Electrochemistry Revision NCERT Class 12 Chemistry

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Electrochemistry Revision Q&A

NCERT Class 12 Chemistry • Study Guide & MCQ Quiz

Class 12 JEE / NEET

Short Answer Q&A (100 Questions)

1. Q1: Define Electrochemistry.
   A: The study of the relationship between chemical energy and electrical energy.
2. Q2: What is an electrochemical cell?
   A: A device that converts chemical energy into electrical energy or vice-versa.
3. Q3: What is a Galvanic (or Voltaic) cell?
   A: A cell where a spontaneous redox reaction generates electrical energy.
4. Q4: What is an Electrolytic cell?
   A: A cell where electrical energy is used to drive a non-spontaneous chemical reaction.
5. Q5: What is the sign of the anode in a Galvanic cell?
   A: Negative (site of oxidation).
6. Q6: What is the sign of the anode in an Electrolytic cell?
   A: Positive (site of oxidation).
7. Q7: What is the function of a salt bridge?
   A: To complete the inner circuit and maintain the electrical neutrality of the solutions.
8. Q8: Define EMF of a cell.
   A: The maximum potential difference between two electrodes when no current is drawn.
9. Q9: Write the relation between ΔrG° and E°cell.
   A: ΔrG° = -nFE°cell.
10. Q10: What is the unit of conductance (G)?
    A: Siemens (S) or ohm^-1.
11. Q11: What is the unit of conductivity (κ)?
    A: Siemens per meter (S m^-1) or S cm^-1.
12. Q12: Define cell constant (G*).
    A: The ratio of distance between electrodes (l) to the area of cross-section (A). (G* = l/A).
13. Q13: How does conductivity of an electrolyte vary with dilution?
    A: It decreases (due to fewer ions per unit volume).
14. Q14: How does molar conductivity (Λm) of a strong electrolyte vary with dilution?
    A: It increases slightly (due to decreased inter-ionic attraction).
15. Q15: State Kohlrausch's Law.
    A: At infinite dilution, each ion contributes independently to the total molar conductivity.
16. Q16: How is the degree of dissociation (α) calculated for a weak electrolyte?
    A: α = Λm / Λm°.
17. Q17: Write the simplified Nernst equation at 298 K.
    A: Ecell = E°cell - (0.0591/n) log Q.
18. Q18: What is the value of Ecell at equilibrium?
    A: Zero (Ecell = 0).
19. Q19: Write the relation between E°cell and the equilibrium constant (Kc).
    A: E°cell = (0.0591/n) log Kc (at 298 K).
20. Q20: State Faraday's First Law of Electrolysis.
    A: Mass deposited is directly proportional to the quantity of electricity passed (W ∝ Q).
21. Q21: What is the value of Faraday constant (F)?
    A: 96487 C mol^-1.
22. Q22: How many moles of electrons are required to deposit 1 mol of Ca from Ca²⁺?
    A: 2 moles of electrons (2F).
23. Q23: Define Primary Battery.
    A: A battery that cannot be recharged once the chemicals are exhausted.
24. Q24: Give an example of a primary battery.
    A: Dry cell (Leclanché cell) or Mercury cell.
25. Q25: Define Secondary Battery.
    A: A battery that can be recharged by passing current in the reverse direction.
26. Q26: Give an example of a secondary battery.
    A: Lead storage battery or Nickel-Cadmium cell.
27. Q27: What is the overall cell reaction in a Mercury cell?
    A: Zn(Hg) + HgO(s) → ZnO(s) + Hg(l).
28. Q28: Why is the voltage of a mercury cell constant throughout its life?
    A: Because the overall reaction does not involve any ion whose concentration changes.
29. Q29: What is the electrolyte in a lead storage battery?
    A: 38% solution of H₂SO₄.
30. Q30: What is the material of the anode in a fully discharged lead storage battery?
    A: Lead sulphate (PbSO₄).
31. Q31: Define a Fuel cell.
    A: A Galvanic cell that converts the energy of combustion of fuels directly into electrical energy.
32. Q32: Give an example of a fuel cell.
    A: H₂-O₂ Fuel Cell.
33. Q33: What is the by-product of an H₂-O₂ fuel cell?
    A: Water (H₂O).
34. Q34: What is Corrosion?
