LH Physical IIIb / 03 33681 Chemistry Summer Examinations 2021

1.         Giving full justification, suggest the best technique (or combination of techniques) and experimental strategy that could be used to investigate each of the following (a) to (e):

[Do not describe all the details of how the methods work from your notes. Instead you should focus on the key features of the experiment that make it suitable for the situation and explain why other methods may be less suited.]

(a)    the absolute configuration of phenylalanine in crystalline form; (3 marks)

(b)    the structure of the complex formed when RhCl(PPh3)3  attaches to an amorphous polymer; (3 marks)

(c)    the distribution of copper and nickel atoms in the doped perovskite crystalline phase LaNi(1-x)CuxO3 and whether this structure is representative of the bulk sample;  (4 marks)

(d)    the structure of a mixed self-assembled monolayer of octadecanethiol and octanethiol on Au(111); (3 marks)

(e)     whether carbon monoxide is adsorbed on the surface of iridium catalyst particles as C and O atoms or as CO molecules, and whether any adsorbed CO molecules lie parallel or perpendicular to the particle surface. (3 marks)

2.         (a)    A Au(110) surface was modified with iodide ions, with diameter d = 0.2 nm.

After surface modification, STM images of the modified surface were obtained. The expected interatomic distance of the atoms is 0.28 nm and 0.55 nm; however, the STM image shows that the actual distance between the atoms is 0.28 nm and 1.10 nm.

(i)   Explain the phenomenon, fully justifying your answer and illustrating your answer with an appropriate sketch of the surface structure.  (3 marks)

(ii)  Describe how the surface energy of the observed surface differs from the surface energy of the expected surface. Your answer should include the appropriate mathematical expressions and a full justification. (4 marks)

(iii) In order to obtain the STM image of the Au(110) surface modified with iodide ions, a bias potential of 0.3 V was applied between the tip and the surface. When the iodide ions were removed from the surface, only 0.2 V    was required to obtain the image. Explain the phenomenon and fully justify your answer. (3 marks)

(b)    A Pd(100) surface, with work function 4.5 eV, was modified with long-chain alkyl thiols, resulting in a change in the work function of - 1.04 eV.

(i)   Is the process chemisorption or physisorption?  Justify your answer. (2 marks)

(ii)  Explain why the surface energy of the modified surface is different from that of the unmodified surface. (2 marks)

(iii) The temperature desorption programme experiment of the modified surface shows that when the coverage was θ = 0.6, the desorption temperature of the molecules was 350 K and when the coverage was θ = 1, the desorption temperature of the molecules was 400 K.  Explain the phenomenon and fully justify your answer. (2 marks)