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Saturated Hydrocarbons and Fractional Distillation

All carbon atoms can only form upto 4 bonds with other atoms. All of the carbon atoms in any alkane has 4 single bonds, therefore the akanes are saturated. Alkanes have a general formula of...

Cn H2n+2

Cycloalkanes are also saturated hydrocarbons, but due their ring-like structure the formula is slightly different..

Cn H2n

Crude oil is made up of many alkanes joined together, fractional distillation is used to seperate this long chain into smaller, more useful alkanes. This is done by adding heated crude oil into a fractionating column. Different alkanes have different meltinging points. Going up the column the temperature decreaces and as the vapors cool they are drained off. Alkanes with the highest melting points are drained off at the bottom, and ones with the lowest at the top.

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Products if fractional distillation arent always in high demand, so to reduce waste and cost the larger hydrocarbons are 'cracked' into smaller ones. This is done by seperating the C-C bonds in the chains. 

An example of this is the cracking of Decane to give Ethane and Octane.

  • C10 H22 --> C2 H6 + C8 H18

There are 2 types of cracking, Thermal cracing and Catalytic cracking.

Thermal cracking involves lots of high temperature and pressures to produce lots of useful unsaturated hydrocarbons (like poly ethene), known as alkenes.

Catalytic cracking involves the use of a zeolite catalyst, this keeps the temperatures and pressures down. The use of this catalyst saves money and time because the activation energy for the reaction is lower than in thermal cracking and it is speeded up. Catalytic cracking produces fuels for cars and aromatic hydrocarbons.

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Probems with combustion

Alkanes are oxidised to form the products carbon dioxide and water. When all of the reactant are used up to make these products it is known as a compleate combustion. Comustion reactions of alkanes can provide lots of useful energy from a small amoun of suel, this is why we need them.

Sometimes when alkanes are oxidised there isnt enough oxygen for the reaction to combust completely. This causes the products to be changed to water and CARBON MONOXIDE. Carbon monoxide is a poisonous gas but due to its structure if it is inhaled by humans it can bind with their heamaglobin, preventing oxygen being carried around the body.

Fuels in car engines are burned, but when the temperature and pressures are very high the oxygen in the air and the nitrogen in the engine react and cause oxides of nitrigen which contributes to smog.

When fuels are burned the sulfur of the fossil fuel creates sulfur dioxide, this causes acidic rain. Sulfur dioxide can be removed with the use of calcium oxide.

Green house gasses are what the earth uses to keep the planet warm enough for us to inhabit. CO2 is a green house gas. with the amount of CO2 in the atmosphere increacing, the earth is getting warmer. This is the green house effect.

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