Crude Oil and Fractional Distillation

Crude oil and its fractionation is a key topic in the Cambridge 0620 organic chemistry syllabus. Questions test the principle of fractional distillation, the names and uses of the fractions, trends in physical properties, and the economics of cracking.

What is crude oil?

Crude oil is a mixture of hydrocarbons — compounds containing only carbon and hydrogen. Most of these hydrocarbons are alkanes (saturated hydrocarbons with single C-C bonds).

Because crude oil is a mixture, its components can be separated by physical methods. No chemical reaction is needed.

How fractional distillation works

The fractionating column is a tall tower that is hottest at the bottom (~400 °C) and coolest at the top (~25 °C).

Process

1. Crude oil is heated in a furnace until most of it vaporises
2. The hot vapour enters at the bottom of the fractionating column
3. As vapours rise, the temperature decreases
4. Hydrocarbons with higher boiling points condense lower in the column (where it is hotter)
5. Hydrocarbons with lower boiling points rise higher before condensing
6. Each fraction is collected at a different level (called a tray)

The key principle: separation works because different hydrocarbons have different boiling points due to different chain lengths and therefore different strengths of intermolecular forces.

The main fractions

Listed from top (coolest) to bottom (hottest) of the column:

Fraction	Carbon atoms	Boiling range	Uses
Refinery gases	C₁–C₄	Below 25 °C	Fuel for heating and cooking (LPG)
Petrol (gasoline)	C₅–C₁₀	25–75 °C	Fuel for cars
Naphtha	C₅–C₁₀	75–150 °C	Feedstock for making chemicals/plastics
Kerosene	C₁₁–C₁₅	150–240 °C	Jet fuel, heating fuel
Diesel	C₁₅–C₂₀	240–350 °C	Fuel for lorries, buses, trains
Fuel oil	C₂₀–C₄₀	350–400 °C	Fuel for ships, power stations
Bitumen	C₄₀+	Above 400 °C	Road surfacing, roofing

Trends in properties

As chain length increases (going down the column):

Property	Trend	Reason
Boiling point	Increases	Longer chains have stronger intermolecular forces
Viscosity	Increases	Longer chains tangle more, flow less easily
Flammability	Decreases	Harder to vaporise, so harder to ignite
Colour	Darker	Short chains are colourless; long chains are dark brown/black

These trends are frequently tested on Paper 1 and Paper 2.

Cracking

There is greater demand for short-chain fractions (petrol, gases, alkenes) than for long-chain fractions (fuel oil, bitumen). Cracking converts surplus long-chain molecules into shorter, more useful ones.

Thermal cracking

Long-chain alkane is heated to high temperature (~700–1000 °C) under pressure with steam.

Catalytic cracking

Long-chain alkane vapour is passed over a hot catalyst (aluminium oxide or zeolite) at ~500 °C. This method uses a lower temperature than thermal cracking.

Products of cracking

Cracking always produces:

• A shorter-chain alkane (for petrol)
• An alkene (for making polymers and other chemicals)

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)

The alkene product is identifiable because cracking produces unsaturated molecules (containing C=C double bonds), whereas the original crude oil fractions are saturated (only single bonds).

Testing for alkenes (cracking product)

Add bromine water (orange/brown) to the product. If an alkene is present, the bromine water is decolourised (turns colourless). Alkanes do not decolourise bromine water.

Environmental issues

The combustion of crude oil fractions produces pollutants covered in environmental chemistry:

• CO₂ — greenhouse gas, climate change
• CO — toxic, from incomplete combustion
• SO₂ — acid rain
• NOₓ — acid rain, smog
• Soot (C) — respiratory problems

Worked exam question

Explain why petrol is collected near the top of the fractionating column and bitumen at the bottom. [3]

Petrol contains short-chain hydrocarbons with weak intermolecular forces and a low boiling point [1]. These molecules remain as vapour until near the top of the column where it is cool enough for them to condense [1]. Bitumen contains very long-chain hydrocarbons with strong intermolecular forces and a high boiling point, so it condenses near the bottom where the temperature is highest [1].

Common exam mistakes

1. Saying fractional distillation is a chemical reaction — it is a physical separation process. No bonds within molecules are broken.
2. Confusing cracking with fractional distillation — fractional distillation separates existing molecules; cracking breaks large molecules into smaller ones (a chemical reaction).
3. Forgetting that cracking produces an alkene — candidates often write two alkane products. At least one product must be an alkene.
4. Stating that crude oil contains “different elements” — crude oil is a mixture of different compounds (hydrocarbons), not different elements.