What are fuels?

Fuels are chemicals which burn, giving out energy.
The reaction is described as exothermic (energy releasing), and the gas oxygen is used up in the process.

 
Combustion

A reaction in which oxygen is used up as energy is released is known as a combustion reaction. This is another word for the process of burning something.

The combustion of carbon to form carbon dioxide

 
Hydrocarbons

Many of the fuels that we use are fossil fuels that were formed millions of years ago from material that was once living. There are three common fossil fuels - Coal, oil and natural gas

The chemicals that mostly make up oil and natural gas are called hydrocarbons. They are given this name because these compounds are made up using the elements hydrogen and carbon only.

When hydrocarbons burn in a plentiful supply of oxygen, the carbon 'part' of the hydrocarbon effectively joins with the oxygen to form carbon dioxide and the hydrogen 'part' of the hydrocarbon effectively joins with the oxygen to form hydrogen oxide, which is better known as water.

When this combustion takes place in a plentiful supply of oxygen, we call it complete combustion. This can be summarised by using a general word equation for all hydrocarbons:

hydrocarbon

oxygen

carbon dioxide

water

• The carbon dioxide that is formed during the combustion of a hydrocarbon can be tested with lime water. When carbon dioxide is passed through lime water, it changes the lime water from colourless to 'milky' white.

• The water that is formed can be tested by measuring the freezing point and boiling point and comparing it to that of pure water, or it can be tested by using cobalt chloride paper which turns from blue to pink in the presence of water.
• The fact that water (hydrogen oxide) is produced proves that the fuel contains hydrogen.
• The fact that carbon dioxide is produced proves that the fuel contains carbon.

The products of burning a hydrocarbon can be identified as follows:

 
Pollution problems with burning hydrocarbons

When we burn hydrocarbons in a plentiful supply of oxygen, we produce water and carbon dioxide.

• Carbon dioxide is a gas which causes global warming (greenhouse effect).

The main pollution problems associated with the combustion of hydrocarbons occur when the fuel is burned in a poor or limited supply of oxygen. This is called incomplete combustion. When this happens carbon moxoxide gas is produced in addition to particles of carbon (soot).

• Carbon monoxide is a poisonous gas that permanently attaches to the red blood cells in our body preventing the transfer of oxygen in our bloodstream.
• Soot particles (carbon) are harmful when breathed in as they can be deposited in our lungs, which can eventually make breathing more difficult.

When we burn fossil fuels (which are mainly hydrocarbons), there is also a small amount of sulphur compounds present. This sulphur, on combustion with oxygen, forms sulphur dioxide (a poisonous gas) that can dissolve in the rain to form sulphurous acid - also known as acid rain.

Fuels need oxygen from the air to burn. In petrol engines, the spark provides the activation energy to ignite the fuel. Unfortunately the spark can also cause nitrogen and oxygen molecule to combine to make nitrogen oxides.

These are poisonous gases which add to the acid rain problem. The same gases are formed when a lightning bolt passes through the air.

 
Reducing pollution from car engines

A special exhaust system called a catalytic converter was introduced in the UK in 1991. This reduced the quantities of harmful gases such as nitrogen oxide, carbon monoxide and unburnt hydrocarbons, by changing them into less harmful gases such as nitrogen and carbon dioxide.

Engines which use more air and less fuel also help prevent the formation of carbon monoxide as the fuel has sufficient oxygen to be completely.
You can quickly test your knowledge of the above information.

 
Fractional Distillation

Crude oil is a mixture of many compounds which are similar to each other in chemical makeup and chemical properties, but differ in size of molecule.
As a result the different molecules have different melting and boiling points.

Distillation will separate two liquids that are mixed together if they have different boiling points.

When there are more than two substances present, process called fractional distillation is used.

The term 'fractional' refers to parts obtained by splitting the crude oil mixture.

Fractions are groups of hydrocarbons that have similar boiling points and so are collected at the same position of the fractionating tower.

The fraction with the lowest boiling point evaporates first and can be separated from fractions with higher boiling points.

This is done in the oil industry using very tall towers called fractionating towers as shown in the photograph.

Fractions with the lowest boiling point are collected higher up the tower.

 
Properties of the fractions

The fractions with the lowest boiling point (first fractions obtained) are pale liquids while as the boiling point increases the fractions get darker in colour.

The residue contains molecules with boiling points higher than 350°C and is a black solid called bitumen that is used to cover road surfaces (road tar).

The table below shows how the properties flammability, evaporation and viscosity change as the boiling point of the fraction increase.

• The bigger the molecules, the higher the boiling point, the lower the flammability and the greater the viscosity.

• Big molecules are more strongly attracted to each other. Higher temperatures are needed to separate the molecules when they are boiled.

• Big molecules don't react as quickly with oxygen and so are less flammable.

• The stronger attractions between the bigger molecules means that liquids containing bigger molecules don't flow as easily as liquids with smaller molecules.

• Longer molecules also tangle together more and are more viscous.

New words and their meanings

FUEL - a substance which burns releasing energy

EXOTHERMIC - a reaction in which energy is released

COMBUSTION - a reaction of a substance with oxygen, giving out energy

FOSSIL FUELS - fuels such as coal and oil that were formed millions of years ago by the decompostion of material that was once living (such as trees and animals)

HYDROCARBONS - compounds containing carbon and hydrogen only

COMPLETE COMBUSTION - when there is sufficient oxygen for a fuel to burn completely.

LIME WATER - a solution of calcium hydroxide that turns milky in the presence of carbon dioxide. This is the test for carbon dioxide.

COBALT CHLORIDE PAPER - This paper can be used to test for water. It turns from blue to pink when water is present.

POLLUTION - substances which damage the environment

INCOMPLETE COMBUSTION - when there is insufficient oxygen for a fuel to burn completely.

ACID RAIN - when gases such as sulphur dioxide and carbon dioxide dissolve in water in the clouds, they form acids (sulphurous acid and carbonic acid), which can cause damage to the environment..

CATALYTIC CONVERTER - a device on cars, containing platinum, to reduce harmful gases such as carbon monoxide, nitrogen oxides and unburnt hydrocarbons.

FRACTIONAL DISTILLATION - separation of a mixture of substances into parts or fractions based the fact that they have different boiling points

FRACTION - a mixture of substances that have similar boiling points and are collected together during fractional distillation.

FLAMMABILITY - refers to how easily a substance burns e.g. a flammable substance burns easily

VISCOSITY - describes the thickness of a liquid e.g. a viscous liquid does not run easily.