Hydrocarbons

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Hydrocarbons

A hydrocarbon is any compound a compound formed from only carbon and hydrogen atoms.
Alkanes are simple hydrocarbons, with the general fromula CnH2n+2. The alkanes are a homologous series - a group of organic compounds that react in a similar way. Alkanes are saturated compounds , each carbon atom forms four single covalent bonds. The first four are methane, ethane, propane and butane.
As the lenth of the carbon chain changes so does the properties:
The shorter the hydrocarbon, the less viscous it is.
Hydrocarbons with shorter carbon chains are also more volitile (lower boiling point).
The shirter the chain, the more flammable it is.
These properties affect what fuels they are used for, e.g short chains can be used as 'bottled gasses'.
Compleate combustion releases lots of energy, along with carbon dioxide and water:
Hydrocarbon + Oxygen -> Carbon Dioxide + Water (+ energy)
Both carbon and hydrogen are oxidised.
Hydrocarbons are used as fuels due to how much energy they release when they combust completely.

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Fractional Distillation

Crude oil is a fossil fuel made mostly from plankton but also the remains of other plants and animal. Over millions of years, with high temperature and pressure, the reamains turn to crude oil, which can be drilled up from the rocks where its formed. Crude oil finite and non-renewable.
Crude oil is made of different hydrocarbkns, mostly alkanes, which can be seperated using fractional distillation:
The oil is heated until most of it has turned into a gas. The gasses ebter a ftractionating coloumb where the liquid is drained off.
In the column, theres a temperature gradient (getting colder a you go up). The longer hydrocarbkns have high boiling points, therfore they condense back into liquids and drain out of the coloumn early on.
You end up with the crude oil seperated out into different fractions, each one containing a mixture of hydrocarbkns with a similar number of carbon atoms.
Approximate number of carbonsin the hydrocarbons in that fraction:
LPG (liquified petroleum gas) - 3
Petrol- 8
Kerosene - 15
Diesel - 20
Heavy fuel oil - 40

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Uses of crude oil

Crude oil provides the fuel for most modern transport, for example diesel oil, kerosene, heavy fuel oil and LPG.
The petrochemical industry uses some hydrocarbons as feedstock to make new compounds for use in things such as polymers, solvents, lubricants and detergents.
All the products you get from crude oil are examples of organic compounds (compounds containing carbon atoms). The variety is caused as carbon atoms can bond to form different groups, called the homologous series. These groups containsimilar compounds with similar properties, for example alkanes and alkenes.

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Cracking of Crude Oil

Short-chain hydrocarbons are flammable so make good fuels and are in high demand. However, long chains of hydrocarbons form thick, gloopy liquids like tar so are more useful when they are broken down, this is done using cracking. As well as alkenes like fuel, cracking produces alkenes. Alkenes are a lot more reactive than alkanes, therefore they're used as a starting material when making lots of other compounds and can be used as polymers.
Bromine water can be used to test for alkenes:
When orange bromine water is added to an alkane, no reaction will happen and it will stay bright orange. However, if it's added to an alkene the bromine reacts to from a colourless compound.
Cracking is a thermal decomposition reaction - breaking down molecules by heating them. The first step is to vaporise the hydrocarbons to vaporise them. The vapour is then passed over a hot powdered aluminium oxide catalyst. The long chain molecules split apart on the surface of the specks of catalyst, this is catalytic reaction. You can also crack hydrocarbons if you vaporise them, mix them with steam and then heat them to a very high temperature, this is called steam cracking.

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