GCSE Combined Science Crude oil, fuels & alkenes revision

Crude oil formed over millions of years as plankton and other tiny marine organisms died, sank into mud on sea beds, and were buried and gently cooked without air. Because the recipe needs geological time, oil is a finite resource — we burn it far faster than the planet refills it. Every drop is a mixture of hydrocarbons: compounds built from hydrogen and carbon atoms only, differing in chain length.

Fractional distillation sorts the mixture. Crude oil is heated so most of it evaporates, then fed into a fractionating column that is hot at the bottom and cooler towards the top. Vapours rise until they reach a temperature at which they condense; each condensation level drains off as a fraction. Short chains have low boiling points, so they rise high before condensing (gases and petrol near the top); long chains condense low down (heavy oils, bitumen at the base).

The alkanes are the saturated hydrocarbons: every carbon–carbon link is a single bond (“saturated” = holding as many hydrogen atoms as possible). They obey CnH2n+2. Give a chain six carbons and the formula writes itself: 2 × 6 + 2 = 14 hydrogens, so C₆H₁₄ (hexane). The first four to memorise: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.

Burn a hydrocarbon in plenty of oxygen and you get complete combustion: carbon dioxide + water. Propane, the camping-stove gas: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O — count every atom: 3 carbons, 8 hydrogens, 10 oxygens on each side. With a limited oxygen supply, combustion goes incomplete: toxic carbon monoxide (CO) forms instead of carbon dioxide, and soot (carbon particles) may appear. Carbon monoxide is silent and deadly because it binds to haemoglobin in blood instead of oxygen, starving the body of oxygen — which is why faulty boilers and blocked flues kill.

Distillation gives plenty of long-chain fractions but far too few short ones — yet it is petrol, not bitumen, that the world queues for. Cracking fixes the imbalance: long alkane molecules are split into shorter alkane and alkene molecules using heat and a catalyst. It also produces alkenes, which are far more reactive than alkanes and valuable as chemical feedstock.

Balancing a cracking equation is atom bookkeeping. Take tetradecane: C₁₄H₃₀ → C₈H₁₈ + 3C₂H₄. Carbons: 8 + 3×2 = 14 ✓. Hydrogens: 18 + 3×4 = 30 ✓. The unknown product’s formula comes from whatever atoms are left over — and if it contains a C=C double bond, it is an alkene.

Alkenes are unsaturated — their carbon-to-carbon double bond could hold more hydrogen atoms — and they follow CnH2n: ethene C₂H₄, propene C₃H₆. The test: shake with orange bromine water. An alkene’s double bond adds bromine atoms across itself and the liquid goes colourless; a saturated alkane leaves it orange. One test tube, instant family diagnosis.

Burning fossil fuels releases carbon dioxide, the main greenhouse gas driving global warming; carbon monoxide, the toxic gas from incomplete combustion; soot, which blackens buildings and worsens breathing problems; and, from sulfur impurities and hot nitrogen in engines, sulfur dioxide and oxides of nitrogen, which dissolve in rain to make acid rain that damages forests, lakes and stonework.

Alternatives exist. Biofuels such as biodiesel are made from plant material, so they are renewable — new crops regrow each season, and the carbon dioxide they release on burning was recently taken in by the plants. They still need land, fertiliser and processing, and they still emit CO₂ when burned. Hydrogen is the cleanest burner of all: 2H₂ + O₂ → 2H₂O — only water leaves the exhaust, with no carbon dioxide at all. The catch is making, storing and distributing the hydrogen.