Hydrocarbons and Crude Oil
What are Hydrocarbons?
Hydrocarbons are organic compounds containing only carbon and hydrogen atoms. They are the main components of fossil fuels (crude oil, natural gas, coal).
Crude Oil
Crude oil is a complex mixture of many different hydrocarbon molecules. It is a finite resource (non-renewable) — it formed from the remains of ancient marine organisms over millions of years.
On its own, crude oil is not very useful — it must be separated into fractions (groups of hydrocarbons with similar chain lengths and boiling points) before use.
Fractional Distillation
Fractional distillation separates crude oil into useful fractions based on their boiling points.
Process:
- Crude oil is heated → vapourised
- Vapours rise up a fractionating column (which has a temperature gradient — hot at bottom, cool at top)
- Each fraction condenses at a different height (where temperature equals its boiling point) and is collected
Key fractions (from bottom/hot to top/cool):
| Fraction | Carbon chain length | Boiling point range | Main use |
|---|---|---|---|
| Bitumen (residue) | C₄₀+ | >350 °C | Road surfacing, roofing |
| Fuel oil | C₂₀–C₄₀ | 250–350 °C | Ships, power stations |
| Diesel | C₁₅–C₂₅ | 220–250 °C | Cars, trucks |
| Kerosene (jet fuel) | C₁₂–C₁₆ | 175–220 °C | Aircraft |
| Naphtha | C₅–C₁₂ | 75–175 °C | Feedstock for chemicals |
| Petrol (gasoline) | C₄–C₁₂ | 40–75 °C | Cars |
| Refinery gas | C₁–C₄ | <40 °C | Cooking/heating (LPG) |
Trend: Longer carbon chain → higher boiling point, higher viscosity (thicker), less flammable, darker colour.
Alkanes
Alkanes are a homologous series of saturated hydrocarbons (single C−C bonds only, general formula CₙH₂ₙ₊₂).
| Name | Formula | Structure |
|---|---|---|
| Methane | CH₄ | Single C surrounded by 4 H |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
Structural formulae show how atoms are bonded (WJEC regularly asks for these or displayed formulae).
Combustion
Complete combustion (excess oxygen): Hydrocarbon + O₂ → CO₂ + H₂O (only)
- Produces maximum energy
- Blue flame
Example — methane: CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion (limited oxygen): Hydrocarbon + O₂ → CO (carbon monoxide) + H₂O, and/or soot (carbon particles)
- Less energy released
- Yellow/orange smoky flame
- Produces carbon monoxide (CO) — a toxic, colourless, odourless gas that binds to haemoglobin, preventing oxygen transport → can be fatal in enclosed spaces
Environmental effects of burning fossil fuels:
- CO₂ → greenhouse gas → global warming / climate change
- SO₂ (from sulfur impurities) → dissolves in rain → acid rain → damages ecosystems, buildings
- NOₓ (formed at high temperatures: N₂ + O₂) → acid rain + photochemical smog
- Carbon particulates (soot) → respiratory problems; climate effects
- CO → poisoning
WJEC Wales Context
The former Milford Haven oil refinery and Pembrokeshire refineries have featured in Welsh GCSE context questions on crude oil processing and environmental impact of the fossil fuel industry.
AI-generated · claude-opus-4-7 · v3-wjec-combined-science