AP Chemistry — Unit 5: Kinetics
Reaction energy and catalysts
Every reaction has to climb an energy barrier — the activation energy, Eₐ — before reactants can become products. A catalyst provides a different pathway with a lower barrier, which speeds the reaction up in both directions. What a catalyst cannot do is change how much energy the reaction releases or absorbs overall. Move the sliders and watch which parts of the diagram respond.
What to take away: the peak height above the reactants is Eₐ for the forward reaction; the peak height above the products is Eₐ for the reverse. The gap between the two flat levels is ΔH, and it depends only on where reactants and products sit — never on the barrier between them. So adding a catalyst lowers both activation energies and leaves ΔH untouched. A reaction that is exothermic (ΔH < 0) stays exothermic no matter how good the catalyst is.