UNIMOLECULAR SURFACE REACTION: MECHANISM, INHIBITION AND ACTIVATION ENERGY

Unimolecular surface reaction may involve a reaction between a molecule A of the reactant and vacant site S on the surface
Surface reaction involving single adsorbed molecules and therefore term as unimolecular and are treated by Langmuir adsorption isotherm

1. TUMKUR UNIVERSITY Department of Studies and Research in Organic Chemistry Seminar presented on, “UNIMOLECULAR SURFACE REACTION: MECHANISM, INHIBITION AND ACTIVATION ENERGY” Presented by Lakshmeesha M Register No.: P11AZ22S104043 I M.Sc., I Semester Under the Guidance The coordinator, Pruthviraj K Dr. Suresh.D Faculty DOS & R in organic chemistry DOS & R in Organic Chemistry Tumkur University, Tumkur . Tumkur University, Tumakuru .

2. CONTENTS Introduction Unimolecular surface reaction Mechanism Inhibition Activation energy Conclusion Reference

3. Unimolecular surface reaction Unimolecular surface reaction may involve a reaction between a molecule A of the reactant and vacant site S on the surface Generally decomposition reactions are come under this category For example decomposition of ammonia on tungsten In general for unimolecular reaction of the type A P Rate will be expressed as Rate =k[A] where k is rate constant of reaction

4. Mechanism Surface reaction involving single adsorbed molecules and therefore term as unimolecular and are treated by Langmuir adsorption isotherm as follows The rate is proportional to the fraction of surface that is covered and is thus V= k =k K[A] 1+K[A] …… (1) where k is constant The dependence of rate on[A] is shown in figure below  

5. Graph Variation of rate with concentration for simple unimolecular process

6. At sufficiently high concentration of A ,the rate is independent of the concentrations which means the kinetics are zero order At low concentration , when K[A] << 1 the kinetics are first order 1. Example the decomposition of ammonia on tungsten This was investigated by Hinshel wood and Burk they used commercial tungsten – filament lamp as their reaction vessel , under the conditions of their experiment the reaction was zero order 2 .Example The decomposition of phosphine on glass ,this reaction was investigated by Van’t Hoff and Kooij who found zero order

7. Inhibition When a substance other than the reactant is adsorbed on the surface with the result that the effective surface area and therefore the rate are reduced Suppose that the substrate ‘ A ‘ is undergoing a unimolecular reaction on a surface and that a non reacting substrate ‘I ‘ known as an inhibitor or a poison ,is also adsorbed. As per competitive adsorption the fraction of the surface covered by A is = [A]/(1+ [A]+ ) where and are the adsorption constants for A and I. The rate of the equation equals to k , is Thus V=k [A]/(1+ [A]+ ………(2) In the absence of inhibitor this equation reduces to V=k [A]/1+ [A]……………….(3)  

8. Special case : when the surface is covered only sparsely by the reactant but is covered fairly fully by the inhibitor, >> 1+ [A] The rate is then V= k [A]/ (4)  

9. Activation energy The rate constant k appearing in equation (1) is expected to obey the Arrhenius equation to a good approximation ……….(5) where is the activation energy corresponding to the reaction of the adsorbed species that is, to the reaction S+ products T he temperature dependence of the equilibrium constant K, if it is expressed as a concentration equilibrium constant, will follow the analogous van ’t Hoff relationship ………(6) W here is the standard molar internal energy change in the adsorption process.  

10. If the reactant pressure is low,rate expression equation (1) reduces to V=k K[A]….(7) And the first order rate co-efficient is given by =k K…..(8) From equation (5 )and( 6) it follows that ………(9) …..(10)  

11. The apparent activation energy is thus If the pressure is high the rate equation is simply V=k[A]…..(11) The observed activation is then T hese relationship are illustrated by the potential energy diagram    

12. Graph Energy diagram for a unimolecular reaction

13. Conclusion Unimolecular surface reaction are widely used in many industrial applications such as catalysis,electrocatalysis and adsorption There also usefull for the dynamic behavior of molecules on surfaces and elucidating the underlying mechanisms of the reaction

14. References Principles of physical chemistry – Puri,Sharma,Pathania Page number 1155 to 1157 Chemical kinetics-Keith J.Laidler page no (244-248)

15. Thank you