$word_{count}$: 200

Activation energy (Ea) is the energy barrier between reactants and the transition state (activated complex). On the energy profile: Ea(forward) = peak - reactant energy. Ea(backward) = peak - product energy. $delta_H$ = Ea(forward) - Ea(backward). Exothermic: Ea(fwd) < Ea(bkwd). Endothermic: Ea(fwd) > Ea(bkwd). The transition state is the highest-energy point — unstable, cannot be isolated, exists for ~10^-13 s. A catalyst provides an alternative reaction pathway with lower Ea. Key properties: (1) lowers Ea for both forward and reverse reactions equally, (2) does not change $delta_H$ or $K_{eq}$, (3) is not consumed (regenerated), (4) speeds attainment of equilibrium without shifting position. Types: homogeneous (same phase — H+ catalyst in ester hydrolysis), heterogeneous (different phase — Fe in Haber process, Pt in catalytic converters). Enzyme catalysis: extraordinarily efficient, reduces Ea by 30-60 kJ/mol, giving rate increases of 10^6 to 10^12. Even a 10 kJ/mol reduction in Ea increases rate ~55x at 300 K.