In order for a reaction to take place, molecules of the reactants must come in contact with each other. The collision theory of chemical kinetics states that the rate of a reaction is proportional to the frequency of the collisions between the molecules. Not only must the molecules come in contact with each other to react, but their energies must be great enough to overcome the repulsive force that molecules experience at extremely small separations. When the molecules collide, they form a transition state, during which the molecules form a large, loosely bound conglomerate. The conglomerate is called an activated complex. The activated complex is an unstable intermiediate step where bonds break and form to generate the products or degenerate back to reactants. Since an activated complex is not stable, it requires a large amount of energy to force the molecules close enough together to attain this form. The required energy is known as the activation energy, as given by the Arrhenius equation. The collision theory provides the theoretical basis for the Arrhenius equation.
