Energy level diagrams are used to depict energy changes that occur during a chemical reaction. It shows relative energy levels of the products and reactants when energy is released (exothermic reactions) as well as when energy is gained (endothermic reactions). Exothermic reaction In an exothermic reaction, energy is given out to the surroundings. Thus, there is a decrease in energy level from reactants to products. Endothermic reaction The energy level increases in an endothermic reaction because energy is taken into the system from the surroundings. Energy level diagrams from both reactions are shown below:
A horizontal line represents the reactants' energy. It goes downward in exothermic reactions as energy is being released whereas it rises vertically as energy is absorbed in endothermic reactions. A horizontal line after energy changes represents the product's energy level which is on a lower level than reactants in the exothermic reactions and on a higher level in endothermic reactions.