UPSC Combined Geo-Scientist, Mains · 2020 · Mains Paper II
Q1(p). HI photolysis yield
Question
Given HI+hν→H+I, H + HI → H₂ + I and I + I → I₂, find the quantum yield.
Answer in brief
For HI disappearance, Φ=2; for either H₂ or I₂ formation, Φ=1.
Key points
- One absorbed photon dissociates one HI molecule, generating H and I atoms.
- The H atom consumes a second HI to form H₂ and another I atom. Recombination of the two I atoms forms one I₂.
- The sum is 2HI+hν→H₂+I₂. Hence two reactant molecules disappear per photon if all listed steps occur quantitatively and competing deactivation is negligible.
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.
CoversHI photolysis yieldPhotochemistry
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