4.1.4Guiding Spaceship Earth Towards a Sustainable Future
Using Fuels and Fractions — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 15
Section 1 of 7Combustion of hydrocarbons
Complete combustion
- Complete combustion With plenty of oxygen, carbon and hydrogen are both oxidised, giving carbon dioxide and water and releasing energy.
- ExampleCH4 + 2O2 → CO2 + 2H2O.
Incomplete combustion
- Incomplete combustion With limited oxygen, carbon monoxide or soot (carbon) forms as well as water.
- Carbon monoxide is a toxic gas that stops blood carrying oxygen.
Burning impurities
- Burning impurities Fuels containing sulfur release sulfur dioxide, which causes acid rain.
- Hot engines also make oxides of nitrogen.
Section 2 of 7Choosing a fuel
Properties that matter
Energy released
- Energy released How much heat is given out per kilogram or per litre.
Ease of ignition
- Ease of ignition Short-chain fuels ignite easily but are hazardous to store.
Pollution
- Pollution Fuels that produce less soot, carbon monoxide or sulfur dioxide are cleaner.
Cost and storage
- Cost and storage A gas must be compressed; liquids are easier to transport.
Section 3 of 7Cracking
1
Long-chain hydrocarbons from the heavier fractions are heated to vaporise them.
2
The vapour is passed over a hot catalyst (such as aluminium oxide) or mixed with steam at a high temperature.
3
The long molecules thermally decompose, breaking into shorter, more useful molecules.
4
The products are shorter alkanes (used as fuels) and alkenes (used to make polymers).
5
Example: C10H22 → C8H18 + C2H4.
Section 4 of 7Why cracking is done
1Supply and demand
- Supply and demand Demand for petrol and short-chain fuels is higher than the amount in crude oil.
2Convert surplus
- Convert surplus Cracking changes surplus long chains into shorter ones.
3Alkenes produced
- Alkenes produced These are reactive and are the feedstock for plastics.
Section 5 of 7Alkane and alkene test
| Test with bromine water | Alkane (saturated) | Alkene (unsaturated) |
|---|---|---|
| Result | Stays orange-brown | Decolourised (goes colourless) |
| Reason | No C=C double bond to react with | Bromine adds across the C=C bond |
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Complete combustion | Burning in plenty of oxygen to give carbon dioxide and water. |
| Incomplete combustion | Burning in limited oxygen, giving carbon monoxide or carbon and water. |
| Cracking | Breaking down long-chain hydrocarbons into shorter, more useful molecules. |
| Alkene | An unsaturated hydrocarbon containing a carbon-carbon double bond. |
| Thermal decomposition | Breaking down a compound using heat. |
Section 7 of 7Exam technique
1
Balancing the equation with the wrong numbers of oxygen; count oxygen on both sides after balancing C and H.
2
Saying carbon monoxide is a greenhouse gas that causes global warming; its danger is toxicity.
3
Describing cracking as a chemical change that adds hydrogen; it only splits molecules.
4
Stating that bromine water turns clear for an alkane. It stays orange-brown.
5
Calling the products of cracking all alkanes; at least one is an alkene.
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