Crude Oil & Fuels

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FreeScienceLessons · AQA ChemistryCrude Oil and Fractional Distillation

Cognito · AQA ChemistryHydrocarbons, cracking and combustion

Key facts

Hydrocarbons and crude oil

Hydrocarbon = compound containing only hydrogen and carbon.
Crude oil = mixture of hydrocarbons with different chain lengths. Must be separated to be useful.
Fractional distillation — crude oil heated, vapours rise up the column, fractions condense at different heights depending on boiling point.
Shorter chains: lower boiling point, more volatile, more flammable, less viscous, lighter colour.
Longer chains: higher boiling point, less volatile, more viscous, darker.
Fractions (top to bottom): refinery gas → petrol → naphtha → kerosene → diesel → fuel oil → bitumen.

Alkanes

Alkanes — saturated hydrocarbons, general formula CₙH₂ₙ₊₂.
Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
Complete combustion (excess oxygen): hydrocarbon + O₂ → CO₂ + H₂O. Clean blue flame.
Incomplete combustion (limited oxygen): produces CO (toxic) and/or carbon (soot). Yellow/orange flame.
Cracking — thermal decomposition of long-chain alkanes into shorter, more useful molecules. Uses high temperature + catalyst (or steam).
Produces alkenes (useful for making polymers) and shorter alkanes (fuels).

Exam questions — 4 questions · 11 marks

1 markFractional distillation principleAQA 8462 P2 style

How are different fractions separated in fractional distillation of crude oil?

A Different fractions have different colours so they are separated using chromatography

B Different fractions have different densities so they sink to different levels

C Different fractions have different boiling points — shorter chains condense higher up the column where it is cooler

D Different fractions react with different reagents added to the column

4 marksCracking equation and purposeAQA 8462 P2 style

Explain what cracking is and why it is used in the oil industry. Write a word equation and a balanced symbol equation for the cracking of decane (C₁₀H₂₂) into octane (C₈H₁₈) and ethene (C₂H₄). (4 marks)

Hint: Cracking = breaking long chains. Give the reason (supply/demand mismatch). Balance the equation: C on each side must match, H on each side must match.

+40 XP

1 markIncomplete combustionAQA 8462 P2 style

Methane burns in a limited supply of air. Which products are formed?

A Carbon dioxide and water only

B Carbon monoxide and water, and possibly carbon particles

C Nitrogen dioxide and water

D Carbon dioxide only

3 marksProperties of fractionsAQA 8462 P2 style

Petrol and bitumen are both obtained from crude oil. Compare their properties and explain why they are different, in terms of molecular structure. (3 marks)

Hint: Compare chain length → intermolecular forces → boiling point → viscosity → flammability. Short chains have weaker forces.

+30 XP

Hydrocarbons and crude oil

Alkanes

Module complete! 🎉