Question 3

1

a. Molecular hydrogen, H₂, undergoes dissociative chemical adsorption (chemisorption) on Ni-

metal surfaces via an activated process. Experimentally, it is found that the enthalpy change for

the chemical adsorption (AHca) is -148 kJ mol-¹ and the activation energy barrier, Ead is

12 kJ mol-¹

i. Explain what are meant by the following terms (one sentence for each):

dissociative chemical adsorption

activated adsorption process.

(3 marks)

ii. Describe how you can tell from a Lennard-Jones plot whether a process is activated or not.

(2 marks)

iii. Figure 3.1 shows a partially complete schematic potential energy profile (Lennard-Jones

plot) to represent the chemical adsorption of H₂(g) onto the Ni metal. Given that

Ead = 12 kJ mol-¹, complete the potential energy profile by adding a second curve,

with appropriate labels, to represent the physical adsorption of H₂(g) onto Ni. Include

arrows and labels on your sketch to indicate the values Ead, AHca, and Edes. (Note a

copy of Figure 3.1 is provided in the Assessment area as a png file. You should paste a

scanned image of your completed plot into your answer).

(5 marks)/npotential energy (not to scale)

O

HH

1+1

"Ni Ni

2*Ni + H(g) + H(g)

distance from

surface

Figure 3.1 A partially complete schematic potential-energy profile (Lennard-Jones plot)

which shows the chemical adsorption of H₂(g) onto the Ni metal.

iv. Briefly explain how your plot accounts for the fact that adsorbed hydrogen atoms are formed

on the Ni surface without prior dissociation of molecular hydrogen in the gas phase. (No

more than 100 words).

(4 marks)

v. Write an expression for the activation energy for desorption (Edes) of hydrogen molecules

from the Ni surface and calculate the value of Edes.

(3 marks)/nb. The selective oxidation of butan-2-one, B, produces diacetyl, D, over a vanadium phosphorus

oxide catalyst by way of the following reaction:

CH3COC₂H5 (g) + O₂(g) = CH₂ COCOCH3(g) + H₂O(g)

B

D

Under certain experimental conditions the reaction appears to involve a Langmuir-Hinshelwood

mechanism, with non-dissociatively adsorbed reactants:

B(g) = B(ad)

O2(g) → 0₂ (ad)

B(ad) + O₂ (ad) → 'free 'products

where 'free' products represents CH3COCOCH3(g) + H₂O(g) which both desorb as quickly

as they are formed.

dрB

dt

dpo2

dt

i. Identify which of the reactions (3.1 -3.3) in the mechanism is the rate-limiting step if the

reaction were to obey a Langmuir-Hinshelwood mechanism.

(1 mark)

ii. Write down the rate equation for this reaction and hence show that the theoretical rate

equation is given by:

=

ke

bврвbо₂Ро₂

(1+bBPB +bo₂po₂) ²

(3.1)

where ke is the theoretical rate constant.

You should explain why it is not necessary to include terms for the products in your

equation.

(7 marks)

(3.2)

(3.3)

(3.4)/niii. At high partial pressure of butan-2-one, the experimental rate equation is given by:

dpo2

dt

PO₂

PB

where KR is the experimental rate constant.

Show how this is consistent with the theoretical rate equation (Equation 3.4). [Hint:

Remember to relate KR to ke in your answer.]

(4 marks)

iv. What other experimental condition would enable reduction of the theoretical rate equation to

be consistent with this experimental rate equation (Equation 3.5) (one sentence)?

(1 mark)

dрB

dt

=

KR

(3.5)