 So, what determines the activation energy of a reaction? In order to turn reactants into products, the bonds holding the reactant molecules together have to be broken so that the atoms can be rearranged to form the product molecules. The new molecules can't be formed until the old molecules break up. So, the thing that determines the activation energy is how strong are the bonds in the reactants. A reaction of which strong bonds have to be broken will have a large activation energy. Quite a lot of extra energy will be needed to break those reactant bonds and get things going. Without some kind of helping hand to provide this energy, like an added flame for instance, then the rate of this reaction is going to be slow. Conversely, reactants with weak bonds, which fall apart easily, may undergo a reaction with a small activation energy. And because the bonds that need to be broken are quite weak, the rate of the reaction will be higher because it's more likely that the bonds will get broken and the reaction can proceed more quickly. So, the activation energy is a way of determining how much energy is needed for a collision to be successful. Say two molecules collide, but the energy of the collision is less than the activation energy, then the molecules will simply bounce off each other and continue on unchanged. However, if the collision is more energetic than the activation energy, then the bonds of the reactant molecule will break and the atoms will be free to rearrange to form the product molecule.