Activation energy (Ea) = energy barrier that reactants must overcome. On the energy profile: Ea(forward) = E_{activated}_{complex} - $E_{reactants}$. Ea(backward) = E_{activated}_{complex} - $E_{products}$. $delta_H$ = Ea(forward) - Ea(backward). For exothermic reactions: Ea(forward) < Ea(backward). For endothermic reactions: Ea(forward) > Ea(backward). A catalyst provides an alternative pathway with lower Ea. It lowers Ea for BOTH forward and reverse reactions by the same amount. It does NOT change $delta_H$, equilibrium constant K, or equilibrium composition. It only increases the rate of attaining equilibrium. Enzyme catalysis: very high specificity, lock-and-key model, much lower Ea than inorganic catalysts.