Guiding Spaceship Earth: Carbon Chemistry — AQA Combined Science: Synergy
This topic covers the forms of carbon, hydrocarbons in crude oil, fractional distillation and cracking.
Bonding and structure in forms of carbon
Carbon forms several structures with different properties:
- Diamond — each carbon bonded to four others in a giant covalent structure; very hard, high melting point, does not conduct.
- Graphite — each carbon bonded to three others in layers that slide; soft, slippery; has delocalised electrons so it conducts electricity.
- Graphene — a single layer of graphite; very strong and conducting.
- Fullerenes — molecules of carbon arranged in tubes or spheres (e.g. buckminsterfullerene C₆₀); used in drug delivery, lubricants and catalysts.
Hydrocarbons in crude oil
Crude oil is a finite resource formed from ancient sea organisms. It is a mixture of compounds, most of which are hydrocarbons (made of hydrogen and carbon only). The main hydrocarbons are alkanes (general formula CₙH₂ₙ₊₂; saturated, only single bonds): methane, ethane, propane, butane.
As molecules get larger: boiling point rises, viscosity increases, and flammability decreases.
Fractional distillation of crude oil
Crude oil is separated into fractions by fractional distillation:
- The oil is heated and evaporates.
- Vapours rise up a column that is hot at the bottom, cooler at the top.
- Each fraction condenses where the temperature matches its boiling point — larger molecules lower down, smaller ones higher up.
Fractions (refinery gases, petrol, kerosene, diesel, fuel oil, bitumen) are used as fuels and as feedstock for the petrochemical industry. Burning hydrocarbons (combustion) is exothermic: hydrocarbon + oxygen → carbon dioxide + water. Incomplete combustion produces carbon monoxide and soot.
Cracking hydrocarbons
Cracking breaks long-chain hydrocarbons into smaller, more useful molecules using heat and a catalyst (catalytic cracking) or steam (steam cracking). It produces smaller alkanes (fuels) and alkenes.
Alkenes (general formula CₙH₂ₙ) contain a carbon–carbon double bond (C=C), so they are unsaturated and more reactive. They turn bromine water from orange to colourless (the test for a double bond) and are used to make polymers.
Cracking is important because it converts surplus long-chain molecules into the short-chain fuels and alkenes that are in high demand.
Exam tips
- Link each form of carbon to its structure and properties (especially graphite's conductivity).
- Define a hydrocarbon precisely and learn the alkane trends (boiling point, viscosity, flammability).
- Describe fractional distillation in steps, linking condensing height to boiling point.
- Remember the bromine water test (orange → colourless for alkenes) and why cracking is done.