For the reaction

Let r_a= rates of adsorption ..........................................(g moles/s.gm of catalyst)

r_s = rates of surface reaction................................... ...(g moles/s.gm of catalyst)

r_d =rates of desorption ................................................(g moles/s.gm of catalyst)

C_AS= concentration of adsorbed A on surface ..............(g moles/gm of catalyst)

C_BS= concentration of adsorbed B on surface ..............(g moles/gm of catalyst)

C_CS= concentration of adsorbed C on surface ..............(g moles/gm of catalyst)

C_DS= concentration of adsorbed D on surface.............. (g moles/gm of catalyst)

C_o= total concentration of active sites on surface........ (g moles/gm of catalyst)

C_v= concentration of vacant sites on surface............... (g moles/gm of catalyst)

C_A= concentration of A in bulk gas phase .....................(g moles/cm³ )

Similarly C_B,C_C ,C_D are concentration of B,C,D in bulk gas phase respectively.

Let the reaction follows the mechanism given below ;

(1)	--- Adsorption of reactant A on surface vacant site S
(2)	--- Adsorption reactant B on surface vacant site S
(3)	--- Surface reaction between adsorbed A and B
(4)	--- Desorption of product C from surface creating a vacant site
(5)	--- Desorption of product D from surface creating a vacant site

Among the various steps described, the slowest step controls the overall rate of reaction and the other remaining steps are assumed to be at near equilibrium conditions. This approach greatly simplifies the overall rate expression, reducing the number of rate constants and equilibrium constants to be determined from experimental data. Further each step in this method is assumed to be elementary and the number of sites is conserved in each step.