Capstone: Name that molecule Triple
The whole of the second half of C7 is built on functional groups: the part of a molecule that decides how it reacts, and which family it belongs to. Given a name or a condensed formula, you should be able to place any small organic molecule into the right homologous series — alkane (C–C and C–H only, CnH2n+2), alkene (a C=C double bond, CnH2n), alcohol (the –OH group) or carboxylic acid (the –COOH group). Sort each molecule below into its family.
Drag each molecule into a box — or tap it to step through the boxes. Then press Check.
- Crude oil, hydrocarbons & alkanes: crude oil is a finite resource — the remains of an ancient biomass (mainly plankton) buried in mud. Hydrocarbons contain hydrogen and carbon only; most are alkanes — saturated (single bonds only), general formula CnH2n+2, first four methane, ethane, propane, butane. A homologous series shares a general formula and each member differs by CH2.
- Fractional distillation: separates crude oil into fractions of similar chain length by evaporation then condensation. Large molecules (high boiling point) condense low in the hot column; small molecules (low boiling point) rise higher before condensing. It is a physical separation — no bonds inside molecules break.
- Properties & combustion: as molecules get larger, boiling point and viscosity rise and flammability falls (stronger intermolecular forces — a C2 idea). Complete combustion oxidises both elements → carbon dioxide + water; incomplete combustion (limited air) also gives toxic carbon monoxide and carbon (soot).
- Cracking: breaks long-chain alkanes into smaller, more useful alkanes + alkenes (catalytic or steam cracking) — meeting demand for short-chain fuels and supplying alkenes for polymers. Bromine water tests for the C=C: alkenes turn it orange → colourless; alkanes leave it orange.
- Alkenes T: unsaturated hydrocarbons with the C=C functional group, general formula CnH2n (ethene, propene, butene, pentene). Their reactions are all addition across the C=C — with hydrogen (nickel catalyst → alkane), steam (catalyst → alcohol) and the halogens (room temperature → di-halo compound).
- Alcohols T: functional group –OH (methanol, ethanol, propanol, butanol). They react with sodium (→ hydrogen), burn, dissolve to near-neutral solutions and are oxidised to carboxylic acids; uses are fuels, solvents and drinks. Ethanol is made by fermentation of sugars (yeast, ≈30 °C, no air → ethanol + CO2) or by hydration of ethene (steam + catalyst).
- Carboxylic acids T: functional group –COOH (methanoic, ethanoic, propanoic, butanoic; ethanoic acid = vinegar). They react with carbonates (→ CO2), form acidic solutions, and react with alcohols to make esters. H They are weak acids — only partially ionised, so a higher pH than a strong acid of the same concentration.
- Polymers T: in addition polymerisation, alkene monomers join at the opened C=C — the repeating unit has the same atoms as the monomer and nothing else forms. H In condensation polymerisation, monomers carry two functional groups (e.g. a diol + a dicarboxylic acid → a polyester) and a small molecule (usually water) is lost at every link.
- Natural polymers T: amino acids (an –NH2 and a –COOH) condense into polypeptides and proteins. DNA is two polymer chains wound into a double helix, each built from four different monomers called nucleotides; starch and cellulose are both polymers of sugars (glucose).
And that’s the carbon trail complete: from plankton buried in mud, through the fractionating column and the cracker, to fuels, alcohols, acids, esters and polymers — ending with the natural polymers that make the chemistry of life. For Paper 2, pair these notes with C6 — Rates & Equilibrium: equilibrium conditions and catalysts questions love to use the ethene-to-ethanol reaction as their context, and cracking borrows the same rate ideas. Next, C8 — Chemical Analysis turns to identifying these compounds — testing for pure substances, separating mixtures by chromatography, and the lab tests for gases and ions.