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).