HookYour petrol was made by snapping bigger molecules in half
Distil a 159-litre barrel of crude oil and simple separation hands you perhaps a fifth of it as petrol-range molecules. Yet an American refinery routinely sells almost half of every barrel as petrol — around 19 of the 42 gallons. The difference is not pumped out of the ground; it is manufactured, by heating the heavy, unwanted molecules until their carbon chains snap into the short, valuable ones drivers actually buy. A refinery is not a giant sieve. It is a disassembly plant, and cracking — the deliberate breaking of big hydrocarbons — is the reason the world's fuel supply matches the world's demand.
The raw material is ancient: crude oil is the compressed remains of marine plankton, buried in mud and cooked over millions of years, which is why it is a finite resource — we spend in decades what formed over geological time. Chemically it is a mixture of hundreds of hydrocarbons, most of them alkanes. C7 is the shortest chemistry section in Trilogy and it is built from exactly four ideas: what the molecules are, how boiling points sort them in a fractionating column, why their properties follow chain length, and how cracking rebalances supply against demand. Combined Science keeps organic chemistry gentle — alkanes and alkenes only, no alcohols, no polymers — so every one of these marks is bankable.
ModelAlkanes — carbon's chain-building habit
A hydrocarbon is a compound of hydrogen and carbon only — and 'only' is doing real work in that definition, because a molecule with even one oxygen atom in it fails the test. Most of the hydrocarbons in crude oil are alkanes: chains of carbon atoms joined entirely by single bonds, with hydrogen filling every remaining slot. Because no more atoms can be squeezed in, alkanes are described as saturated.
Every alkane obeys one general formula: \(\mathrm{C_nH_{2n+2}}\). The first four are the ones you must know by name — methane CH₄ (natural gas), ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀ (bottled camping gas). A family like this — same general formula, similar reactions, properties that change steadily as the chain grows — is called a homologous series, and the 'change steadily' part is what makes the rest of this section predictable rather than memorised.
Predict the formula of the alkane with 8 carbon atoms. Substitute n = 8 into \(\mathrm{C_nH_{2n+2}}\): hydrogen count = 2(8) + 2 = 18, so the formula is \(\mathrm{C_8H_{18}}\) — octane, squarely in the petrol fraction. Now the reverse skill: is \(\mathrm{C_6H_{12}}\) an alkane? For n = 6 an alkane needs 2(6) + 2 = 14 hydrogens; this molecule has only 12, so it is not an alkane — it fits \(\mathrm{C_nH_{2n}}\), the alkene pattern, and it would decolourise bromine water. Two hydrogens short is the fingerprint of one double bond.
MechanismFractional distillation — a tower that sorts by boiling point
Crude oil is useless as it comes — a black soup of mixed chain lengths — so the refinery's first job is separation. The oil is heated until most of it vaporises and fed into the base of a fractionating column that is deliberately kept hot at the bottom and cool at the top. Vapours rise, cool as they climb, and each one condenses at the height where the column temperature falls to its boiling point. Long-chain molecules, with high boiling points, condense almost immediately near the base; short-chain molecules climb far higher before turning liquid; the very shortest never condense at all and leave the top as gases.
The result is a set of fractions — mixtures of hydrocarbons with similar chain lengths and boiling ranges, drawn off at different heights. Top to bottom: refinery gases, petrol, kerosene for jet engines, diesel, heavy fuel oil for ships and power stations, and bitumen for road surfaces at the very base. Notice this is a physical process: no bonds inside any molecule are broken, which is exactly what separates distillation from cracking. Most fractions are burned as fuels, but a slice becomes feedstock for the petrochemical industry — the starting point for solvents, lubricants, detergents and polymers. Look around any room: the paint, the carpet, the phone case and the medicine packet all trace back up this one tower, which is why chemists say modern life runs on the barrel twice — once as energy, once as material.
ModelProperties follow the chain — and combustion cashes them in
Three properties slide smoothly along the homologous series, and all three follow from molecule size. As chains get longer, the boiling point rises — bigger molecules attract each other more strongly, so more energy is needed to pull them apart. Methane boils at −162 °C; octane at about 126 °C; the C20-and-up molecules in candle wax are solid at room temperature. Viscosity rises with chain length too — long chains tangle, so heavy fractions pour like treacle while petrol splashes like water. Flammability runs the other way: short-chain hydrocarbons vaporise easily and ignite readily, which is precisely why the short fractions are the prized fuels.
Burning is where the value is released. In complete combustion, a hydrocarbon reacts with plentiful oxygen and both elements are fully oxidised: every carbon atom leaves as carbon dioxide, every hydrogen atom as water, and energy is transferred to the surroundings. Fuel + oxygen → carbon dioxide + water is the sentence; the skill examiners actually test is balancing it.
