Fuels and Earth science
Crude oil and fractional distillation
Crude oil is a mixture of hydrocarbons (mainly alkanes) formed from the remains of ancient marine organisms over millions of years. It is a finite, non-renewable resource.
Fractional distillation of crude oil separates it into fractions based on boiling point ranges. The crude oil is heated and vapours rise up a fractionating column that has a temperature gradient (hot at bottom, cool at top). Components condense at their own boiling point and are collected.
| Fraction | Carbon chain length | Boiling point range | Uses |
|---|---|---|---|
| Petroleum gases | C₁–C₄ | Below 25°C | LPG fuel, chemical feedstock |
| Petrol (gasoline) | C₅–C₁₀ | 25–75°C | Car fuel |
| Naphtha | C₈–C₁₃ | 75–120°C | Chemical feedstock |
| Kerosene (paraffin) | C₁₀–C₁₆ | 120–240°C | Jet fuel, heating |
| Diesel oil | C₁₄–C₁₉ | 240–350°C | Lorries/diesel engines |
| Fuel oil | C₂₀–C₃₀ | 350–400°C | Ships, power stations |
| Bitumen | >C₃₀ | Above 400°C | Road surfacing |
Trend: longer chain → higher boiling point, higher viscosity, lower volatility, less easily ignited.
Alkanes and combustion
Alkanes (CₙH₂ₙ₊₂): methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
Complete combustion (excess O₂): CₓH₂ₓ₊₂ + O₂ → CO₂ + H₂O (only) Incomplete combustion (limited O₂): produces CO (toxic), C (soot/particulates), and/or H₂O.
Edexcel Core Practical CP6 — Testing combustion products:
- Pass combustion products through cobalt chloride paper: turns blue to pink → water present.
- Pass products through limewater: turns milky → CO₂ present.
Cracking
Demand for short-chain fractions (petrol, etc.) exceeds supply; heavy fractions are in excess. Cracking breaks long-chain alkanes into shorter, more useful hydrocarbons including alkenes.
Thermal cracking: high temperature (>700°C), high pressure; produces mostly alkenes. Catalytic cracking: moderate temperature (~450°C), zeolite catalyst; produces a mix including branched/cyclic hydrocarbons and alkenes.
Example: C₁₂H₂₆ → C₈H₁₈ + C₂H₄ + C₂H₄ (octane + ethene + ethene)
Alkenes produced are feedstock for making polymers and other chemicals.
Earth's atmosphere and climate change
Current atmosphere (by volume)
- Nitrogen N₂: ~78%
- Oxygen O₂: ~21%
- Argon Ar: ~0.9%
- Carbon dioxide CO₂: ~0.04% (rising)
Early Earth and development of the atmosphere
Early atmosphere (~4 billion years ago): mainly CO₂ and water vapour (from volcanic outgassing), with some N₂, CH₄, NH₃, H₂S; very little O₂.
How the atmosphere changed:
- Oceans formed as Earth cooled; CO₂ dissolved into oceans and formed carbonate rocks → CO₂ levels decreased.
- Algae/cyanobacteria evolved (photosynthesis): 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ → O₂ levels rose.
- O₂ reacted with methane/ammonia → N₂ increased.
Greenhouse gases and climate change
Greenhouse gases: CO₂, CH₄, N₂O, H₂O vapour. They absorb infrared radiation emitted by Earth's surface and re-emit it in all directions, keeping the planet warmer than it would be otherwise — the greenhouse effect.
Human activities increasing greenhouse gases: burning fossil fuels (CO₂), deforestation (less CO₂ absorbed), livestock farming (CH₄), landfill sites (CH₄), rice paddies (CH₄).
Effects of climate change: rising sea levels (ice melting), extreme weather, habitat loss, ocean acidification (CO₂ + H₂O → H₂CO₃, which reduces pH).
Carbon footprint: total greenhouse gases emitted by an activity/individual/product, expressed in CO₂ equivalents.
⚠Common mistakes
- Cracking produces alkenes, not more alkanes: one product is always an alkene (unsaturated).
- Fractional distillation vs simple distillation: fractional uses a column (temperature gradient); simple does not.
- Greenhouse effect is natural: it is essential for life; the problem is the enhanced greenhouse effect from anthropogenic emissions.
- CO vs CO₂: incomplete combustion makes CO (toxic, odourless); complete combustion makes CO₂.
AI-generated · claude-opus-4-7 · v3-edexcel-chemistry