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Rates of Reaction

Energy - Exothermic and Endothermic.

What do Exothermic and Endothermic mean?

A chemical reaction always has a change in energy.

In a reaction going from reactants to products, either

1) heat is given out - called exothermic
or
2) heat is taken in - called endothermic.


The large majority of chemical reactions are exothermic.
Exothermic reactions give out heat to their surroundings.

Some physical processes are associated with a change in
energy. For example, melting and boiling are endothermic
but freezing and condensing are exothermic.

Breaking bonds (overcoming the force of attraction)
requires energy. You have to put heat in - it is endothermic.
This is why melting and boiling are endothermic.

 Making bonds gives out energy - it is exothermic.
This is why freezing and condensing are exothermic.

In a chemical reaction you need to put energy in
to break the bonds that exist in the reactants. You
get energy out when new bonds are formed to
make the products. The bonds are covalent or ionic.

If you get out more energy than you have to put in,
then overall the reaction is exothermic.
This is what normally occurs - see some examples.

If you have to put in more energy than you get out,
then the reaction is endothermic - see some examples.

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