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Energy Level Diagrams.
The change in energy can
be plotted against
the progress of a reaction, as the reactants turn into products.
Going from reactants to the top
of the curve,
you are going up the energy
scale.
Energy (heat) is being put in to break
bonds in the reactants.
At the top of the curve, the
bonds in the reactants have been broken.
The amount of energy put in to break these
bonds
is called the activation energy.
The activation energy
is the minimum amount of energy needed for the
reaction to occur.
A catalyst may
work by lowering the activation energy for
a reaction.
Going from the top of the curve to
the products,
you are going down the energy
scale.
Energy (heat) is given out as bonds
form in the products.
The reactants are higher
up the energy scale than are
the products.
The amount of energy (heat) you need to put
in (the activation energy)
is less than the amount of energy
(heat) you get
out.
This is a typical exothermic
reaction.
The difference in energy
levels between the reactants and the products
is given the symbol DH (pronounced
'delta H').
This is the amount of heat given out
(or taken in) during the reaction.
For an exothermic reaction, DH is negative.
For an endothermic reaction,
DH is
positive.
Rates of
Reaction Catalysts and
Energy Search Questions 
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Copyright © 2008 Dr. Colin France. All Rights Reserved.