Desorption of a product as the rate determining step

In this case pseudo-equilibrium is assumed for the surface reaction and for the adsorption-desorption processes involving A and B. By similar methods to those employed in deriving equation 3.81, it can be shown that the rate of chemical reaction is  

In principle it is possible to write down the rate equation for any rate determining chemical step assuming any particular mechanism. To take a specific example, the overall rate may be controlled by the adsorption of A and the reaction may involve the dissociative adsorption of A, only half of which then reacts with adsorbed B by a Langmuir-Hinshelwood mechanism. The basic rate equation which represents such a process can be transposed into an equivalent expression in terms of partial 

Langmuir-Hinshelwood mechanism (adsorbed A reacts with adsorbed B) 

PaPb y [1+KAPA + KbPb+ KrPr)1 PaPb - y 1 + KaPa + KbPb + KKfPAP, 

The expreuion for the rate of reaction in terms of partial pressures is proportional to the driving force divided by the adsorption term. 

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