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Name: Emily M.
Status: student
Age: 14
Location: N/A
Country: N/A
Date: Sunday, December 08, 2002

What is the difference between an endo and exothermic reaction. What are some examples? Like where does the energy for each come from? I am told the energy is in the chemical bonds, but I am also told that a bond is at low potential. It cannot be at high and low energy at the same time. Please help!


Net endo-energetic reactions require energy to proceed. Net exo-energetic reactions release energy as they proceed.

If bonds are to be broken, activation energy must be supplied. When new bonds are formed, energy is released.

Overall, the energy to drive chemical reactions is either input as activation energy at the start or it comes from energy released as atoms rearrange into other kinds of ions or molecules.

ProfHoff 537

Endothermic reactions absorb heat. Exothermic give off heat. Dilution of ammonium chloride is an example of an endothermic reaction. This is the active ingredient in chemical "ice packs" you can obtain in a pharmacy. Other "reactions" are melting and boiling which also absorb heat to happen, although you may not consider them "chemical" reactions. Combustion is a typical exothermic reaction -- any type of burning.

An exothermic reaction occurs if the energy of the bonds formed in the products are stronger (lower energy) than the bonds broken in the reactants. Endothermic reactions require heat. If there is no external source, the reaction "gets" the heat by cooling to a lower temperature. These reactions are driven by the change in the configuration of the atoms. If the atoms in the product molecules have a less orderly structure than the atoms in the reactants, these reactions will occur even if the "cost" some energy to happen.

Vince Calder

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