Catalyst: substance that changes reaction rate without being consumed. Positive catalyst: increases rate (lowers E_a). Negative catalyst (inhibitor): decreases rate (increases E_a). Key facts: catalyst does NOT change delta_G, delta_H, K_eq, or equilibrium composition. Only increases the rate of reaching equilibrium. Catalyst provides an alternative pathway with lower activation energy. Homogeneous catalysis: catalyst in same phase (H+ in esterification). Heterogeneous catalysis: catalyst in different phase (Fe in Haber process, Pt in contact process, Ni in hydrogenation).