UPSC Civil Services (Mains), Chemistry optional · 2018 · Paper I
Q3(c)(i). Galvanic cell and concentration cell
Question
Distinguish between Galvanic cells and concentration cells with an example each.
Answer in brief
In a galvanic (chemical) cell the two electrodes are different and a chemical reaction gives the e.m.f. (E°≠0; Daniell cell, 1.10 V). In a concentration cell the two electrodes and electrolytes are the same and differ only in concentration (or pressure); the e.m.f. comes from the transfer of matter from high to low concentration (E°=0).
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
| Feature | Galvanic (chemical) cell | Concentration cell |
| Electrodes | two different couples | the same couple on both sides |
| Source of e.m.f. | a chemical (redox) reaction | transfer of electrolyte (or gas) from higher to lower concentration (pressure) |
| E°_(cell) | not zero (from the two E° values) | zero |
| e.m.f. | E=E°-(RT)/(nF)ln Q | E=(RT)/(nF)ln(c₂)/(c₁); falls to zero when c₁=c₂ |
| Example | Daniell cell: Zn|Zn²⁺|Cu²⁺|Cu, E°=1.10 V; Zn + Cu²⁺ → Zn²⁺ + Cu | Ag|Ag⁺(c₁)|Ag⁺(c₂)|Ag with c₂>c₁; net change Ag⁺(c₂) → Ag⁺(c₁) |
| Types | primary and secondary cells, fuel cells | electrolyte concentration cells (above) and electrode concentration cells (H₂ at two pressures, amalgams of two concentrations); with or without transference |
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All parts of Question 3
- 3(a)(i) First law and the cooling of rising clouds5 marks
- 3(a)(ii) Reversible work along two different paths10 marks
- 3(b) Two phases in thermal equilibrium have the same temperature10 marks
- 3(c)(i) Galvanic cell and concentration cell5 marks
- 3(c)(ii) Transport number of Ag⁺ by the Hittorf method10 marks
- 3(d) Capillary action of water and mercury in tubes of two radii10 marks