 So, let's think about reactions with high and low activation energies and what that means. Imagine two identical flasks of some reactant sitting on the lab bench. I'm going to draw an energy diagram on the left and a Maxwell-Boltzmann diagram on the right for each of them. The two flasks are at the same temperature, so they must have identical Maxwell-Boltzmann distributions. In the first flask, I'm going to mix up the reactant with something else and it's going to undergo some reaction which happens to have a very high activation energy. So the energy diagram for this reaction could look like this. And on the Maxwell-Boltzmann distribution, the activation energy would be up here somewhere. And you can see that in this case, only a tiny proportion of molecules have sufficient energy to successfully react, so this reaction will proceed slowly. But now, consider mixing in the second flask this same reactant with something that will cause a different chemical reaction, one that has a low activation energy. The energy diagram for this reaction looks like this, and the activation energy would be somewhere here on the Maxwell-Boltzmann diagram. And you can see that because the minimum energy required is now much lower, a much greater proportion of molecules in the sample have sufficient energy to react, and so this reaction has a high rate. It proceeds quickly. So let's go back to our original question. What is necessary for a reaction to occur between two molecules? The essence of it is that they must collide with the right orientation and energy, and the chances of that happening vary greatly depending on the reaction and the conditions. As we said, many collisions may happen before two molecules collide energetically enough to surpass the activation energy and react. If successful collisions happen frequently, then the reaction will proceed quickly, which is the same as saying that the rate of reaction is high. If successful collisions are rare, then the reaction is very slow. In other words, the rate of reaction depends on how frequently successful collisions occur. So to get to the heart of the matter, we should now rephrase our original question and instead ask, what affects the chances of a successful collision occurring? And it turns out that the variables that affect the chances of a successful collision occurring are surface area, concentration, pressure, temperature, and the presence of a catalyst. And in the next video, we'll see how they do that.