6.2.2Global Challenges
Cracking — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 12
Part 1 of 4Alkanes and alkenes
Saturated alkanes
- Saturated alkanes Alkanes contain only single carbon–carbon bonds, so each carbon holds as many hydrogen atoms as it can.
- General formulaCnH2n+2, for example hexane C6H14.
Unsaturated alkenes
- Unsaturated alkenes Alkenes contain a C=C double bond, so they hold fewer hydrogen atoms than the alkane with the same number of carbons.
- General formulaCnH2n, for example ethene C2H4.
Why the difference matters
- Why the difference matters Short-chain alkanes are runny and burn easily, so they make good fuels.
- Alkenes are far more reactive than alkanes and are the starting point for making polymers and alcohols.
Part 2 of 4Why cracking is needed
1Crude oil is fractionated
- Crude oil is fractionated Fractional distillation separates crude oil into fractions, but the proportions are fixed by the oil's composition.
2Supply does not match demand
- Supply does not match demand There is more of the long-chain, heavy fractions (fuel oil, bitumen) than customers want to buy.
- There is less of the small, light fractions (petrol, kerosene) than the market demands.
3Cracking converts surplus
- Cracking converts surplus Large molecules are broken into smaller, more useful ones, so the surplus is turned into high-demand products.
4Extra useful products
- Extra useful products Alkenes and sometimes hydrogen are made as well, supplying the chemical industry.
Part 3 of 4Two methods of cracking
| Feature | Catalytic cracking | Steam cracking |
|---|---|---|
| Conditions | Hydrocarbon vapour at roughly 450–700 °C passed over a hot catalyst | Hydrocarbon vapour mixed with steam and heated to a very high temperature |
| Catalyst | Powdered aluminium oxide (alumina) or silica | No solid catalyst; the steam dilutes the hydrocarbon |
| Type of reaction | Thermal decomposition: covalent bonds break on the catalyst surface | Thermal decomposition at a higher temperature |
| Typical products | A mixture of shorter alkanes and alkenes | Mainly alkenes (and hydrogen) |
Part 4 of 4Writing cracking equations
Rule
- Rule Count atoms on both sides: the total carbon and hydrogen must be equal, and the products must be an alkane plus one or more alkenes.
Example 1
- Example 1 Decane to octane and ethene: C10H22 → C8H18 + C2H4.
Example 2
- Example 2 Hexane to butane and ethene: C6H14 → C4H10 + C2H4.
Energy
- Energy Cracking is endothermic: energy is needed to break the strong covalent bonds.
Key terms
| Term | Meaning |
|---|---|
| Cracking | The thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules. |
| Saturated | A hydrocarbon with only single carbon–carbon bonds. |
| Unsaturated | A hydrocarbon containing at least one C=C double bond. |
| Catalyst | A substance that speeds up a reaction but is not used up. |
| Fraction | A mixture of hydrocarbons with similar boiling points separated from crude oil. |
| Thermal decomposition | A compound breaking down into simpler substances when heated. |
Avoid these mistakes
1
Saying cracking removes carbon atomsthe atoms are only rearranged, so the equation must balance for both C and H.
2
Writing a cracking equation with two alkanes as products: one product must be an alkene (CnH2n).
3
Calling cracking exothermicit requires a continuous energy supply, so it is endothermic.
4
Describing bromine water as turning 'clear' for an alkene: it decolourises from orange to colourless, and a plain alkane leaves it orange.
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