Hydrocarbons, fuels and organic chemistry
Hydrocarbons and crude oil
Hydrocarbons are compounds containing only hydrogen and carbon. Crude oil is a mixture of many hydrocarbons formed from the remains of ancient marine organisms (a finite/non-renewable resource).
Fractional distillation of crude oil
Crude oil is separated by fractional distillation in a fractionating column:
- Crude oil is heated at the bottom; vapours rise.
- The column is cooler at the top and hotter at the bottom.
- Fractions condense at different levels depending on their boiling point, which depends on molecular size (chain length).
Key fractions (from top to bottom of column):
| Fraction | Carbon chain | BP range | Uses |
|---|---|---|---|
| Gases (LPG) | C₁–C₄ | <25 °C | Camping gas, calor gas |
| Petrol | C₅–C₁₀ | 25–75 °C | Fuel for cars |
| Naphtha | C₅–C₁₀ | 75–150 °C | Chemical feedstock |
| Kerosene | C₁₀–C₁₆ | 150–250 °C | Jet fuel |
| Diesel | C₁₅–C₂₅ | 250–350 °C | Lorries, trains |
| Heavy fuel oil | C₂₀–C₇₀ | 350–500 °C | Ships, power stations |
| Bitumen | C₇₀+ | >500 °C | Road surfacing, roofing |
Trends in hydrocarbons: longer chain → higher boiling point (stronger London dispersion forces), more viscous, less flammable, darker colour.
Alkanes
Alkanes are saturated hydrocarbons: only single C−C and C−H bonds.
General formula: CₙH₂ₙ₊₂
| Name | Formula | Structural formula |
|---|---|---|
| Methane | CH₄ | CH₄ |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃(CH₂)₂CH₃ |
Complete combustion (excess oxygen): CₙH₂ₙ₊₂ + excess O₂ → CO₂ + H₂O
Incomplete combustion (limited oxygen): → Carbon monoxide (CO) — toxic; or soot (C particles)
Alkenes
Alkenes are unsaturated hydrocarbons: contain at least one C=C double bond.
General formula: CₙH₂ₙ
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
Test for unsaturation: add bromine water (orange). Alkenes decolourise it (addition reaction across the C=C). Alkanes do NOT decolourise bromine water.
Ethene + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)
Cracking
Cracking breaks long-chain alkanes into shorter, more useful molecules. It produces alkanes (for petrol) and alkenes (for plastics).
- Thermal cracking: high temperature (~500 °C) and pressure — produces mainly alkenes.
- Catalytic cracking: lower temperature (~500 °C) with a zeolite catalyst, lower pressure — produces branched alkanes (better petrol) and alkenes.
Example: C₁₆H₃₄ → C₈H₁₈ + C₄H₈ + C₄H₈
Addition polymerisation
Alkenes can polymerise through their C=C bonds. Each monomer adds to the growing chain — no atoms are lost (100% atom economy).
Poly(ethene): n CH₂=CH₂ → (−CH₂−CH₂−)ₙ Poly(propene): n CH₂=CHCH₃ → (−CH₂−CH(CH₃)−)ₙ PVC: n CH₂=CHCl → (−CH₂−CHCl−)ₙ
Representing polymers: the repeat unit is drawn in brackets with n outside and bonds through both ends of the brackets.
Environmental issues: most addition polymers are non-biodegradable. Solutions: recycle, incinerate for energy recovery, develop biodegradable alternatives.
Combustion and fuels — environmental impact
Products of combustion of fossil fuels:
- CO₂: greenhouse gas → climate change
- CO: toxic (binds to haemoglobin)
- Particulate carbon (soot): health problems; global dimming
- SO₂ (from sulfur impurities): dissolves in rain → sulfurous/sulfuric acid → acid rain → damages stonework, kills aquatic life
- NOₓ (nitrogen oxides from high-temperature combustion): smog, acid rain
Biofuels (e.g. biodiesel, bioethanol): derived from plant material; considered carbon neutral (CO₂ released = CO₂ absorbed during growth). WJEC requires evaluation of biofuels vs fossil fuels.
Common examiner traps
- Alkane vs alkene bromine water test: alkanes do NOT react (no colour change); alkenes DO (decolourise).
- General formula: alkanes CₙH₂ₙ₊₂; alkenes CₙH₂ₙ. Don't confuse.
- Cracking makes alkenes (unsaturated), not just shorter alkanes.
- Polymer repeat unit: in structural formulae the double bond disappears — it is now two single bonds (one into each adjacent monomer in the chain).
- Carbon neutral ≠ zero carbon: biofuels release CO₂ on burning; they are called carbon neutral because that CO₂ was recently absorbed from the atmosphere. Net CO₂ change ≈ 0, not zero emissions.
AI-generated · claude-opus-4-7 · v3-wjec-chemistry