Crude Oil and Fuels

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Fuels from Crude Oil

Crud oil is a mixture of many different compounds. E.g. hydrocarbons. it is a liquid when found. It is also very dark and has a strong smell.

It is formed from the remains of small animals and plants that died and fell to the bottom of the sea.

It is useful because it can be used for petrol which helps people get around.It is used for everyday substances. 

A mixture consists of two or more elements or compounds not chemically combined together.

formation of crude oil:

  • Layers of dead sea creatures settle on the sea bed.
  • Layers of sedimentary rock build up on top.
  • The heat and pressure from these rocks, along with the absence of oxygen mean that oil and gas are formed over millions of years.

A hydrocarbon contains only carbon and hydrogen. Most hydrocarbons in crude oil are alkanes.

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Alkanes contain only carbon and hydrogfen bonded together by single bonds.

Alkanes are saturated, which means they are atoms held together by single bonds. E.g. Methane = CH4. 

CnH2n + 2 - General formula for alkanes.


1           Meth-

2           Eth-

3           Prop-

4           But-

5           Pent-

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Fractional Distillation

Crude oil is seperated into fractions at refineries using fractional distillation. This can be done because the boiling point of a hydrocarbon depends on the size of its molecule. The larger the molecule, the higher the boiling point of the hydrocarbon .

The crude oil is vapourised and fed into a fractionating column. This is a tall tower that is hot at the bottom and gets cooler going up the column.

Inside the column there are many trays with holes to allow gases through. the vapors move up the column getting cooler as they go up. The hydrocarbons condense to liquids when they reach the level whcih is theoir boiling point. Different liquids lay on the differend fractions so it is easy for them to be collected.

The top of the column contains hydrocarbons with the smallest molecules and lowest boiling points and at the bottom arre hydrocarbons with the highest boiling points.

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Burning Fuels

When pure hydrocarbons burn completely they are oxidised to carbon dioxide and water. However, the fuels we use are not always burned completely. They may also contain other substances.

Equation for the complete combustion of ethane:

Ethane + Oxygen ----- Carbon Dioxide + water

In a limited supply of air incomplete combustionmay produce carbon monoxide. Carbon may also be produced and some of the hydrocarbons may not burn. This produced slid particles that contain soot and unburnt hydrocarbons called particulates.

Most fossil fuels contain sulfur compounds. When the fuel burns these sulfur compounds produce sulfur dioxide. Sulfur dioxide causes acid rain.

At the high temperatures produced when fuels burn, oxygen and nitrogen in the air may combine to form nitrogen in thr air may combine to form nitrogen oxides. Nitrogen oxides also cause acid rain.

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Alternative Fuels

Biofuels are made from plant or animal products and are renewable. Biodiesel can be made from vegetable oils extracted from plants.

There are advantages to using biodiesel. For example, it makes little contribution to carbon dioxide levels. This is because the carbon doxide given off when it burns was taken from the atmosphere by plants as they grew.

There are also disadvantages, for example the plants that are grown for biodiesel take up large amounts of land.

Ethanol made from sugar cane or sugar beet is a biofuel. It is a liquid and so can be stored and distributed like other liquid fuels. It can be mixed with petrol.

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