Rates of
Reaction Catalysts and
Energy Search Questions 
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.
If you have to put in more energy
than you get out,
then the reaction is endothermic but
except in rare circumstances, the
reaction doesn't
happen.
Rates of
Reaction Catalysts and
Energy Search Questions
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