They are found deep within the earth. (Fossil fuels are the main cause of pollutions, they do not form quickly, and they are non-renewable resources.)
When energy is needed in order for the reaction to happen, then that reaction is known as endotermic. When the reaction has as a result energy then it is exotermic. An example of an endotermic reaction would be photosynthesis, for an exotermic: combustion.
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This chemical reaction can also be written as
2C2H6+7O2-->4CO2+6H2O
This is a chemical reaction because reactants are converted into products. This is the combustion of ethane where ethane and oxygen have been converted into carbon dioxide and water