 In the last video we looked at why activation energy is so important in determining rate. Collisions between molecules will only result in a reaction if their energy is greater than the activation energy. So what do we need to understand now? Well we need to know what determines the energy of a collision. The measure we're looking for is kinetic energy, the energy of movement. The equation for kinetic energy is a half mv squared, half mass times the square of velocity. So a larger mass or a higher velocity will give an object more kinetic energy. Think about a car crash. There are two main things that affect how bad a crash is. One is the mass of the vehicles being run into by a semi-trailer is much worse than being run into by a hatchback. The other is the speed they're travelling at. High speed crashes are worse than low speed crashes. Both of these facts are true because for higher masses and higher velocities more energy is involved. It's the same with molecules. Heavy, fast molecules collide with greater energy than light, slow molecules. Now for a given reaction we can't do anything about the mass of the molecules, it's fixed. So the variable we're really interested in is how fast the molecules are going. So let's explore the kinetic energy of molecules a little. You know from junior science that as you increase the temperature of a substance the molecules move faster. This is the reason behind phase changes from solid to liquid and liquid to gas. But even within one phase a liquid say, as the temperature increases so does the speed of the molecules. So if we perform a reaction at a higher temperature it means that the molecules taking part in the reaction are moving faster and so their kinetic energy is increasing and their collisions are more violent. Here's the thing though. At a particular temperature not all molecules have the same speed. Some will be moving slowly and some will be moving quickly. If you measure the speeds of molecules in a liquid or a gas sample at a particular temperature you'll find that there will always be some that are slow and some that are fast. And since we know that faster speeds correspond to higher kinetic energies we can produce a graph that shows the frequency of particles for all possible values of kinetic energy. This graph corresponds to a mathematical distribution known as the Maxwell-Boltzmann distribution that has this particular shape. It starts at zero zero since there are no molecules that are sitting completely still meaning their velocity is zero. And it peaks at some particular kinetic energy. This is the most common kinetic energy or rather the energy that the largest number of molecules have. And then it tails off since very few molecules will have extremely large kinetic energies. Now have a think about some mixture of reactants sitting at room temperature. In that sample quite a few molecules will be travelling slowly. If these slow molecules collide they won't react because their collisions will not be energetic enough. However some proportion of molecules may be travelling fast enough such that if they do happen to collide in the right orientation they will do so with sufficient energy for the reaction to be successful. So we can redraw the Maxwell-Boltzmann distribution to show that for some generic reaction a certain proportion of molecules will have enough energy to react while the rest will not. And the amount of energy that divides the haves from the have-nots the minimum energy necessary for a collision to cause a reaction that is the activation energy of that reaction. Remember the amount of energy needed to break the reactant bonds so the atoms can react.