UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper II
Q1(o). Uranyl-sensitised decomposition
Question
For UO₂²⁺ + (COOH)₂ → UO₂²⁺ + CO + CO₂ + H₂O: (i) what does the reaction indicate? (ii) what is the role of UO₂²⁺? Explain.
Answer in brief
It represents photosensitised decomposition of oxalic acid; uranyl acts as the light-absorbing sensitiser and is regenerated in the stated net reaction.
Key points
- The substrate decomposes to CO, CO₂ and water. Uranyl appears on both sides, so it is not consumed stoichiometrically in the given net scheme.
- The intended photochemical interpretation is excitation of uranyl by absorbed light, followed by interaction with oxalic acid that initiates decomposition and regenerates the sensitiser.
- The net equation alone does not specify every electron-transfer or radical intermediate.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversUranyl-sensitised decompositionPhotochemistry
All parts of Question 1
- 1(a) Critical-point inflection5 marks
- 1(b) Boyle and inversion temperatures5 marks
- 1(c) Accuracy at high diffraction angle5 marks
- 1(d) Composition change and non-expansion work5 marks
- 1(e) Van't Hoff plots5 marks
- 1(f) Potassium sulfate activities5 marks
- 1(g) Reaction order and rate constant5 marks
- 1(h) Catalytic rate enhancement5 marks
- 1(i) Electrochemical potential of zinc ions5 marks
- 1(j) Open-circuit emf and equilibrium5 marks
- 1(k) Hermitian operators5 marks
- 1(l) Probabilities of quantum states5 marks
- 1(m) Hydrogen rotational spectra5 marks
- 1(n) Wavelength and the einstein5 marks
- 1(o) Uranyl-sensitised decomposition5 marks
- 1(p) HI photolysis yield5 marks