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 𝜃 of surface that is covered and
is thus
V= k𝜃 =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
𝜃 =𝑘𝐴[A]/(1+ 𝑘𝐴[A]+𝑘𝐼[𝐼])
where 𝑘𝐴 and 𝑘𝐼 are the adsorption constants for A and I.
The rate of the equation equals to k𝜃, is Thus
V=k𝑘𝐴[A]/(1+ 𝑘𝐴[A]+𝑘𝐼[𝐼]………(2)
In the absence of inhibitor this equation reduces to
V=k 𝑘𝐴[A]/1+ 𝑘𝐴[A]……………….(3)

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

9. Activation energy
The rate constant k appearing in equation (1) is expected to obey the
Arrhenius equation to a good approximation
ⅆ 𝒍𝒏 𝒌
ⅆ𝑻
=
𝑬𝟐
𝑹𝑻𝟐……….(5)
where 𝑬𝟐is the activation energy corresponding to the reaction of the
adsorbed species that is, to the reaction 𝐴 − 𝑆 S+ products
The temperature dependence of the equilibrium constant K, if it is expressed
as a concentration equilibrium constant, will follow the analogous van ’t Hoff
relationship
ⅆ 𝑙𝑛 𝑘
ⅆ𝑇
=
Δ𝑈0
𝑅𝑇2………(6)
Where Δ𝑈0
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 𝑘1 is given by
𝑘1=k K…..(8)
From equation (5 )and( 6) it follows that
ⅆ ln 𝑣
ⅆ𝑇
=
ⅆ ln 𝑘1
ⅆ𝑇
=
ⅆ ln 𝑘
ⅆ𝑇
+
ⅆ ln 𝐾
ⅆ𝑇
………(9)
=
𝐸2+Δ𝑈0
𝑅𝑇2 …..(10)

11. The apparent activation energy 𝐸𝑎is thus
If the pressure is high the rate equation is simply
V=k[A]…..(11)
The observed activation is then𝐸2
These relationship are illustrated by the potential energy diagram
𝐸2 + Δ𝑈0

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