Fossil fuels and petroleum fractions
Coal (mainly carbon), natural gas (mainly methane) and petroleum (a hydrocarbon mixture) are fossil fuels formed from ancient organisms over millions of years; they are non-renewable. Petroleum is separated by fractional distillation into fractions with specific uses: refinery gas → camping gas; gasoline → cars; kerosene → jet fuel; diesel → lorries; fuel oil → ships; bitumen → roads. Longer chains down the list raise boiling point and viscosity but lower flammability.
Complete vs incomplete combustion
Complete combustion (excess oxygen) oxidises a hydrocarbon fully to carbon dioxide and water ONLY, with a clean blue flame — e.g. CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete combustion (limited oxygen) gives carbon monoxide and/or carbon (soot) as well as water, with a yellow, smoky flame. Water is produced in BOTH cases. CO is a toxic gas that binds to haemoglobin; soot is a particulate pollutant. Oxygen availability decides which products form.
(Extended) Fractional distillation of petroleum
Fractional distillation separates petroleum because its hydrocarbons have different boiling points. The oil is heated and vaporises; vapours rise up a column that is hot at the bottom and cooler at the top. Each hydrocarbon condenses where the temperature matches its boiling point and is drawn off as a liquid. Short chains collect near the top, long chains near the bottom (bitumen as residue). It is a PHYSICAL separation — molecules are unchanged.
Drawn from real examiner reports.
Vague fraction uses ("just fuel")
Each fraction needs its SPECIFIC named use; a vague "fuel" or "fuel for cars" does not score. Refinery gas is bottled/camping gas (LPG); kerosene is jet/aviation fuel; diesel powers lorries and buses; fuel oil is for ships and power stations; bitumen is for road surfacing and roofing, not a fuel at all. "Fuel for cars" is credited only for gasoline (petrol).
W22 Paper 11 Q35; W22 Paper 12 Q35; S22 Paper 31 Q6(c) — named fraction uses poorly recalled across all three sittings; vague answers flagged as insufficient.
Combustion products swapped
Complete combustion always gives carbon dioxide and water and ONLY these; CO and soot never come from it. Incomplete combustion (limited oxygen) gives carbon monoxide and/or carbon (soot) — but water is STILL produced. Two frequent errors: writing CO₂ as an incomplete-combustion product, and saying incomplete combustion makes no water.
Cracking ≠ fractional distillation
Cracking is a CHEMICAL process — long-chain hydrocarbons are broken into shorter-chain alkanes and alkenes by heating with a catalyst. Fractional distillation is a PHYSICAL separation that leaves molecules unchanged; calling cracking "separating petroleum" scores zero. Conditions: high temperature AND a catalyst — "high pressure" is a Haber condition, not cracking.
Cracking confused with distillation: W22 Paper 31 Q1(b)(i); W22 Paper 32 Q6(d)(i); S22 Paper 31 Q6(d)(i). Conditions ("high pressure" instead of "catalyst"): W22 Paper 11 Q38; S22 Paper 31 Q6(d)(ii).
Refinery gas vs natural gas
These are different. Natural gas is mainly methane, extracted directly from underground, and is one of the three fossil fuels. Refinery gas is the lightest FRACTION from the distillation of petroleum (short-chain alkanes like propane and butane, used as LPG/bottled gas). Both are short-chain alkane gases, but one is a raw fossil fuel and the other a petroleum fraction.
Chain-length trend reversed
As the hydrocarbon chain gets LONGER (down the fractions from refinery gas to bitumen), boiling point and viscosity INCREASE and flammability DECREASES, so the fraction is less useful as a fuel. Candidates often invert this, claiming long chains are more flammable or boil lower. Anchor it: refinery gas (short, volatile) versus bitumen (long, viscous, hard to ignite).
Balancing combustion equations
Write the skeleton with CO₂ and H₂O as the only products. Balance carbon first (the CO₂ coefficient), then hydrogen (H₂O), and oxygen LAST. If oxygen comes out fractional, multiply through — e.g. 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O. Recount every atom at the end.
Naming a fraction's specific use
For "state the use of [fraction]", give the exact named use, never a generic "fuel". Refinery gas → camping gas (LPG); gasoline → cars; kerosene → jet fuel; diesel → lorries; fuel oil → ships; bitumen → road surfacing. "Fuel for cars" scores only for gasoline.
Describing incomplete combustion
For the products of incomplete combustion, give carbon monoxide (CO) and/or carbon (soot) PLUS water — water is always produced. Then state the condition: it happens when oxygen is limited. A yellow, smoky flame signals incomplete combustion in a question.
(Extended) Explaining fractional distillation
Give the reasoning chain: petroleum is heated and vaporises; vapours rise up a column that is hot at the bottom, cooler at the top; each fraction condenses where the temperature equals its boiling point and is drawn off. Short chains condense high up, long chains low down.
Fossil fuel — a fuel formed over millions of years from the compressed and heated remains of ancient organisms. Coal, natural gas and petroleum are the three fossil fuels in the 0620 syllabus. They are non-renewable resources — they cannot be replaced at the rate at which they are used.
Petroleum (crude oil) — a mixture of many hydrocarbons (mainly alkanes) of different chain lengths. It is separated into useful fractions by fractional distillation.
Fraction — a mixture of hydrocarbons with similar boiling points, collected together during fractional distillation.
Hydrocarbon — a compound containing carbon and hydrogen only.
Complete combustion — combustion in excess oxygen, producing carbon dioxide () and water () only.
Incomplete combustion — combustion when oxygen supply is limited, producing carbon monoxide () and/or carbon (soot, ) in addition to water.
Cracking — a chemical process in which long-chain hydrocarbon molecules are broken down into shorter-chain molecules by heating with a catalyst (or at very high temperature alone). Products include shorter-chain alkanes and alkenes.
Name the THREE fossil fuels that students must know for Cambridge 0620 and state the main composition of each.
Natural gas is mainly methane (). When methane burns completely in excess oxygen it releases energy.
(a) Write a balanced symbol equation for the complete combustion of methane. Include state symbols.
(b) State ONE observation that indicates complete rather than incomplete combustion is occurring.
