CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT
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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.
In the example above the combustion reaction of methane and oxygen to form carbon dioxide and water is shown broken into steps to show the whole 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 the second step, 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, the various atoms form new bonds in new molecules. 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). 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 that are exothermic. Less energy is required to break old bonds than is given off in the process of forming new bonds.