Some reactions take place in a single step. We can represent this using an energy profile . An activated complex (or transition state ) forms between reactant and product. This is not a 'real' substance in the sense that it can be isolated and put in a test tube. But based on various pieces of experimental evidence it is the chemist's model of how the reaction occurs. The energy 'hump' shows how much energy reacting molecules must have for a 'successful' collision, . one that leads to reaction. The formation of an activated complex requires energy to bring molecules together in the correct orientation. Therefore, it is always an endergonic reaction. The energy required is called the activation energy (E a ).