Crude oil, hydrocarbons and alkanes
Crude oil is a finite, fossil-fuel resource formed over millions of years from the remains of ancient sea creatures (plankton). It is a mixture of hydrocarbons, mostly alkanes.
Hydrocarbons — the basics
A hydrocarbon is a compound made only of hydrogen and carbon atoms. Most hydrocarbons in crude oil are alkanes — saturated chains of carbon atoms with single bonds only.
Alkanes have the general formula CₙH₂ₙ₊₂.
The first four alkanes
| Name | Formula | Displayed |
|---|---|---|
| Methane | CH₄ | tetrahedral 4 H around C |
| Ethane | C₂H₆ | H₃C–CH₃ |
| Propane | C₃H₈ | H₃C–CH₂–CH₃ |
| Butane | C₄H₁₀ | H₃C–CH₂–CH₂–CH₃ |
Mnemonic for the order: Meth, Eth, Prop, But (then Pent, Hex, Hept, Oct, Non, Dec).
Saturated vs unsaturated
- Saturated: only single bonds (alkanes).
- Unsaturated: contains at least one double or triple C=C bond (alkenes — see C7.4).
Properties of alkanes
Alkanes are generally:
- Colourless (small ones).
- Insoluble in water.
- Combust in air to give CO₂ and H₂O (complete) or CO + soot (incomplete).
- Unreactive with most reagents at room temperature (don't react with acids, alkalis, halogens in dark).
Trends as chain length increases
| Property | Trend |
|---|---|
| Boiling point | Increases (more intermolecular forces) |
| Viscosity | Increases (longer chains tangle) |
| Flammability | Decreases (harder to vaporise large molecules) |
Why ancient origin matters
Crude oil is non-renewable on human timescales. Once burned, it's gone. This drives the search for renewable alternatives (C10.1).
⚠Common mistakes
- Confusing the formula CₙH₂ₙ₊₂ (alkane) with CₙH₂ₙ (alkene).
- Saying alkanes are reactive — they are unusually stable.
- Forgetting the prefix order: methane (1 C), ethane (2 C), propane (3 C), butane (4 C). Not "1, 2, 3, 4 = a, b, c, d".
- Treating crude oil as a compound — it's a mixture.
Links
Sets up C7.2 (fractional distillation), C7.3 (combustion), C7.4 (cracking and alkenes).
AI-generated · claude-opus-4-7 · v3-deep-chemistry