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Essentials of Chemical Reaction Engineering by H. Scott Fogler

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Why is there an activation energy?

If the reactants are free radicals that essentially react immediately on collision, there usually isn’t an activation energy. However, for most atoms and molecules undergoing reaction, there is an activation energy. A couple of the reasons are that in order to react:

  1. The molecules need energy to distort or stretch their bonds so that they break and now can form new bonds.
  2. The molecules need energy to overcome the steric and electron repulsive forces as they come close together.

The activation energy can be thought of as a barrier to energy transfer (from kinetic energy to potential energy) between reacting molecules that must be overcome. The activation is the minimum increase in potential energy of the reactants ...

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