UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I
Q1(c). F-centres: one colour by every route; effect on density
Question
(i) F-centre defect can be introduced in several ways. Regardless of the method used, the colour produced in any particular crystal is always same. Explain the reasons. (ii) How would the formation of F-centre affect the density of the crystal?
Answer in brief
(i) Every method leaves the same defect, one electron trapped in an anion vacancy, and the size of that cavity (the host lattice) fixes the light it absorbs. (ii) The density falls slightly.
Key points
- An F-centre (from Farbe, colour) is an anion vacancy that has trapped one electron, held by the six surrounding cations.
- Alkali-metal vapour, X-rays or γ-rays, and electrolysis of the hot crystal differ only in where the electron comes from.
- The trapped electron behaves like a particle in a box of the size of the anion site, so its absorption depends only on the host lattice.
- A longer cation–anion distance gives a larger box and closer levels, so the band moves to longer wavelength: NaCl yellow, KCl violet, KBr blue-green.
- Density decreases: a lattice site holds an electron in place of a heavy halide ion; with metal vapour the volume grows by a formula unit but mass by one metal atom only.
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.
CoversF-centreFarbe centremetal excess defectanion vacancynon-stoichiometric defectcolour centre
All parts of Question 1
- 1(a) Average and most probable distance of the 1s electron10 marks
- 1(b) Non-bonding interactions between orbitals of A and B10 marks
- 1(c) F-centres: one colour by every route; effect on density10 marks
- 1(d) ΔG for the formation of ozone in smog10 marks
- 1(e) ΔG° and K for the zinc–copper cell reaction10 marks