Crude Oil & Fractional Distillation

Crude oil is a dark, sticky liquid found underground. It is not a single substance — it is a mixture of mainly hydrocarbons.

A hydrocarbon is a compound made of only hydrogen and carbon atoms. Most of the hydrocarbons in crude oil are alkanes (chains of carbon atoms surrounded by hydrogen).

How it formed. Crude oil formed over millions of years from the remains of tiny marine (sea) organisms — mostly plankton and algae — that died and sank to the sea bed. They were buried under layers of mud and rock, and high pressure and temperature slowly turned them into a mixture of hydrocarbons.

Finite and non-renewable. Because crude oil takes millions of years to form but we use it up very quickly:

• Finite = there is only a limited amount — it will run out.
• Non-renewable = it is being used much faster than it forms, so it cannot be replaced on a useful timescale.

Contrast this with renewable resources (such as wind or solar energy), which are replaced as fast as they are used.

Why is crude oil so useful? The different-sized hydrocarbons in crude oil make excellent fuels (petrol, diesel, jet fuel) and are the starting materials for plastics, medicines and many everyday chemicals. But the hydrocarbons are all jumbled together, so the first job at an oil refinery is to separate them — and that is done by fractional distillation.

Fractional distillation separates crude oil into fractions. A fraction is a group of hydrocarbons with similar boiling points (and similar chain lengths).

It works because different-sized hydrocarbons boil at different temperatures:

• Short chains have low boiling points (they turn to gas easily).
• Long chains have high boiling points (they stay liquid).

The fractionating column. A tall tower that is hot at the bottom and cool at the top — this difference is called a temperature gradient.

How it works — step by step:

1. Crude oil is heated until most of it turns into a vapour (gas).
2. The hot vapours enter the column near the bottom and rise up.
3. As the vapours rise, the column gets cooler.
4. Each fraction condenses (turns back into a liquid) when it reaches a part of the column that is at its boiling point.
5. Short-chain hydrocarbons (low boiling point) rise high up the column before condensing — collected near the top.
6. Long-chain hydrocarbons (high boiling point) condense low down — collected near the bottom.
7. The condensed liquids are piped off at different heights as separate fractions.

The fractions and their uses (top → bottom):

Fraction	Chain length	Main uses
Refinery gases	shortest	Bottled gas (cooking, heating)
Gasoline (petrol)	short	Fuel for cars
Kerosene (paraffin)	medium	Jet fuel; heating
Diesel oil	longer	Fuel for lorries, buses, trains
Fuel oil	long	Fuel for ships and power stations
Bitumen	longest	Surfacing roads and roofs

A handy way to remember the order is the first letters R-G-K-D-F-B (Refinery gases, Gasoline, Kerosene, Diesel, Fuel oil, Bitumen). Try to learn at least three fractions with their uses.

As you go down the fractionating column, the hydrocarbons have longer chains (more carbon atoms). This changes their physical properties in a predictable way. These trends are favourite exam questions.

Going DOWN the column (longer chains):

• Boiling point INCREASES. Longer molecules have stronger forces of attraction between molecules, so more energy is needed to separate them.
• Viscosity INCREASES — the liquid becomes thicker and harder to pour. Petrol is runny; bitumen is almost solid.
• Flammability DECREASES — longer molecules do not turn to vapour as easily, so they are harder to ignite / catch fire less easily.
• Colour gets DARKER — fractions near the top are pale/colourless; near the bottom they are dark brown/black.

Going UP the column the trends simply reverse: shorter chains, lower boiling point, runnier (less viscous), more flammable, paler.

Why does boiling point increase? Hydrocarbon molecules are held to each other by weak forces of attraction. Longer chains have stronger forces between molecules, so it takes more heat energy to pull them apart into a gas — meaning a higher boiling point.

Exam tip: the words examiners want are viscosity (thickness), flammability (how easily it catches fire) and boiling point — and the direction of each trend. Learn them as a set: down the column → higher boiling point, more viscous, less flammable.

Crude oil produces more of the long-chain fractions than we actually need, but there is huge demand for short-chain fuels like petrol. The solution is cracking.

Cracking breaks long-chain alkanes into shorter alkanes and alkenes.

• A long-chain alkane (less useful) is broken down.
• The products are a shorter alkane (e.g. for petrol) + an alkene (a reactive molecule with a C=C double bond, used to make plastics).

Example. Decane can be cracked into octane (used in petrol) and ethene (used to make poly(ethene)):

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Check it balances: carbon 10 = 8 + 2 ✓ and hydrogen 22 = 18 + 4 ✓. In cracking equations, the number of C atoms and H atoms must be the same on both sides.

Why crack? Two reasons:

1. To turn surplus long-chain hydrocarbons into short-chain fuels that are in high demand (e.g. petrol).
2. To make alkenes, which are used to make plastics (polymers).

Cracking vs fractional distillation — do not confuse them:

• Fractional distillation separates the hydrocarbons already in crude oil (no new substances made).
• Cracking breaks molecules apart to make new, smaller molecules.