Explain Langmuir-Hinshelwood kinetics for surface reactions.

Langmuir-Hinshelwood model assumes: surface reaction is rate-limiting, adsorption follows Langmuir isotherm (limited sites, monolayer), and reaction occurs between adsorbed species. Rate expression: r = k*KA*PA*KB*PB / (1 + KA*PA + KB*PB + KP*PP)^2, for reaction A + B -> Products. Denominator accounts for site competition. At low pressures, rate increases with pressure; at high pressures, rate becomes independent (saturation). Model explains why rate passes through maximum with some partial pressures.