Rates of Reaction

Energy - Endothermic and Exothermic.

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 reactions are exothermic, they give heat out.

Some physical processes are associated with a change in energy.
Melting and boiling are endothermic,
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 bonds in the reactants.
You get energy out
when new bonds are formed to make the products.

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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