Unit 1.4 Crude oil and fuels

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4.1 Fuels from Crude oil

Crude oil is formed over millions of years, from buried remains of plants and animals.


We can separate a mixture of liquids by distillation. Distillation of crude oil produces liquids that boil within different temperature ranges, these liquids are called fractions. 

Crude oil is a mixture of many different compounds. Most of the compounds are hydrocarbon molecules which are made up of hydrogen and carbon. 

Many of the hydrocarbons are alkanes, which have the general formula of:


Alkanes contain as many hydrogen atoms as possible in each molecule, therefore we call them saturates hydrocarbons

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4.2 Fractional Distilliation

Crude oil is separated into fractions 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.

The vaporised oil rises up the column into various fractions. The further up the column the cooler it gets.

Fractions with:

Low boiling point has a high volatility; low viscosity; highly flammability.

High boiling point has a low volatility; high viscosity; burns with a black smoke.

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4.3 Burning Fuels

When pure hydrocarbons burn completely they are oxidised to carbon dioxide and water.

In a limits supply of air incomplete combustion may produce carbon monoxide. 

 Carbon may also be produced and some hydrocarbons may not burn. This produces solid particles that contain soot (carbon) and hydrocarbons called particulates. 

Most fossil fuels contain sulphur compounds. When the fuel burns these sulphur compounds produce sulphur dioxide causing acid rain. 

At high temperatures oxygen and nitrogen in the air may combine to form nitrogen oxides, causing acid rain.

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4.4 Cleaner Fuels

Burning any fuel that contains carbon produces carbon dioxide.

Carbon dioxide is a greenhouse gas that many scientists believe id causing global warming.

Incomplete combustion of these fuels produces the poisonous gas carbon monoxide.

It can also produce particulates that reflect the sunlight, causing global dimming.

We can remove harmful substances from waste before they are released into the atmosphere

Exhaust systems of cars are fitted with catalytic converters to remove carbon monoxide and nitrogen oxides.

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4.5 Alternative Fuels

Biofuels are made from plant or animal products and are renewable.


Biodiesel is made from rapeseed oil and other plant oils. It can be used in diesel-powered vehicles without needing any modifications to the engine.


Ethanol, C2H5OH, is not a hydrocarbon because it contains oxygen as well as hydrogen and carbon. However, it is a liquid fuel that burns well. Bioethanol is made by fermenting sugars from sugar cane, wheat and other plants. It cannot be used on its own unless the engine is modified. However, modern petrol engines can use petrol containing up to 10 percent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol.

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4.5 Alternative Fuels

Ethical concerns

There are ethical issues surrounding the use of biofuels. For example, crops that could be used to feed people are used to provide the raw materials for biofuels instead. This could cause food shortages or increases in the price of food. There are other economic issues surrounding the use of biofuels, including:

  • human resources -more people are needed to produce biofuels than are needed to produce petrol and diesel
  • increased income - for farmers
  • lower fuel prices - biofuels limit the demand for fossil fuels, helping to reduce increases in fuel prices.
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4.5 Alternative Fuels


There are environmental issues surrounding the use of biofuels. Biodiesel naturally contains little sulfur. For example, it may be said that they are carbon neutral – the amount of carbon dioxide released when they are used is the same as the amount absorbed by the plants as they grew. If so, this would reduce the production of this greenhouse gas. However, while biofuels produce less carbon dioxide overall, they are not carbon neutral. This is because fossil fuels are used in their production, for example in making fertilisers for the growing plants.

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