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.

Energy Diagram for an Exothermic Reaction

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 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.

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