 In this video, you are going to learn what happens in a reaction, why some reactions are endothermic and some exothermic. When I was younger, I had a model set. Inside the box were the various components and an instruction book containing 80 models to build. On the first day, I built a ship. The day after, I wanted to build a tank, but I could not. Any idea why? I hadn't enough pieces left to build the tank. I had to take my ship to pieces and then use those pieces to build up the tank. You can liken this analogy to what happens in reactions. Consider the reaction between hydrogen and iodine to give hydrogen iodide. The reactants are what react and are on the left hand side of the arrow in the equation. And the products are what are produced in the reaction are on the right hand side of the arrow in the equation. We can represent what happens in this reaction by the diagram. A is the activation energy, which is the energy taken in to break the HH and the II bonds in the reactants. C is the energy given out when the new HI and HI bonds make to form the products. B is the net energy taken in, delta H, during the reaction. This is the energy difference between the energy taken in to break the bonds in the reactants and the energy given out when the new bonds make to form the products. The activation energy is the sufficient energy mentioned in the video Collision Theory and Reactions 1. Now consider the reaction between nitrogen and hydrogen to form ammonia. This is the harbour process. We can represent what happens in this reaction by the diagram. Again A is the activation energy, which is the energy taken in to break the NN and the III HH bonds in the reactants. C is the energy given out when the six new NH bonds make to form the products. B is the net energy given out during the reaction, delta H, which is the energy difference between the energy taken in to break the bonds in the reactants and the energy given out when the new bonds make to form the products. So to recap, breaking bonds takes in energy, making bonds gives out energy. If more energy is taken in to break bonds than is given out when new bonds are made, the reaction is endothermic. If less energy is taken in to break bonds than is given out when new bonds are made, the reaction is exothermic.