 So we can now put this together to draw a diagram that shows how the chemical energy in a reaction changes during the course of the reaction. Our y-axis represents energy and the units we use for energy are usually kilojoules per mole. Kilojoules is a measure of energy. Mole is a measure of how much chemical we've got. We're going to deal with those units in more detail later on in this course. So for now you just need to know that it's kilojoules per mole. So our x-axis is time. This is sometimes called the reaction coordinate. Really all it needs to represent is that this is the beginning of the reaction and this is the reaction proceeding and this is the end of the reaction. Okay, if this is an exothermic reaction then our reactants at the beginning of the reaction have more energy than the products at the end of the reaction. The difference between those two energies is delta H, the change in enthalpy. You can see this is going from high value to a low value, so the change is a negative one and that's what shows that it's exothermic. Finally we need to show how the energy varies during the course of the reaction. What happens is this. First we have to put in the activation energy that will get things started. So the reaction energy increases and increases until it reaches the point where there's sufficient energy for the reactant bonds to start breaking and the atoms to rearrange. At that point product molecules start being made and energy starts being released. So the energy decreases and decreases until all the reactants have been transformed into products and we're at the product energy level. If you look at the difference between the reactant energy and the top of our little curve here that shows you what the value of the activation energy is. That was how much energy had to be put in above the value of the reactants in order to get the reaction started.