Watch first
CognitoAlkanes and crude oil
CognitoCracking of alkanes
Key facts & methods
Alkane homologous series
- General formula: CnH(2n+2).
- Methane CH4, Ethane C2H6, Propane C3H8, Butane C4H10.
- All single C-C bonds (saturated).
- Properties change gradually: boiling point increases with chain length.
- Longer chains: higher boiling point, more viscous, harder to ignite.
Fractional distillation of crude oil
- Crude oil = mixture of hydrocarbons (mainly alkanes) with different chain lengths.
- Heated and vaporised; vapours rise up fractionating column (cooler at top).
- Short chains condense at top (low BP): petrol, LPG. Most useful as fuels.
- Long chains condense at bottom (high BP): bitumen, fuel oil. Less useful.
Combustion of alkanes
- Complete combustion (excess O2): alkane + O2 → CO2 + H2O. Clean blue flame.
- Incomplete combustion (limited O2): produces CO (toxic) and soot (C).
- Carbon monoxide: colourless, odourless, toxic — binds to haemoglobin.
- Combustion products: CO2 contributes to global warming.
Cracking
- Long-chain alkanes are less useful as fuels — supply exceeds demand.
- Cracking: thermal decomposition of long-chain alkanes into shorter molecules.
- Produces shorter alkanes (fuels) AND alkenes (for polymers).
- Catalytic cracking: Al2O3 catalyst, ~450°C.
- Steam cracking: ~850°C, no catalyst.
Exam questions — 4 questions · 10 marks · AQA 8462 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
1 markAlkane formulaAQA 8462
What is the molecular formula of propane?
A C3H6
B C3H8
C C3H4
D C4H10
2 marksCombustion equationAQA 8462
Write a balanced symbol equation for the complete combustion of methane (CH4). (2 marks)
+20 XP
1 markCracking purposeAQA 8462
What is the main purpose of cracking long-chain alkanes?
A Convert alkanes into alcohols
B Break them into shorter, more useful molecules including fuels and alkenes for polymers
C Remove sulfur from crude oil
D Increase boiling point of oil fractions
3 marksFractionating columnAQA 8462
Explain how the fractionating column separates crude oil. Why are shorter-chain hydrocarbons more useful as fuels? (3 marks)
+30 XP
Module complete! 🎉
Score loading...