3.2.1Physical Chemistry
Fractional Distillation of Crude Oil — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 69 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Crude Oil and Its Separation
Crude oil is a mixture of hydrocarbons, mainly alkanes, that is separated rather than reacted.
A mixture
- A mixture Crude oil contains straight-chain and branched alkanes of many different lengths, so it is not very useful until separated.
Fractions
- Fractions A fraction is a group of hydrocarbons with similar chain lengths and therefore similar boiling points.
Physical process
- Physical process Separation depends on differences in boiling point and involves no chemical change.
Boiling point link
- Boiling point link Longer chains have more electrons and stronger London forces, so they boil at higher temperatures.
Part 2 of 4Inside the Fractionating Column
The column is hot at the bottom and cooler at the top.
1
Crude oil is heated in a furnace so that most of it vaporises, and the vapour enters near the bottom of the column.
2
The vapours rise up the column, where the temperature gradually falls.
3
Hydrocarbons with high boiling points condense low in the column and are drawn off there.
4
Hydrocarbons with lower boiling points rise higher before condensing, so smaller molecules are collected nearer the top.
5
The very large molecules never vaporise and leave from the bottom as residue, which includes bitumen.
Part 3 of 4Fractions and Uses
Fractions and Uses
Smaller molecules
- Smaller molecules Short chains are more volatile, flow easily, and burn cleanly, making them useful as fuels.
Larger molecules
- Larger molecules Long chains are viscous and less volatile and are used for lubricating oils, bitumen and as feedstock for cracking.
Demand mismatch
- Demand mismatch More petrol and shorter alkenes are needed than crude oil provides, which leads to cracking of larger fractions.
| Fraction | Approx. carbon atoms | Typical use |
|---|---|---|
| Refinery gases | 1 to 4 | Heating and cooking (LPG) |
| Petrol (gasoline) | 5 to 10 | Fuel for cars |
| Kerosene | 10 to 16 | Aircraft fuel |
| Diesel | 15 to 20 | Fuel for lorries and trains |
| Residue | Over 20 | Bitumen and fuel oil |
Part 4 of 4Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- Fraction A mixture of hydrocarbons with similar boiling points collected at one level in the column.
- Fractional distillation Separation of a liquid mixture into fractions by differences in boiling point.
- Volatility How easily a liquid turns into vapour.
- Hydrocarbon A compound made only of carbon and hydrogen atoms.
Common Mistakes
- Saying that crude oil is a compound; it is a mixture of many hydrocarbons.
- Stating that the column is hotter at the top, when it is hotter at the bottom.
- Explaining separation by differences in chemical reactivity rather than boiling point.
- Forgetting that boiling point rises with chain length because of stronger London forces.
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