Electrochemistry: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. Define cell constant.
2. What is a salt bridge for?
3. State Faraday's first law.
4. Name the cell used in inverters.
5. Why is AC used for conductivity?
6. Write the Nernst equation for Zn²⁺/Zn.
7. Which is the strongest reducing agent: Li, Na, Zn?
8. Give one use of the fuel cell.
9. What is a sacrificial anode?
10. Define limiting molar conductivity.

D. Short answer I (2 marks each)

1. Write the Daniell cell notation and its half-reactions.
2. Find E for Cu²⁺(0.01 M)/Cu (E° = 0.34 V).
3. κ = 0.012 S cm⁻¹ for 0.1 M KCl. Find Λm.
4. How many coulombs deposit 1 mol of Al from Al³⁺?
5. Why does the pH of NaCl solution rise on electrolysis?
6. Find ΔG° for a cell with E° = 1.10 V, n = 2.
7. Distinguish primary and secondary batteries.
8. Find α if Λm = 39 and Λ°m = 390 S cm² mol⁻¹.

Rough work

E. Short answer II (3 marks each)

1. Calculate the emf of Zn | Zn²⁺(0.01 M) ‖ Cu²⁺(0.1 M) | Cu.
2. Find Kc for Zn + Cu²⁺ ⇌ Zn²⁺ + Cu (E° = 1.10 V).
3. R of a cell with 0.1 M KCl (κ = 0.0129 S cm⁻¹) is 100 Ω; with 0.02 M NaCl it is 400 Ω. Find κ and Λm of NaCl.
4. Explain the variation of Λm with √c for strong and weak electrolytes, with a graph.
5. What mass of Ag is deposited by 2 A for 30 min?
6. Write the reactions of the lead storage battery during discharge and charging.

F. Long answer (5 marks each)

1. (a) State Kohlrausch's law. (b) Λ°m of HCl, NaCl and CH₃COONa are 426, 126 and 91 S cm² mol⁻¹. Find Λ°m of CH₃COOH. (c) If Λm of 0.001 M acetic acid is 39 S cm² mol⁻¹, find α and Ka.
2. (a) Derive the relation between E°cell and Kc. (b) Calculate E° and ΔG° for Fe + 2Ag⁺ → Fe²⁺ + 2Ag. (c) Explain rusting as an electrochemical process.

G. HOTS (higher-order thinking)

1. Why is a salt bridge not needed in a lead storage battery?
2. Why does a galvanic cell stop working after some time?
3. Why is Na not obtained by electrolysis of aqueous NaCl?
4. Why is graphite, not iron, used as the anode in many electrolytic cells?

20 MCQs

1. Reduction occurs at the
   (a) anode (b) salt bridge (c) cathode (d) wire
2. E°cell equals
   (a) E°cathode − E°anode (b) E°anode − E°cathode (c) their sum (d) their product
3. Strongest oxidising agent
   (a) Li (b) I₂ (c) Na (d) F₂
4. At equilibrium Ecell is
   (a) E° (b) zero (c) maximum (d) negative
5. ΔG° equals
   (a) nFE° (b) −nFE° (c) −nE°/F (d) F/nE°
6. Unit of κ
   (a) S cm² mol⁻¹ (b) S (c) Ω cm (d) S cm⁻¹
7. On dilution Λm
   (a) increases (b) decreases (c) unchanged (d) becomes zero
8. Charge for 1 mol Cu from Cu²⁺
   (a) 1F (b) 3F (c) 2F (d) 0.5F
9. Cathode in aqueous NaCl electrolysis gives
   (a) Na (b) O₂ (c) Cl₂ (d) H₂
10. Electrolyte of the lead storage battery
    (a) KOH (b) H₂SO₄ (c) NH₄Cl (d) NaCl
11. Constant-voltage cell
    (a) dry cell (b) lead cell (c) mercury cell (d) Daniell cell
12. H₂ electrode at pH 3 (298 K)
    (a) −0.177 V (b) +0.177 V (c) 0 V (d) −0.059 V
13. Λ°m for weak electrolytes is found by
    (a) extrapolation (b) Kohlrausch's law (c) Nernst (d) Faraday
14. Cell constant unit
    (a) cm (b) cm² (c) cm⁻¹ (d) S
15. Metal that protects iron sacrificially
    (a) Cu (b) Sn (c) Ag (d) Mg
16. Fuel cell product
    (a) H₂O (b) CO₂ (c) H₂O₂ (d) O₃
17. Faradays for MnO₄⁻ → Mn²⁺
    (a) 5 (b) 3 (c) 7 (d) 2
18. E° for Daniell cell
    (a) 0.42 V (b) 0.76 V (c) 2.0 V (d) 1.10 V
19. Primary battery
    (a) lead storage (b) dry cell (c) Ni–Cd (d) Li-ion
20. Λm =
    (a) κc (b) c/κ (c) 1000κ/c (d) κ/1000c

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: Zinc displaces copper from CuSO₄ solution. R: E° of zinc is lower than that of copper.
2. A: Λm of CH₃COOH rises steeply at low concentration. R: Its degree of dissociation increases on dilution.
3. A: Ecell is zero at equilibrium. R: E°cell is zero at equilibrium.
4. A: Mercury cell gives a constant voltage. R: No ion concentration changes in its overall reaction.
5. A: Cl₂ is formed at the anode in aqueous NaCl electrolysis. R: Cl⁻ has a higher reduction potential than water.
6. A: A salt bridge keeps the solutions neutral. R: It allows ions to move between the two half-cells.

Key: 1 (a) · 2 (a) · 3 (c) · 4 (a) · 5 (c) · 6 (a)

Answers to the practice set