1.9.2Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Alkanes as Fuels — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 32 of 36 Unit quiz
Section 1 of 5Fractional Distillation of Crude Oil
Fractional distillation separates crude oil into fractions based on boiling point ranges.
Crude oil
- A mixture of many hydrocarbons, mainly alkanes, formed from ancient marine organisms over millions of years.
The column
- Oil is heated and vaporised, then fed into a column that is hot at the bottom and cooler at the top.
Separation
- Vapours rise and condense at the level where the temperature is just below their boiling point.
Fractions
- Refinery gas and petrol come off near the top, then naphtha, kerosene and diesel, with fuel oil and bitumen at the bottom.
Trend
- Fractions with larger molecules have higher boiling points, are more viscous and are less flammable.
Section 2 of 5Cracking
Cracking
Purpose
- Demand for short chains such as petrol is greater than the supply from distillation, so long chains are broken down.
Thermal cracking
- High temperature and pressure break long alkanes into shorter alkanes and alkenes.
Catalytic cracking
- A zeolite catalyst at about 450 °C and slight pressure gives branched alkanes and aromatic compounds, which make better petrol.
Example equation
- C₁₀H₂₂ → C₈H₁₈ + C₂H₄ shows decane splitting into octane and ethene.
Alkene products
- The ethene and other alkenes produced are valuable raw materials for making polymers.
Section 3 of 5Improving Fuel Quality
1
Straight-chain alkanes burn unevenly in engines and cause knocking.
2
Reforming converts straight-chain alkanes into cyclic and aromatic compounds using a platinum catalyst.
3
Isomerisation converts straight chains into branched isomers, which burn more smoothly.
4
The octane number measures resistance to knocking, and higher values mean better performance.
5
Refiners blend fractions so that the final petrol has the required octane rating.
Section 4 of 5Issues with Fossil Fuels
Issues with Fossil Fuels
Finite
- Crude oil takes millions of years to form, so it is a non-renewable resource that will run out.
Climate
- Combustion releases CO₂, a greenhouse gas that contributes to global warming.
Pollution
- Impurities and incomplete burning release sulfur dioxide, carbon monoxide, nitrogen oxides and particulates.
Transition
- These problems have encouraged research into alternative fuels.
Section 5 of 5Exam Checklist & Tips
1.9.2 – Key Points for the Exam
- Fractional distillation Separation of a liquid mixture into fractions by boiling point.
- Fraction A mixture of hydrocarbons with similar boiling points separated from crude oil.
- Cracking Breaking long-chain hydrocarbons into shorter, more useful molecules.
- Reforming Converting straight-chain alkanes into cyclic or aromatic hydrocarbons to improve fuel quality.
- Octane number A measure of how well a fuel resists knocking in an engine.
Common Mistakes
- Saying the column is hottest at the top; it is hottest at the bottom.
- Stating that cracking makes longer molecules.
- Forgetting that thermal cracking also produces alkenes, not only alkanes.
- Describing a fraction as a pure compound.
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