CRUDE OIL is a FINITE, NON-RENEWABLE resource found underground, formed over millions of years from the remains of ancient marine organisms under high pressure and temperature.
Crude oil is a MIXTURE of many different HYDROCARBONS — compounds containing only CARBON and HYDROGEN atoms.
Crude oil is both a FUEL and a FEEDSTOCK:
FUEL — burned to release energy (petrol, diesel, jet fuel, heating oil).
FEEDSTOCK — raw material for making other chemicals (plastics, medicines, dyes, detergents).
Because it is a MIXTURE, the hydrocarbons in crude oil are not chemically bonded — they can be separated by physical methods. The most important method is FRACTIONAL DISTILLATION.
Alkanes — Structure and Formulae
ALKANES are the most common type of hydrocarbon in crude oil.
Properties of alkanes:
Contain only SINGLE C-C bonds and C-H bonds — SATURATED hydrocarbons.
General formula: CₙH₂ₙ₊₂ (where n = number of carbon atoms).
Do not decolourise bromine water — no double bonds to react.
Homologous series — alkanes with increasing chain length:
Methane: CH₄ (n=1, 1 carbon)
Ethane: C₂H₆ (n=2)
Propane: C₃H₈ (n=3)
Butane: C₄H₁₀ (n=4)
Pentane: C₅H₁₂ (n=5)
STRUCTURAL FORMULAE:
Methane: H-C-H (tetrahedral, 4 H around 1 C)
Ethane: H₃C-CH₃ (two CH₃ groups joined)
Propane: H₃C-CH₂-CH₃
The C-H and C-C bonds in alkanes are all SINGLE COVALENT bonds — this makes alkanes relatively unreactive (the bonds are strong and non-polar).
Combustion of Alkanes
The main reaction of alkanes is COMBUSTION — burning in oxygen to release energy.
COMPLETE COMBUSTION (plenty of oxygen):
Alkane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O (methane)
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (propane)
Products: CO₂ and H₂O only — relatively clean burn.
INCOMPLETE COMBUSTION (limited oxygen):
Carbon monoxide (CO) and soot (C — unburned carbon particles) are produced.
CO is TOXIC — colourless, odourless, binds to haemoglobin in red blood cells → prevents oxygen transport.
Soot (particulates) causes respiratory problems and contributes to global dimming.
Why complete combustion is preferred:
More energy released per gram of fuel.
No toxic CO produced.
Less air pollution.
⚠️ Common Mistake
Alkanes are SATURATED — they have only SINGLE bonds and do NOT decolourise bromine water. ALKENES are UNSATURATED (have a C=C double bond) and DO decolourise bromine water. The bromine water test distinguishes saturated (no change) from unsaturated (decolourises).
Crude oil: mixture of hydrocarbons (C and H only). Alkanes: CₙH₂ₙ₊₂, saturated (single bonds only), relatively unreactive. Complete combustion: CO₂ + H₂O. Incomplete combustion: CO (toxic) + soot. Crude oil is both fuel and feedstock.
🎯 Matching Activity — Match the Alkane
Match each alkane to its molecular formula. — drag the symbols on the right to match the component names on the left.
Methane
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Ethane
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Propane
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Butane
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Complete combustion
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CH₄ — 1 carbon, 4 hydrogens
C₄H₁₀ — 4 carbons, 10 hydrogens
C₂H₆ — 2 carbons, 6 hydrogens
C₃H₈ — 3 carbons, 8 hydrogens
Alkane + O₂ → CO₂ + H₂O — clean burn with plenty of oxygen
🎯 Test Yourself
Question 1 of 2
1. An alkane has 5 carbon atoms. What is its molecular formula?
2. Why is incomplete combustion of fuels more dangerous than complete combustion?
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