A catalyst increases reaction rate by providing an alternative pathway with lower Ea. Key properties: (1) Not consumed — regenerated at the end. (2) Does NOT change $delta_H$ or $K_{eq}$. (3) Lowers Ea for both forward and reverse reactions equally. (4) Speeds up attainment of equilibrium without shifting its position. Homogeneous catalysis: catalyst in same phase as reactants (e.g., H+ in ester hydrolysis, NO in lead chamber process). Heterogeneous catalysis: different phase (e.g., Fe in Haber process, Pt/Pd in catalytic converters, V2O5 in contact process). Steps in heterogeneous catalysis: adsorption -> surface reaction -> desorption. Enzyme catalysis: biological catalysts with very high specificity and efficiency. Inhibitors reduce catalyst effectiveness; promoters enhance it.