 Last video, we looked at the conditions necessary for a collision between two molecules to cause a reaction. We're now going to zoom out from our view of individual molecules and consider the energies involved in a reaction from a more macro scale. To do this, we need to revisit the energy diagrams that you learned about in Unit 1. Okay, so we know that for a reaction to occur, the reactant molecules have to collide in the right orientation and with sufficient energy to break bonds. So let's explore the idea of sufficient energy more closely. We need a little revision on the energy changes that occur during a reaction. So we're going to draw an energy diagram. The vertical axis here is energy. The horizontal axis represents the progress of the reaction, or time, if you like. If we draw the enthalpy of the reactants here, remember that we're representing the amount of chemical energy stored in the bonds of the reactant molecules. And the enthalpy of the products shows the energy stored in the bonds of the product molecules. So remember this, we can draw a curve that represents how the energy changes through the reaction. We start with this much chemical energy and then we add some energy in order to get the reaction started and then the reaction emits some energy and we end up with our products. Now there are two important quantities here. Delta H, the change in enthalpy, is the difference in energy between the reactants and the products. If Delta H is negative, it means that energy is emitted, as in this reaction here, and we call the reaction exothermic. If Delta H is positive, the reaction is endothermic. However, the quantity that's directly relevant to rates of reaction is not the enthalpy, but the amount of energy we had to put in over and above the energy of the reactants before the reaction got started. And this is called the activation energy.