A series of animations explaining exothermic and endothermic reactions and how to calculate energy changes taking place during chemical reactions. Temperature changes associated with exothermic and endothermic reactions are shown in the context of familiar reactions. Energy changes are explained in terms of relative energy of reactants and products using energy level diagrams. Worked examples show how to calculate overall energy changes from the total amount of energy required to break all bonds in the reactants and that released when bonds are formed in the products.