Crude oil, alkanes, fractional distillation and cracking
Crude oil is a mixture of many hydrocarbons formed over millions of years from ancient plankton. Fractional distillation separates it into useful fractions, and cracking turns the less useful long chains into fuels and the starting materials for plastics.
Part 1 of 3: Learn it
In short
- Hydrocarbons contain hydrogen and carbon only. Alkanes have the general formula CₙH₂ₙ₊₂.
- Fractional distillation separates crude oil by boiling point.
- Cracking breaks long alkanes into shorter alkanes and alkenes.
Where this is in your specification
Spec points: AQA 4.7.1.1 to 4.7.1.4 (Combined Trilogy 5.7.1), Edexcel Topic 8, OCR C6.2
| Board | Topic: Organic chemistry |
|---|---|
| AQA 8462 | 4.7 (8462); 5.7 (8464) |
| Edexcel 1CH0 | Topics 8 and 9 |
| OCR J248 | C6.2 |
| Eduqas C410 | 10 |
| Cambridge IGCSE 0620 | 11.1 to 11.8 |
| National 5 C813 75 | 2(a) Homologous series; 2(b) Everyday consumer products; 3(b) Plastics |
The alkanes
| Name | Formula |
|---|---|
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
Alkanes are saturated: every carbon is joined to other atoms by single bonds only. The order of the first four names: Monkeys Eat Peanut Butter.
Fractional distillation
- Crude oil is heated until most of it evaporates.
- The vapour rises up a column that is hot at the bottom and cooler at the top.
- Each fraction condenses where the temperature falls below its boiling point and is piped off: short chains (gases, petrol) near the top, long chains (fuel oil, bitumen) near the bottom.
Properties and chain length
As the carbon chain gets longer, the boiling point rises, the liquid becomes more viscous (thicker), and it becomes harder to ignite. That is why short-chain fractions make the best fuels.
Complete combustion of a hydrocarbon in plenty of oxygen makes only CO₂ and H₂O. With too little oxygen, carbon monoxide and soot can form.
Cracking
Long-chain alkanes are less in demand, so they are broken into shorter, more useful molecules. Catalytic cracking passes the hot vapour over a hot catalyst; steam cracking mixes it with steam at a very high temperature. The products include a shorter alkane and an alkene, which has a carbon to carbon double bond.
What is the formula of the alkane with 5 carbon atoms?
Show the answer
C₅H₁₂ (pentane).
Part 2 of 3: See it worked
Worked examples
Example 1
Write the balanced equation for the complete combustion of propane, C₃H₈.
- C₃H₈ + O₂ → CO₂ + H₂O
- 3 carbons make 3CO₂; 8 hydrogens make 4H₂O
- Oxygen atoms needed: 3 × 2 + 4 = 10, so 5O₂
Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Example 2
Decane, C₁₀H₂₂, is cracked to give octane, C₈H₁₈, and one other product. Find the other product.
- Carbon: 10 − 8 = 2
- Hydrogen: 22 − 18 = 4
- C₂H₄ is ethene, an alkene
Answer: C₂H₄ (ethene).
Common mistakes
- Saying fractions separate because they have different melting points. It is boiling points.
- Putting long-chain fractions at the top of the column.
- Calling alkenes saturated. The double bond makes them unsaturated.
- Writing the products of complete combustion as carbon monoxide.
How does viscosity change as the hydrocarbon chain gets longer?
Show the answer
It increases: longer chains make thicker liquids.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What is the formula of the alkane with 5 carbon atoms?
C₅H₁₂ (pentane).
How does viscosity change as the hydrocarbon chain gets longer?
It increases: longer chains make thicker liquids.
Describe the test for an alkene.
Shake with bromine water: it turns from orange to colourless.
Jobs that use this
- Chemical engineer (se abre en otra pestaña)
- Chemist (se abre en otra pestaña)
- Renewable energy engineer (se abre en otra pestaña)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
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