Rates of Reaction
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
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 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
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.
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