Fractional Distillation
A concise revision guide to how crude oil is separated into useful petroleum fractions by fractional distillation, including the role of boiling point, molecular size and id-id forces.
Fractionating Column in Action
Choose a step to follow crude oil from the furnace up the column, and see where molecules of different chain lengths condense and leave.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Separating Liquids by Boiling Point
Start with the GCSE ideas about distillation and boiling points.
Alkanes, Crude Oil and Petroleum Fractions
Alkanes are saturated hydrocarbons. They contain only carbon and hydrogen atoms, and they contain only single carbon-carbon bonds.
The general formula is CnH2n+2, where n is the number of carbon atoms in the molecule.
It fits branched alkanes just as well as straight-chain ones. The only alkanes it does not fit are cycloalkanes, whose carbon chain is joined into a ring.
Alkanes are mainly obtained from crude oil. Crude oil is a complex mixture made up mainly of alkane hydrocarbons.
Petroleum fraction: a collection of hydrocarbons with a similar chain length and therefore a similar range of boiling points.
Key idea: Fractional distillation does not separate crude oil into single pure compounds. It separates crude oil into groups of hydrocarbons with similar boiling point ranges.
Quick Check: Spot the Alkanes
Click every molecular formula that belongs to an alkane.
How Fractional Distillation Separates Crude Oil
Before separation takes place, crude oil is pre-heated so that most of it vaporises. The hot vapours are then fed into a tall fractionating column.
The part that does not vaporise runs to the base of the column and is drawn off there as the residue. That residue is the bitumen fraction, which is why bitumen appears at the bottom of the diagram.
Inside the column, the temperature decreases as you move upwards. This produces a temperature gradient.
As the vapours rise through the column, different hydrocarbons condense at different heights, depending on their boiling points.
The column is fitted with trays, and the liquid that condenses collects on the tray at the level where the temperature has dropped to its boiling point.
Smaller, lower boiling point hydrocarbons leave nearer the top of the column, while larger, higher boiling point hydrocarbons condense lower down.
Important: Fractional distillation is a physical separation process. It does not break covalent bonds and it does not chemically change the hydrocarbons.
Quick Check: Through the Furnace and Up the Column
Put the stages of fractional distillation in the right order.
Watch: an exam question on the fractionating column
Why do long-chain hydrocarbons condense at the bottom of the column? Two answers are ruled out. Which is right?
Why the Fractions Separate
The separation depends on boiling point, which is linked to the size of the hydrocarbon molecules.
- Larger molecules have more electrons and a larger surface area in contact with their neighbours, so the id-id forces between them are stronger and they have higher boiling points.
- Smaller molecules have weaker id-id forces, so they boil at lower temperatures.
- Hydrocarbons with similar molecular size, mass and boiling point condense together to form a fraction.
| Hydrocarbon size | id-id forces | Boiling point | Where it condenses |
|---|---|---|---|
| Smaller hydrocarbons | Weaker id-id forces between molecules. | Lower boiling points. | Nearer the top of the column, where temperatures are lower. |
| Larger hydrocarbons | Stronger id-id forces between molecules. | Higher boiling points. | Lower down in the column, where temperatures are higher. |
Exam focus: Link increasing chain length to stronger id-id forces, higher boiling point and condensation lower in the fractionating column.
Remember: the full name is instantaneous dipole-induced dipole (id-id) forces. They act between all molecules and grow with the number of electrons.
Quick Check: Explain Where Two Alkanes Condense
Explain why two alkanes condense at different heights in the column.
Common Petroleum Fractions
The fractions obtained from crude oil are useful because they contain hydrocarbons with similar properties and similar boiling point ranges. Many fractions are used as fuels because alkanes release large amounts of energy when burned.
| Fraction | Typical position in column | Main use |
|---|---|---|
| Fuel gas | Very near the top. | Bottled gas. |
| Petrol or gasoline | Near the top. | Fuel for cars. |
| Naphtha | Upper part of the column. | Chemicals and feedstock. |
| Kerosene | Middle part of the column. | Jet fuel. |
| Diesel oil | Lower middle part of the column. | Diesel fuel. |
| Fuel oil and lubricating oils | Lower part of the column. | Heavy fuels and lubrication. |
| Bitumen | Bottom of the column, the residue that never vaporised. | Road surfacing and roofing. |
Quick Check: Which Fraction?
Use boiling ranges to place each alkane in its fraction.
Fractional Distillation Is a Physical Process
Fractional distillation separates hydrocarbons by differences in boiling point. It is a physical process, not a chemical reaction.
During vaporisation and condensation, the process overcomes weak id-id forces between molecules. It does not break the strong covalent bonds inside the hydrocarbon molecules.
| Type of force or bond | What happens during fractional distillation? | Exam wording |
|---|---|---|
| id-id forces between molecules | These weak intermolecular forces are overcome when fractions vaporise. | Fractional distillation overcomes intermolecular forces. |
| Covalent bonds within molecules | These bonds are not broken during fractional distillation. | No chemical reaction takes place. |
Remember: If covalent bonds are broken and new molecules are made, the process is chemical. Fractional distillation does not do this.
Quick Check: Separation or Reaction?
Decide quickly whether each change is physical or chemical.
Quick Check: Fractional Distillation Summary
Pull the whole page together by choosing the accurate statement in each round.
Common Exam Mistakes
- Do not say fractional distillation separates hydrocarbons by chemical reaction. It is a physical process.
- Do not say covalent bonds are broken. Fractional distillation overcomes id-id forces between molecules.
- Do not say all hydrocarbons leave the top of the column. Larger hydrocarbons condense lower down.
- Do not confuse fractional distillation with cracking. Cracking breaks larger hydrocarbons into smaller molecules.
- Do not describe a petroleum fraction as one compound. A fraction is a mixture of hydrocarbons with similar boiling points.
Fractional Distillation FAQ
These short answers summarise the key exam points from this page.
What is fractional distillation of crude oil?
Fractional distillation is a physical separation process that separates crude oil into fractions based on different boiling point ranges.
Why do smaller hydrocarbons leave nearer the top of the column?
Smaller hydrocarbons have weaker id-id forces between molecules, so they have lower boiling points and condense nearer the top where the column is cooler.
Why do larger hydrocarbons condense lower down?
Larger hydrocarbons have stronger id-id forces between molecules, so they have higher boiling points and condense lower down where the column is hotter.
Are covalent bonds broken during fractional distillation?
No. Fractional distillation only overcomes weak intermolecular forces between molecules. It does not break covalent bonds within the hydrocarbon molecules.
Is a petroleum fraction a pure compound?
No. A fraction is a mixture of hydrocarbons with similar chain lengths and a similar range of boiling points. It is collected over a temperature range, not at one temperature.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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