Carbon Compounds as Fuels and Feedstock (C7.1)
Crude oil
Crude oil is a mixture of hydrocarbons (compounds of carbon and hydrogen only) formed from marine organisms buried under sediment over millions of years. It is a non-renewable fossil fuel.
Fractional distillation
Crude oil is separated by fractional distillation:
- Crude oil heated → vapour rises up a fractionating column (hot at bottom, cooler at top).
- Different fractions condense at different temperatures (boiling points).
- Shorter hydrocarbon chains = lower boiling point = collected higher up the column.
Fractions from top (lowest bp) to bottom (highest bp):
| Fraction | Carbon chain length | Use |
|---|---|---|
| Refinery gas | C1–C4 | Bottled gas, fuel |
| Petrol (gasoline) | C5–C10 | Car fuel |
| Naphtha | C5–C10 | Chemical feedstock |
| Kerosene (jet fuel) | C10–C16 | Aircraft fuel |
| Diesel | C15–C25 | Vehicles, heating |
| Fuel oil | C20–C70 | Ships, power stations |
| Bitumen/tar | C70+ | Roads, roofing |
Alkanes
Alkanes: saturated hydrocarbons — single C–C bonds only. General formula: CₙH₂ₙ₊₂.
| n | Name | Formula |
|---|---|---|
| 1 | Methane | CH₄ |
| 2 | Ethane | C₂H₆ |
| 3 | Propane | C₃H₈ |
| 4 | Butane | C₄H₁₀ |
Properties trend (as chain length increases):
- Boiling point ↑ (stronger intermolecular forces)
- Viscosity ↑ (thicker)
- Flammability ↓ (harder to ignite)
- Colour darker
Alkenes
Alkenes: unsaturated hydrocarbons — contain at least one C=C double bond. General formula: CₙH₂ₙ.
Ethene: CH₂=CH₂; Propene: CH₃CH=CH₂.
Test for alkene: add bromine water (brown/orange) → decolourises (turns colourless) if alkene present. Alkenes react by addition across the double bond.
Cracking
Cracking breaks large, less useful hydrocarbon chains into smaller, more useful ones. It produces alkenes (used for polymers) as well as shorter alkanes (fuels).
large alkane → smaller alkane + alkene
Example: C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀ (decane → pentane + pentene)
Thermal cracking: very high temperature (>700°C) and pressure.
Catalytic cracking: lower temperature (~550°C), zeolite catalyst. More common industrially.
Combustion
Complete combustion (excess O₂): hydrocarbon + O₂ → CO₂ + H₂O (clean, more energy released)
CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion (limited O₂): produces carbon monoxide (CO) and/or soot C.
CO is toxic (binds to haemoglobin).
Common exam errors
- Saying alkenes are saturated — they are unsaturated (C=C double bond).
- Confusing fractional distillation (mixture separation by boiling point) with cracking (chemical breakdown).
- Saying longer chains are more flammable — they are less flammable.
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