Hydrocarbon: compound of carbon and hydrogen only. Saturated: only single C-C bonds (alkanes). Unsaturated: has a C=C double bond (alkenes) or C≡C triple bond (alkynes). Alkenes are unsaturated because they can still add atoms across the double bond.
Cracking: breaking large alkanes into smaller, more useful alkanes and alkenes. Thermal cracking: high temperature (about 450 to 750 °C) and high pressure. Catalytic cracking: about 550 °C with a zeolite catalyst, which gives more C5 to C10 hydrocarbons. Reasons: supply of petrol demand is larger than the supply from distillation; smaller hydrocarbons are more volatile, burn well and make better fuels; alkenes are raw material for plastics and chemicals.
Elimination reaction: a small molecule (H₂O or HX) is removed from adjacent carbons, giving a double bond. Dehydration: loss of water. Dehydrohalogenation: loss of hydrogen halide.
Fossil fuels: coal, natural gas, petroleum (non-renewable, from remains of ancient organisms). Natural gas is mainly methane. Petroleum (crude oil) is a thick dark mixture of hydrocarbons; the name comes from Latin petra (rock) and oleum (oil).
Dehydration of alcohols (any of three routes): CH₃CH₂OH → CH₂=CH₂ + H₂O using conc. H₂SO₄ at 170 °C, or passing vapour over heated alumina (Al₂O₃) at 340 to 450 °C, or phosphoric acid. Propan-1-ol or propan-2-ol similarly gives propene.
Bromine or bromine water: C₂H₄ + Br₂ → CH₂Br-CH₂Br (1,2-dibromoethane). Orange-brown bromine water turns colourless. This is the test for unsaturation (alkenes and alkynes). Alkanes give no reaction (stays orange-brown).
Cold dilute alkaline KMnO₄ (Baeyer's test): purple colour is discharged and a brown MnO₂ precipitate forms. 3C₂H₄ + 2KMnO₄ + 4H₂O → 3CH₂OH-CH₂OH + 2MnO₂ + 2KOH. Ethene is oxidised to ethane-1,2-diol (ethylene glycol, antifreeze). Alkanes do not react.
Hydrogenation: C₂H₄ + H₂ → C₂H₆ with finely divided nickel at 200 to 300 °C and high pressure (Pt or Pd at room temperature). Propene gives propane.
Polymerisation of ethene gives polythene (Unit 12).
Ethyne (alkyne)
HC≡CH, triple bond. Test with bromine: C₂H₂ + 2Br₂ → CHBr₂-CHBr₂ (tetrabromoethane), so bromine water is decolourised.
Uses: welding and cutting (oxy-acetylene flame at about 3200 °C, the hottest of the fuel gases) and artificial ripening of fruit. Ripening: a packet of calcium carbide in the crate gives ethyne with moisture, CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. Health risk: harmful chemicals, breathing problems.
Distinguishing propane and propene, or hexane and hexene: add bromine water (or alkaline KMnO₄); the alkene decolourises it, the alkane does not.
Fractional distillation of petroleum
Crude oil is heated to about 400 °C in a furnace and the vapour rises up a fractionating tower that is hotter at the bottom and cooler at the top. Each fraction condenses where the temperature matches its boiling point range. Compounds with the highest boiling points condense near the bottom. The method works because the components have different boiling points.
Fractions (top to bottom) with carbon numbers and uses: LPG or refinery gas (C1 to C4, below 20 °C), cooking and heating; petrol or gasoline (C5 to C10, 35 to 70 °C), motor fuel; naphtha (C8 to C12), chemical feedstock for plastics and drugs; kerosene (C10 to C16), jet fuel and heating; diesel or gas oil (C14 to C20), diesel engines; lubricating oil (C20 to C50), lubricants, waxes and polishes; fuel oil (C50 to C70), power stations, ships and home heating; bitumen (above C70), roads and roofing.
From bottom to top: chain length decreases, boiling point decreases, viscosity decreases, volatility increases, flammability increases, and the colour gets lighter. Reason: longer chains have stronger intermolecular forces.
Past-paper questions with model answers
What are fossil fuels? Name three (B 1.5+1.5, D). Natural fuels formed from the remains of ancient organisms over millions of years, non-renewable: coal, natural gas, petroleum.
What is petroleum, why must it be separated (B 1+2): a mixture of hydrocarbons (crude oil). It must be separated because the crude mixture is not directly useful, and each fraction has its own use and boiling range.
What is fractional distillation, two fractions (B, D, M 2+1): separation of crude oil into fractions by boiling point difference; for example petrol (car fuel) and kerosene (jet fuel).
State the observation when an unsaturated hydrocarbon reacts with aqueous bromine (B): the reddish-brown colour disappears (colourless), because bromine adds across the double bond. How can a double bond be identified (D): bromine water test, or alkaline KMnO₄ turns from purple to colourless with brown MnO₂.
KMnO₄ test with balanced equation (M): Baeyer's test above.
Preparation of ethene from bromoethane and ethanol (B), of propene from bromopropane and propan-1-ol (D), dehydration of ethanol and dehydrohalogenation of chloroethane (M): use the equations above (alcoholic KOH; conc. H₂SO₄ or alumina).
Ethene prepared by steam: C₂H₄ + H₂O → C₂H₅OH, phosphoric acid catalyst (B OR). The 2025 paper asked hydrogenation of ethene (C₂H₄ + H₂ → C₂H₆ with nickel).
Sec C: four petroleum fractions with uses (B, 1x4); how properties change from bottom to top, chain length and boiling point (M).
Sec C: uses of ethyne (D): (a) artificial ripening with calcium carbide and water, CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂; (b) welding, because the oxy-acetylene flame reaches about 3200 °C.