CC7.1 — Hydrocarbons and crude oil (Edexcel 1SC0)
Crude oil
Crude oil is a mixture of hydrocarbons formed over millions of years from the remains of marine organisms. It is a finite (non-renewable) resource.
Hydrocarbons: compounds containing only carbon and hydrogen.
Alkanes: saturated hydrocarbons — all C-C single bonds. Formula: CₙH₂ₙ₊₂. Examples: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
Fractional distillation
Crude oil is separated in a fractionating column. Components with similar boiling points are collected as fractions. Shorter carbon chains have lower boiling points and are collected at the top; longer chains at the bottom.
| Fraction | Carbon chain length | Use |
|---|---|---|
| Refinery gas | C1–C4 | Bottled gas, fuel |
| Petrol (gasoline) | C5–C10 | Car fuel |
| Kerosene | C10–C16 | Jet fuel |
| Diesel | C16–C25 | Truck/bus fuel |
| Fuel oil | C25+ | Ships, power stations |
| Bitumen | C70+ | Road surfacing |
Cracking
Long-chain alkanes are in low demand; short-chain alkanes and alkenes are in high demand.
Cracking: thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules.
$$C_{10}H_{22} \to C_8H_{18} + C_2H_4$$
Produces alkenes (unsaturated) — used to make polymers.
Test for alkenes: bromine water — decolourises (orange → colourless) in the presence of a C=C double bond.
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