Energy diagrams are a visual tool to understand the energy changes in chemical reactions. They map the reaction from reactants to products, with the vertical axis showing potential energy. The difference in energy between the start and end points is the enthalpy change (ΔH). A peak in the diagram represents the transition state, and the energy needed to reach it from the reactants is the activation energy (Ea​). For an exothermic reaction, products are at a lower energy level than reactants, resulting in a negative ΔH (heat exits). In an endothermic reaction, products are at a higher energy level, resulting in a positive ΔH (heat enters). A catalyst speeds up a reaction by providing a new pathway with a lower activation energy, effectively making the energy hill shorter without changing the overall ΔH of the reaction.