UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II
Q1(i). KCl salt bridge for a silver–lead cell
Question
Can a salt bridge made from filter paper dipped in KCl solution be used to make a cell consisting of silver and lead half-cells? Justify your answer.
Answer in brief
No. Cl⁻ from the bridge precipitates AgCl and PbCl₂ in the half-cells. Use KNO₃ or NH₄NO₃.
Key points
- The bridge electrolyte must be inert to both half-cell solutions. KCl is not.
- Ag⁺ + Cl⁻ → AgCl↓ (Ksp = 1.8 × 10⁻¹⁰); Pb²⁺ + 2Cl⁻ → PbCl₂↓ (Ksp = 1.7 × 10⁻⁵).
- Precipitation lowers [Ag⁺] and [Pb²⁺] ⇒ the emf drifts from its Nernst value.
- The precipitate blocks the paper ⇒ internal resistance rises.
- KNO₃, NH₄NO₃: no precipitate; ions of equal mobility keep the liquid junction potential small.
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.
Coverssalt bridgeliquid junction potentialgalvanic cellAgCl precipitationKNO3 salt bridge
All parts of Question 1
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- 1(c) Band structure of n-type and p-type semiconductors5 marks
- 1(d) Symmetry elements of orthorhombic Mg₂SiO₄5 marks
- 1(e) Fall of G with temperature at constant pressure5 marks
- 1(f) Z > 1 and f > P for the gas P(Vm − b) = RT5 marks
- 1(g) Overall equation and the role of each species5 marks
- 1(h) log k against pH for specific H⁺ catalysis5 marks
- 1(i) KCl salt bridge for a silver–lead cell5 marks
- 1(j) Equilibrium constant from standard potentials5 marks
- 1(k) Conditions for an acceptable wave function5 marks
- 1(l) No solution with E = 0 for the particle in a box5 marks
- 1(m) Discrete levels and the resonance condition5 marks
- 1(n) Rotational activity of CO and CO₂5 marks
- 1(o) Measuring light intensity with a chemical actinometer5 marks
- 1(p) Why watch hands glow in the dark5 marks