Balance the complete combustion of butane. Skeleton: \(\mathrm{C_4H_{10}} + \mathrm{O_2} \rightarrow \mathrm{CO_2} + \mathrm{H_2O}\). Work in strict order. Carbon first: 4 carbons → \(4\mathrm{CO_2}\). Hydrogen second: 10 hydrogens → \(5\mathrm{H_2O}\). Oxygen last, because it arrives alone: the right side holds 8 + 5 = 13 oxygen atoms, which is 6.5 O₂ — so double everything to clear the half: \[2\mathrm{C_4H_{10}} + 13\mathrm{O_2} \rightarrow 8\mathrm{CO_2} + 10\mathrm{H_2O}\] C, then H, then O, then double if you hit a half — that four-step habit is the entire mark.
MechanismCracking — where supply is forced to meet demand
Distillation gives you the barrel as nature made it, and nature's proportions are wrong: too much heavy fuel oil that nobody wants, not enough petrol that everybody does. The fix is cracking — a thermal decomposition reaction that breaks long-chain alkanes into shorter, more useful molecules. In catalytic cracking, heavy fraction vapours pass over a hot powdered catalyst at around 500–550 °C; in steam cracking, the vapours are mixed with steam and heated to a higher temperature still. Either way the chain snaps.
The products always come in a characteristic pair: a shorter alkane (sellable as fuel) plus one or more alkenes — hydrocarbons carrying a carbon–carbon double bond, general formula \(\mathrm{C_nH_{2n}}\). That double bond makes alkenes far more reactive than alkanes, which is exactly why industry wants them: they are the starting materials for polymers and a long list of other chemicals. The reactivity also gives you the lab test. Shake an unknown liquid with bromine water: an alkene turns it from orange to colourless; an alkane leaves it orange. Write both colours, every time.
Tetradecane, \(\mathrm{C_{14}H_{30}}\), is cracked into hexane plus two identical alkene molecules. Find the alkene. Atom bookkeeping: carbons remaining = 14 − 6 = 8, hydrogens remaining = 30 − 14 = 16, shared between two identical molecules → each is \(\mathrm{C_4H_8}\), butene. So \(\mathrm{C_{14}H_{30}} \rightarrow \mathrm{C_6H_{14}} + 2\mathrm{C_4H_8}\). Check it balances: C: 6 + 8 = 14 ✓; H: 14 + 16 = 30 ✓. And the follow-up the paper loves: which product decolourises bromine water? \(\mathrm{C_4H_8}\) fits \(\mathrm{C_nH_{2n}}\) — the alkene — while hexane fits \(\mathrm{C_nH_{2n+2}}\) and does nothing.
CaseFinite barrels — the evaluation question hiding in C7
Every AO3 question in this section grows from one tension: crude oil took millions of years to form and is being drained in centuries, yet it is simultaneously our dominant fuel and the feedstock for materials we have no easy substitute for. Burn a fraction and it is gone; turn it into a polymer or a medicine and it keeps working. Data questions hand you a table — typical crude yields perhaps 3–10% refinery gas, 15–25% petrol-range molecules, with heavy fractions making up much of the rest — against a demand row that wants far more of the light end. The three-sentence answer that scores: name the mismatch with numbers from the table, state that cracking converts the surplus heavy fractions into petrol-range alkanes plus alkenes, and add the evaluation turn — cracking manages the mismatch but cannot change the fact that the barrel itself is finite.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
C7 sits on Chemistry Paper 2 (8464/C/2F or 2H) and is the most AO1-friendly chemistry section in the qualification — definitions, named alkanes, fraction order and the two tests are pure recall. Bank them, but respect the precision: a hydrocarbon contains hydrogen and carbon ONLY, and dropping 'only' drops the mark.
Trend questions are three-link chains, and the mark scheme pays per link: chain length → strength of attraction between molecules (or ease of vaporising) → the property in the question. Jumping straight from 'longer chain' to 'higher boiling point' with no middle step is the standard lost mark. For combustion equations, balance C, then H, then O last, and double everything if oxygen lands on a half — show the doubled equation, not your fractional draft. Cracking equations are pure atom bookkeeping: subtract carbons, subtract hydrogens, divide among identical products, then verify both counts in the margin.
Observation marks demand both ends of the change: bromine water goes 'orange to colourless', never 'clear', never just 'changes colour'. And when the AO3 supply-and-demand data question appears, quote actual numbers from the table before naming cracking as the fix — application marks in this section are awarded for used data, not gestured-at data.