Fossil Fuels Hydrocarbons Combustion  Elmhurst College
Natural Gas Gasoline Compounds  Combustion Energy  Chemistry Department
Gasoline Distill Oil Refinery Convert Oil Refinery  Virtual ChemBook

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 Energy of Combustion

 The combustion of all fossil fuels follows a very similar reaction:

Fossil Fuel (any hydrocarbon source) plus oxygen yields
carbon dioxide and water and ENERGY.

The world and modern society are driven by the need to produce energy to make products (manufacturing), to move around (transportation), to heat homes and buildings, and to create light (electricity). At least 75% of these needs are met by the combustion of fossil fuels. Energy is stored in chemical compounds in the bonds that bind atoms to each other.

CH4[g] + 2 O2[g] -> CO2[g] + 2 H2O[g] + ENERGY

A chemical reaction occurs by the rearrangement of atoms and molecules in the reactant (starting) molecules and the end product molecules. Some bonds are broken while others are reformed. The process of breaking and forming bonds results in a net energy needed or given off for a reaction.

In the example above and to the left, the combustion reaction of methane and oxygen to form carbon dioxide and water is shown broken into steps to show the entire energy "using" and "forming" process. First it takes energy to break bonds, all four of the C-H bonds in methane must be broken. The energy units are kilojoules, a positive sign means that the process is endothermic or energy is required to break the bonds.

In a similar fashion, two diatomic oxygen molecules are broken apart which requires more energy. Now all of the individual atoms in the reactant molecules have been broken apart.

On the right side of the diagram in a second step, the various atoms form new bonds in new molecules of carbon dioxide and water. The formation of new bonds is an exothermic process where heat is given off. Again the energy given off is totaled to form new bonds in carbon dioxide and water molecules.

Finally, the overall reaction yields an excess of energy given off -802 kj. (the minus sign means that this is an exothermic process). In more familiar units this is equivalent to 191 kilocalories per 16 grams of methane. This is a little more than the 150 calories in a can of Coke.

The excess of energy given off is mainly in the form of heat. Chemical energy stored in the bonds of molecules is transformed into heat and light energy. Most chemical reactions are of this type and thus are exothermic. Less energy is required to break old bonds than is given off in the process of forming new bonds.