AQA GCSE Chemistry (8462) · Paper 2
🛢️ Organic Chemistry
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Crude oil & alkanes
Crude oil formed over millions of years from the buried remains of plankton compressed under mud, making it a finite fossil fuel. It is a mixture of hydrocarbons, compounds made of hydrogen and carbon only.
Most of these hydrocarbons are alkanes, the simplest hydrocarbon family. Alkanes have the general formula CₙH₂ₙ₊₂ and contain all single bonds, so they are described as saturated. The first four are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀); a common memory aid for this order is 'Monkeys Eat Peanut Butter'. A homologous series is a family of compounds with the same general formula and similar chemical reactions, where each member has one more CH₂ group than the last. A displayed formula shows every atom and every bond in a molecule.
Fractional distillation
The oil is vaporised and fed into a column that is hot at the bottom and cool at the top. As the vapours rise, each fraction condenses at its own level, so molecules with similar boiling points collect together. Small molecules stay as a gas for longest and collect near the top, while the largest molecules condense immediately near the bottom.
As the carbon chains get longer, the boiling point increases, the viscosity increases so the liquid becomes gloopier, and the flammability decreases. This is why short-chain hydrocarbons make the most valuable fuels. Burning any hydrocarbon completely produces carbon dioxide and water.
Cracking & alkenes
Demand for short-chain fuels is greater than the supply, so longer chains are cracked: broken down into a shorter alkane, a useful fuel, plus an alkene. There are two methods of cracking: catalytic cracking, which passes hot vapour over a catalyst, and steam cracking, which mixes the vapour with steam at a high temperature.
Alkenes contain a C=C double bond. They have the general formula CₙH₂ₙ, are described as unsaturated, and are more reactive than alkanes, which makes them the building blocks for the polymer industry. The first four, needed at Triple only, are ethene, propene, butene and pentene. Alkenes burn with smoky flames because combustion is incomplete.
The test for an alkene is to shake the sample with orange bromine water: an alkene turns it colourless, while an alkane leaves it orange.
At Triple only, alkenes undergo addition reactions, in which the double bond opens and atoms simply add on. Adding hydrogen, with a nickel catalyst, produces an alkane; adding steam produces an alcohol; adding a halogen produces a dihalide. Nothing is left over, because everything adds onto the molecule.
Alcohols to polymers (Triple) Separate science only
A functional group is the reactive part of a molecule that defines which family it belongs to: the –OH group gives alcohols, the –COOH group gives carboxylic acids, and the C=C double bond gives alkenes.
Fermentation produces ethanol when yeast is added to a sugar solution at around 30°C with no oxygen present (anaerobic conditions), producing ethanol and carbon dioxide.
Polymers
Nature's polymers
Carbohydrates are made of carbon, hydrogen and oxygen: sugars such as glucose are the monomers, and starch and cellulose are their polymers. Proteins are polymers of amino acids, which carry both –COOH and –NH₂ groups and are joined by condensation reactions into polypeptides. DNA is made of two strands of nucleotide monomers wound together into a double helix.