Global challenges — atmosphere, climate change, resources, polymers, organic chemistry and fuels
Earth's atmosphere — composition and changes
The current atmosphere is approximately: 78% nitrogen (N₂), 21% oxygen (O₂), ~1% argon (Ar), ~0.04% carbon dioxide (CO₂), plus water vapour (variable).
Early atmosphere (billions of years ago): mainly carbon dioxide and water vapour (from volcanic outgassing), with small amounts of methane and ammonia. Very little oxygen.
How the atmosphere changed:
- Water vapour condensed as Earth cooled → formed oceans.
- Carbon dioxide dissolved in oceans and was incorporated into sedimentary rocks and fossil fuels → CO₂ levels fell.
- Photosynthesis by early algae/plants released oxygen → O₂ levels rose.
- Ozone (O₃) formed from oxygen in the upper atmosphere → shielded Earth from UV radiation → life on land became possible.
Greenhouse effect and climate change
Greenhouse gases (CO₂, CH₄, water vapour, N₂O) absorb infrared radiation emitted by Earth's surface and re-emit it in all directions — some back towards Earth. This greenhouse effect keeps Earth warm enough for life.
Enhanced greenhouse effect: human activities (burning fossil fuels, deforestation, agriculture, cement production) increase greenhouse gas concentrations → more heat trapped → global warming → climate change.
Evidence for climate change: rising average global temperatures, ice core data showing historical CO₂/temperature correlation, sea level rise, melting polar ice, shifts in animal migration patterns.
Impacts of climate change: more extreme weather events; flooding of low-lying areas; species extinction; disruption to agriculture.
Reducing greenhouse gas emissions: renewable energy (solar, wind, hydroelectric); carbon capture and storage (CCS); increased energy efficiency; biofuels (carbon neutral if sustainable); electric vehicles.
Carbon chemistry and fuels
Crude oil is a fossil fuel — a mixture of hydrocarbon compounds. Fractional distillation separates it into fractions by boiling point range.
| Fraction | Carbon chain length | Uses |
|---|---|---|
| Petrol (gasoline) | C₄–C₁₂ | Car fuel |
| Kerosene (jet fuel) | C₁₀–C₁₆ | Aircraft |
| Diesel | C₁₄–C₂₀ | Trucks, buses |
| Heavy fuel oil | C₂₀–C₃₀+ | Ships, power stations |
| Bitumen | C₄₀+ | Road surfacing |
Shorter carbon chains → lower boiling point, more volatile, more flammable, less viscous.
Complete combustion of a hydrocarbon (excess oxygen): produces CO₂ + H₂O only. Incomplete combustion (limited oxygen): produces CO + C (soot) + H₂O. Carbon monoxide is toxic — binds to haemoglobin irreversibly, reducing oxygen transport.
Cracking: breaking long-chain hydrocarbons into shorter, more useful ones using heat and a catalyst.
- Produces alkenes (unsaturated) which are used to make polymers.
- Example: C₁₀H₂₂ → C₅H₁₀ + C₅H₁₂ (a pentene + pentane)
Alkanes and alkenes
Alkanes (CₙH₂ₙ₊₂): saturated hydrocarbons; all C−C single bonds. Relatively unreactive. Burn in air (combustion).
- Methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
Alkenes (CₙH₂ₙ): unsaturated hydrocarbons; contain at least one C=C double bond. More reactive than alkanes.
- Ethene (C₂H₄), propene (C₃H₆).
- Test for alkenes: bromine water (orange-brown) is decolourised by alkenes (addition reaction); alkanes do NOT decolourise bromine water.
Addition polymerisation: alkene monomers join end-to-end, opening the C=C bond to form long-chain polymers.
- n(CH₂=CH₂) → (−CH₂−CH₂−)ₙ (polyethylene/poly(ethene))
- n(CH₂=CHCH₃) → poly(propene)
Polymers and sustainability
Thermosoftening polymers (thermoplastics): e.g. poly(ethene), PVC. Soften on heating — polymer chains can slide; no cross-links between chains. Can be remoulded and recycled.
Thermosetting polymers (thermosets): e.g. Bakelite, melamine. Do NOT soften on heating — extensive cross-links between chains prevent movement. Cannot be remoulded.
Environmental issues with polymers: most plastics are non-biodegradable, derived from finite fossil fuels, cause environmental pollution. Solutions: mechanical recycling, chemical recycling (cracking back to monomers), bioplastics, reducing use.
Earth's resources and sustainable chemistry
Life cycle assessment (LCA) evaluates the environmental impact of a product from raw material extraction → manufacture → use → disposal.
Alloys: mixtures of a metal with other elements to improve properties (hardness, strength, corrosion resistance).
- Steel (Fe + C): harder than iron.
- Brass (Cu + Zn): harder than copper, good for instruments.
- Bronze (Cu + Sn): harder, for statues and coins.
Corrosion: metals reacting with oxygen and/or water. Iron rusts (hydrated iron(III) oxide) when both oxygen AND water are present; this is an electrochemical redox process. Prevention: painting, galvanising (zinc coat), sacrificial protection (zinc anode), stainless steel (chromium-iron alloy forms protective oxide layer).
AI-generated · claude-opus-4-7 · v3-ocr-chemistry