Indian Forest Service (Mains), Chemistry optional · 2021 · Paper I
Q1(d). Desilverisation of lead
Question
Explain the application of phase rule in desilverization of lead.
Answer in brief
The Pattinson process removes nearly pure Pb crystals from a Pb-rich melt, enriching silver in the remaining liquid until the Pb–Ag eutectic is reached.
Key points
- Pb–Ag is treated as a simple eutectic system with negligible solid mutual solubility; at fixed pressure, F = C − P + 1 = 3 − P.
- On cooling a lead-rich melt below its liquidus, solid Pb separates; liquid plus Pb has P = 2, F = 1.
- Removing Pb crystals repeatedly makes the residual melt richer in Ag, moving down the Pb-side liquidus towards the eutectic.
- At about 303 °C and 2.6 mass% Ag, liquid, Pb and Ag coexist: P = 3, F = 0; fractional crystallisation cannot enrich it further.
- Silver is recovered from the enriched fraction by an additional separation, traditionally cupellation.
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.
CoversPattinson processdesilverisation of leadPb–Ag eutecticphase rulereduced phase rule
All parts of Question 1
- 1(a) Frenkel disorder in silver bromide8 marks
- 1(b) Changing the enthalpy reference8 marks
- 1(c) C–F and C–I potential-energy curves8 marks
- 1(d) Desilverisation of lead8 marks
- 1(e) Equilibrium constant of a Zn–Ag cell8 marks