A fuel is a substance which stores energy and then releases that energy to its surroundings, usually when it it burned. Examples of fuels are wood, coal, petrol, diesel and ethanol. But, what is it about fuels that allow this storage and release of energy, and what makes a fuel efficient and useful?
Oxygen is Required
Most fuels currently in use release their energy through a process called combustion. In this process the fuel reacts with oxygen producing new compounds and releasing energy as heat. The energy is released by the chemical reaction due to bonds being broken and other being formed.
Many fuels, especially fossil fuels, are hydrocarbons – they are compounds of carbon and hydrogen – although some also contain some oxygen, like ethanol. When a hydrocarbon burns it reacts with oxygen to produce carbon dioxide and water. For example, when methane (CH4) burns, each molecule reacts with two molecules of oxygen to form one molecule of carbon dioxide and two molecules of water.
Enthalpy Change of Combustion
The scientific term for the amount of energy produced by the combustion of fuel is the Enthalpy Change of Combustion. It is measured in joules per mole and is the amount of energy produced by a fixed number of molecules of the fuel.
In a chemical reaction bonds between atoms are broken and others are formed. Breaking bonds requires energy, while making bonds releases energy. Different types of bonds require different amounts of energy to be broken, and release different amounts of energy when they are formed. These quantities are called the bond enthalpies, and can be combined to find an overall enthalpy of combustion in a combustion reaction.
So, for example, when methane is burned, four carbon-hydrogen (C-H) bonds and two oxygen-oxygen (O=O) double bonds are broken. Then, two carbon-oxygen (C=O) double bonds and four hydrogen-oxygen (H-O) bonds are formed.
The bond enthalpies of each bond are:
- C-H 413 kJ/mol
- O=O 498 kJ/mol
- C=O 805 kJ/mol
- H-O 464 kJ/mol
So, this reaction requires 2448 kJ/mol to break all the bonds, but will then release 3466 kJ/mol in forming the new bonds. This means that the overall enthalpy change of combustion of methane is -1018 kJ/mol. It is negative because energy is released overall.
Energy Density
In order to decide which is the best fuel, however, there is a more important quantity to consider. This is the energy density of a fuel – or the amount of energy released per kilogram of fuel. Clearly, larger hydrocarbon molecules, like octane or hexadecane, will have higher enthalpy change of combustion per molecule, but each molecule also weighs more.
So, lighter fuels, like methane, have higher energy density, and so are most efficient when the requirement to transport it is factored in. In fact, the fuel with the highest energy density is hydrogen, which is one reason it is considered to be a fuel of the future.
Sources:
D. Denby et al, Chemical Storylines, Heinemann, 2008
Bond Enthalpy and Mean Bond Enthalpy, WebChem.net, Accessed 19th December, 2010
Simon Davies - Simon Davies has lived a varied life so far After completing a BSc in Chemistry at the University of Bristol, UK, he worked as a ...
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