a first order reaction in component A. High concentration, then . It is a zeroth order reaction in component A. Bimolecular reaction Langmuir–Hinshelwood mechanism In this mechanism, suggested by Irving Langmuir in 1921 and further developed by Cyril Hinshelwood in 1926, two molecules adsorb on neighboring sites and the adsorbed molecules undergo a bimolecular reaction: A + S ⇌ AS B + S ⇌ BS AS + BS → Products The rate constants are now ,,, and for adsorption/desorption of A, adsorption/desorption of B, and reaction. The rate law is: Proceeding as before we get , where is the fraction of empty sites, so . Let us assume now that the rate limiting step is the reaction of the adsorbed molecules, which is easily understood: the probability of two adsorbed molecules colliding is low. Then , with , which is nothing but Langmuir isotherm for two adsorbed gases, with adsorption constants and . Calculating from and we finally get . The rate law is complex and there is no clear order with respect to either reactant, but we can consider different values of the constants, for which it is easy to measure integer orders: Both molecules have low adsorption That means that , so . The order is one with respect to each reactant, and the overall order is two. One molecule has very low adsorption In this case , so . The reaction order is 1 with respect to B. There are two extreme possibilities for the order with respect to A: At low concentrations of A, , and the order is one with respect to A. At high concentrations, . The order is minus one with respect to A. The higher the concentration of A, the slower the reaction goes, in this case we say that A inhibits the reaction. One molecule has very high adsorption One of the reactants has very high adsorption and the other one doesn't adsorb strongly. , so . The reaction order is 1 with respect to B and −1 with respect to A. Reactant A inhibits the reaction at all concentrations. The following reactions follow a Langmuir–Hinshelwood mechanism: 2 CO + O2 → 2 CO2 on a platinum catalyst. CO + 2H2 → CH3OH on a ZnO catalyst. C2H4 + H2 → C2H6 on a copper catalyst. N2O + H2 → N2 + H2O on a platinum catalyst. C2H4 + ½ O2