 This is the last video in this first introduction section of Unit 1. We're going to tie things up by looking at reaction energy diagrams. Energy diagrams allows to represent both qualitatively and quantitatively what happens to energy during a chemical reaction. Recall from the last video that we could classify reactions into exothermic and endothermic reactions. In an exothermic reaction, as the reaction proceeds, energy is released. This means that some energy that had been stored in the chemical bonds of the reactants is now transformed into heat or light energy and is released to the surroundings. And that means that the amount of chemical energy in the products must be less than the energy that was stored in the reactants, and the difference between those values is the energy that's released as heat or light. During an endothermic reaction, energy is absorbed from the surroundings. This means that energy that was in the surroundings as heat, or occasionally light, is now absorbed by the chemicals and incorporated into the chemical bonds of the products. In turn, this means that the amount of chemical energy in the product molecules must be greater than that in the reactants, with the extra energy having been absorbed from the surroundings to make up the difference.