3.1 Activation Energy

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Another aspect of the collision theory describes what happens to reacting particles as they change from the reactant state to the product stage. When the particles collide, the theory (transition state theory) tells us that a temporary, unstable grouping of particles forms - bonds are in the process of both being broken and being formed. This unstable association of particles is called the activated complex or transition state. This unstable complex is at a high energy level.

The activated complex is similar to the reaction intermediates discussed in Section 2.2, but it is an even more unstable state.

The amount of energy required to reach the activated complex is called the activation energy, E_a. Activation energy is essentially what we called threshold energy when we discussed kinetic energy diagrams.

These terms - activated complex and activation energy - will make more sense when we revisit a concept we last examined in our unit on Thermochemistry (Section 2.1) - potential energy diagram