Hydrocarbons and crude oil
Crude oil and hydrocarbons
Crude oil is a finite mixture of mostly hydrocarbons — compounds of carbon and hydrogen only. It formed over millions of years from ancient plankton buried under heat and pressure.
Alkanes — saturated hydrocarbons
General formula: CₙH₂ₙ₊₂. Single bonds only (saturated). First four:
| Name | Formula | Boiling point trend |
|---|---|---|
| Methane | CH₄ | lowest |
| Ethane | C₂H₆ | rising |
| Propane | C₃H₈ | rising |
| Butane | C₄H₁₀ | highest |
Trends as chain length increases: higher boiling point (more intermolecular forces), more viscous, less flammable.
Combustion
Complete: hydrocarbon + O₂ → CO₂ + H₂O. Releases lots of energy — used as fuels. Incomplete (limited O₂): produces CO (toxic) and soot (carbon).
Fractional distillation
Crude oil is heated to ~350 °C and the vapour rises up a fractionating column with a temperature gradient. Fractions condense at different heights:
| Fraction (top → bottom) | Approx. carbons | Use |
|---|---|---|
| Refinery gases | 1–4 | Cooking gas |
| Petrol | 5–10 | Car fuel |
| Kerosene | 10–16 | Aircraft |
| Diesel | 14–20 | Trucks |
| Bitumen | 30+ | Roads |
Shorter chains = lower BP, condense higher up; longer chains = high BP, condense lower.
Cracking
Long-chain alkanes are heated and passed over a catalyst → break into shorter alkanes and alkenes.
decane → octane + ethene C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Why crack? Demand for short-chain (petrol, polymers) exceeds supply; long-chain fractions are surplus.
Alkenes — unsaturated hydrocarbons
General formula: CₙH₂ₙ. Contain at least one C=C double bond. Test: decolourise orange bromine water → colourless (the bromine adds across the double bond).
Alkenes are the building blocks for addition polymers (e.g. ethene → poly(ethene)).
OCR exam tip
If the question asks why fractional distillation works, write about boiling point and temperature gradient — not "they go to different places."
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves