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Crude oil is one of the most important natural resources on Earth. It is a complex mixture of thousands of different hydrocarbons formed over millions of years from the remains of ancient marine organisms. Understanding what crude oil is, what hydrocarbons are, and how the properties of hydrocarbons change with molecular size is fundamental to this topic.
Crude oil is a finite resource — once we use it all, it cannot be replaced on any human timescale. It is formed from the remains of tiny marine organisms (plankton, algae) that died and sank to the ocean floor millions of years ago. Over time, layers of sediment buried these remains, and the heat and pressure transformed them into crude oil trapped in porous rock.
Crude oil is a mixture of many different hydrocarbons. Because it is a mixture (not a compound), the components can be separated by physical methods — specifically, fractional distillation.
A hydrocarbon is a molecule that contains only hydrogen and carbon atoms. There are no other elements present.
The simplest hydrocarbons are the alkanes — a homologous series of saturated hydrocarbons with the general formula:
CnH2n+2
Where n = number of carbon atoms.
| Name | Molecular formula | Structural formula | Number of C atoms |
|---|---|---|---|
| Methane | CH₄ | CH₄ | 1 |
| Ethane | C₂H₆ | CH₃CH₃ | 2 |
| Propane | C₃H₈ | CH₃CH₂CH₃ | 3 |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ | 4 |
| Pentane | C₅H₁₂ | CH₃(CH₂)₃CH₃ | 5 |
| Hexane | C₆H₁₄ | CH₃(CH₂)₄CH₃ | 6 |
| Octane | C₈H₁₈ | CH₃(CH₂)₆CH₃ | 8 |
| Decane | C₁₀H₂₂ | CH₃(CH₂)₈CH₃ | 10 |
graph TD
A["Hydrocarbons"] --> B["Saturated\n(single C–C bonds only)"]
A --> C["Unsaturated\n(contains C=C double bonds)"]
B --> D["Alkanes\nGeneral formula: CₙH₂ₙ₊₂"]
C --> E["Alkenes\nGeneral formula: CₙH₂ₙ"]
D --> F["Examples: methane, ethane,\npropane, butane"]
E --> G["Examples: ethene, propene,\nbutene"]
As the number of carbon atoms in a hydrocarbon increases, the physical properties change in a predictable pattern:
| Property | Short chains (1–4 C) | Medium chains (5–12 C) | Long chains (13+ C) |
|---|---|---|---|
| State at room temperature | Gas | Liquid | Solid (waxy) |
| Boiling point | Low | Medium | High |
| Viscosity | N/A (gases) | Low (runny) | High (thick/viscous) |
| Flammability | Very flammable | Flammable | Less easily ignited |
| Colour | Colourless | Colourless to yellow | Dark brown/black |
The trend in boiling points is explained by intermolecular forces: longer molecules have stronger London dispersion forces (van der Waals forces) between them because they have more electrons and a larger surface area for contact. More energy is needed to overcome these forces, so the boiling point is higher.
Question: An alkane has 7 carbon atoms. Determine its molecular formula.
Using CₙH₂ₙ₊₂ where n = 7:
Question: A molecule has the formula C₄H₈. Is it an alkane or an alkene? Explain your answer.
If it were an alkane (CₙH₂ₙ₊₂ with n = 4): C₄H₁₀ Since C₄H₈ has fewer hydrogen atoms than C₄H₁₀, it is not an alkane. The formula matches CₙH₂ₙ (with n = 4), so it is an alkene. It contains a carbon-carbon double bond and is unsaturated.
Question: Predict which has the higher boiling point: pentane (C₅H₁₂) or decane (C₁₀H₂₂). Explain your answer.
Answer: Decane has the higher boiling point. Decane molecules are larger (more carbon and hydrogen atoms), giving them a larger surface area. This means the intermolecular forces (London dispersion forces) between decane molecules are stronger. More energy is needed to overcome these forces and separate the molecules, so the boiling point is higher.
Common GCSE exam mistake: Writing that longer chain hydrocarbons have higher boiling points because they have "stronger bonds." The covalent bonds within the molecules are not what is being broken during boiling. It is the intermolecular forces BETWEEN molecules that must be overcome, and these are stronger for longer molecules.
Crude oil provides:
However, crude oil is a finite, non-renewable resource. Burning it releases CO₂ (contributing to climate change) and other pollutants. Finding sustainable alternatives is one of the great challenges of the 21st century.
The alkanes form a homologous series — a family of compounds that share the same general formula, have similar chemical properties, and show a gradual trend in physical properties as the chain length increases.
Key features of any homologous series:
Question: Nonane (C₉H₂₀) is an alkane. Predict its state at room temperature and whether it would be more or less viscous than pentane. Explain your reasoning.
Answer: Nonane has 9 carbon atoms, which places it in the medium-chain range (5–12 C). It would be a liquid at room temperature (like petrol). It would be more viscous than pentane (C₅H₁₂) because it has a longer chain. Longer chains have stronger intermolecular forces (London dispersion forces) due to the greater surface area for contact between molecules, making the liquid thicker and harder to pour.
| Name | Formula | Mᵣ | Boiling point (°C) | State at 25 °C |
|---|---|---|---|---|
| Methane | CH₄ | 16 | −162 | Gas |
| Ethane | C₂H₆ | 30 | −89 | Gas |
| Propane | C₃H₈ | 44 | −42 | Gas |
| Butane | C₄H₁₀ | 58 | −1 | Gas |
| Pentane | C₅H₁₂ | 72 | 36 | Liquid |
| Hexane | C₆H₁₄ | 86 | 69 | Liquid |
| Heptane | C₇H₁₆ | 100 | 98 | Liquid |
| Octane | C₈H₁₈ | 114 | 126 | Liquid |
Notice the clear trend: boiling point increases steadily with chain length. The transition from gas to liquid at room temperature occurs between butane (bp −1 °C, just below room temp) and pentane (bp 36 °C, above room temp).