A problem with the collision model for reaction rates is that the experimental activation energies are much lower than the energy needed to break all the chemical bonds in the reacting species. The transition state model is one way to account for this problem

Rather than breaking all the bonds in the reacting species, an intermediate called a activated complex is formed. This is a highly unstable molecule that quickly decays either to products or back into the reactants. It is difficult to say exactly what an activated complex looks like: they don't exist long enough for people to do experiments on them.

One example is the reaction between CO and NO₂. A guess as to what the activated complex is shown below.

A generic reaction diagram is shown below. The y axis is energy, the x axis is the "reaction coordinate", a way of plotting how far along the reaction is. At the left are the reactants: the activated complex is at the center and the products are on the right. The energy needed for the reactants to become an activated complex is shown as E_a, and the overall change in energy is shown as DH

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