2c3e50,color:#fff
style B fill:#e74c3c,color:#fff
style C fill:#e67e22,color:#fff
style D fill:#27ae60,color:#fff
style E fill:#2980b9,color:#fff
style F fill:#8e44ad,color:#fff
Exam Tip: You must be able to explain the trend in boiling points. Longer chains have stronger intermolecular forces (more points of contact between molecules), so more energy is needed to separate them, giving a higher boiling point.