Alkanes as fuels

Most of the fractions formed through crude oil are used for fuels. For example:

• liquefied petroleum gas for camping and cooking gas
• petrol for car, motorbike and machine fuel
• kerosine for aeroplane fuel, lamps and ovens
• diesel for lorry fuel and central heating systems
• fuel oil for ship fuel and power stations
• wax for candles

The main function behind a fuel is to produce useful energy. Hydrocarbons, in particular alkanes, react with oxygen present in the air to produce carbon dioxide and water in a process called combustion. This reaction requires a spark or heat to begin as alkanes are not reactive.

Heat energy is then released making this reaction exothermic. This heat energy can then be used for:

• direct heating like central heating
• mechanical energy for cars, lorries and ships
• electrical energy in power stations.

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A Level Chemistry Revision – Pollution issues

Carbon dioxide itself is not poisonous and is found naturally in the atmosphere. However, due to the enormous amount of hydrocarbons burnt over the years the level of carbon dioxide has risen to dangerous levels. This has led to global warming in which heat is prevented from leaving the atmosphere due to a build-up of carbon dioxide and other compounds, collectively known as greenhouse gases. Global warming has already had disastrous effects on ecosystems, most noticeably the ice caps.

Other greenhouse gases produced through the combustion of hydrocarbons include:

• water vapour
• carbon monoxide
• sulphur dioxide
• oxides of nitrogen

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Incomplete combustion and carbon monoxide

As you now know, carbon dioxide and water is produced through the combustion of hydrocarbons. This is called complete combustion. If there is not enough oxygen present, however, carbon monoxide or carbon is formed and not carbon dioxide. This is known as incomplete combustion.

Where the air supply is limited, as in internal combustion engines, incomplete combustion is more likely. This can be an issue due to:

• less energy is produced than with complete combustion
• carbon monoxide is poisonous
• carbon particles can lead to breathing difficulties and cancer

In this respect, it is important for health as well as efficiency to ensure a good air supply is maintained when burning hydrocarbon fuels.

There are certain times, however, when incomplete combustion is useful. For example, closing the air hole of a Bunsen burner to produce a yellow flame which is more visible and produces a more gentle heat.

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Sulphur dioxide

A lot of crude oil also creates the waste product sulphur dioxide. Attempts are made to lower the sulphur content through special treatments however some sulphur still remains.

When sulphur burns it produces sulphur dioxide which dissolves in rain water leading to acid rain. Acid rain damages buildings and kills plants, tree and fish.

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Oxides of nitrogen

Fuels tend to be burned in air rather than pure oxygen, 80% of which is nitrogen. Nitrogen is not reactive but at a high temperature and the in presence of a spark some reacts with oxygen producing nitric oxide and nitrogen dioxide.

• When nitrogen oxides combine with hydrocarbons not yet burned they produce photochemical smog.
• Like sulphur dioxide, nitrogen dioxide dissolves in rain water and contributes to acid rain.

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A Level Chemistry Revision – Unburned hydrocarbons

Some hydrocarbons are vaporised but escape before being burned. These are toxic and, if inhaled, can lead to cancer. They also combine with nitrogen oxides to create photochemical smog.

Reducing pollution levels

Various techniques, two of which are highlighted below, have been developed to reduce the polluting effects of hydrocarbon combustion.

Flue gas desulphurisation

Acidic gases, like sulphur dioxide, are absorbed by alkaline materials, like calcium oxide (lime), which line the factory chimneys. The resulting substance can then be formed into gypsum which is used to create plaster

Catalytic converters

A lot of cars now use catalytic converters which convert some harmful gases existing in car exhaust into ones which are less harmful. For example, carbon monoxide, nitrogen oxides, and unburned hydrocarbons.

Two main reactions occur in a catalytic converter:

• Carbon monoxide is converted into carbon dioxide and nitrogen monoxide into nitrogen.
• Unburned hydrocarbons and nitrogen monoxide are converted into carbon dioxide, nitrogen and water.

However, although technology can be used to limit the damage caused by burning hydrocarbons, ultimately our reliance on fossil fuels needs to be reduced. Although difficult, this can be achieved by using alternative forms of energy and looking for ways in which the Western world can reduce its energy consumption.