What is the activation energy of a reaction, and how is this energy related to the activated complex of the reaction?

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The activation energy of a chemical reaction is the energy barrier that must be overcome for the reactants to convert to the products.  You can plot the energy of a reaction versus time.  For most reactions, the energy level rises from the reactants to reach a peak and then falls down to the products.  The difference in energy between the reactants and the peak of the curve is the measure of the activation energy.  The highest part of the energy plot is the transition state complex.  This is the high energy complex that represents a transition state, or a fleeting species that is a mixture of the reactants and products.  This transition state is what you are referring to with the term activated complex.  So the energy value of this activated complex represents the highest energy value that you use to calculate the activation energy of a reaction.

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