    A: The slow decay of a metal due to its reaction with air and moisture (an electrochemical process).
35. Q35: Write the chemical formula of rust.
    A: Fe₂O₃·xH₂O (Hydrated ferric oxide).
36. Q36: What is Galvanisation?
    A: Coating iron with a layer of Zinc to prevent rusting.
37. Q37: Name the electrode whose standard potential is conventionally taken as zero.
    A: Standard Hydrogen Electrode (SHE).
38. Q38: What is the cell reaction at the anode during the discharge of a dry cell?
    A: Zn(s) → Zn²⁺ + 2e^-.
39. Q39: What happens to ΔrG when Ecell is positive?
    A: ΔrG is negative (spontaneous).
40. Q40: What is the condition for a non-spontaneous cell reaction?
    A: Ecell is negative (or ΔrG is positive).
41. Q41: Why is the corrosion of iron quicker in saline water?
    A: Salt acts as an electrolyte, increasing the conductivity and rate of the electrochemical process.
42. Q42: Why does Λm of weak electrolytes increase steeply on dilution?
    A: Due to the increase in the degree of dissociation (α).
43. Q43: Which metal is a stronger reducing agent, A (E° = -2.5 V) or B (E° = -1.0 V)?
    A: Metal A (more negative E°).
44. Q44: How is the potential of a single electrode measured?
    A: By connecting it to the SHE to form a complete cell.
45. Q45: What is the minimum voltage required for the electrolysis of water?
    A: E°cell ≈ -1.23 V.
46. Q46: What is the relationship between resistance (R) and cell constant (G*)?
    A: κ = G*/R.
47. Q47: What is the unit of the cell constant (G*) in SI units?
    A: m^-1.
48. Q48: What does Λm° represent?
    A: Limiting molar conductivity (molar conductivity at infinite dilution).
49. Q49: Give the SI unit of Λm.
    A: S m² mol^-1.
50. Q50: What is the role of ZnCl₂ in a dry cell?
    A: It forms a complex with NH₃ to prevent pressure build-up.
51. Q51: State Faraday's Second Law in terms of equivalent mass (E).
    A: Mass deposited (W) is proportional to equivalent mass (W ∝ E) for the same Q.
52. Q52: What is the charge required for the reduction of 1 mol of MnO₄^- to Mn²⁺?
    A: 5 Faradays (5F).
53. Q53: What is the formula to calculate the mass deposited (W)?
    A: W = ZIT, where Z is the electrochemical equivalent.
54. Q54: What is electrochemical equivalent (Z)?
    A: The mass of the substance deposited by 1 C of charge (Z = E/F).
55. Q55: What is the anode in the H₂-O₂ fuel cell?
    A: Porous carbon rod through which H₂ gas is passed.
56. Q56: What is the electrolyte in the H₂-O₂ fuel cell?
    A: Hot aqueous KOH or NaOH.
57. Q57: What metal is used for sacrificial protection of iron?
    A: Zinc (Zn).
58. Q58: Name a noble metal used as an electrode in SHE.
    A: Platinum (Pt).
59. Q59: What is the standard concentration for ions in E°?
    A: 1 M (Molar).
60. Q60: What is the standard pressure for gases in E°?
    A: 1 bar (or 1 atm).
61. Q61: What does the notation Zn | Zn²⁺ represent?
    A: A half-cell of zinc metal dipped in Zn²⁺ solution.
62. Q62: What does the double vertical line (||) represent in cell notation?
    A: A salt bridge or porous partition separating two half-cells.
63. Q63: Write the cell notation for the Daniell cell.
    A: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s).
64. Q64: Give one reason why Li is the strongest reducing agent in aqueous solution.
    A: Extremely high hydration energy of the Li⁺ ion.
65. Q65: What is the condition for Ecell > 0?
    A: ΔrG < 0 and Kc > 1.
66. Q66: What happens to the pH near the cathode during the electrolysis of aqueous NaCl?
    A: It increases (becomes basic due to OH^- formation).
67. Q67: What is the primary product at the anode during the electrolysis of aqueous NaCl?
    A: Cl₂ gas.
68. Q68: What is the primary product at the cathode during the electrolysis of aqueous NaCl?
    A: H₂ gas.
69. Q69: What does the term electrolysis of brine refer to?
    A: Electrolysis of concentrated aqueous NaCl solution.
70. Q70: What happens to the concentration of H₂SO₄ during the recharging of a lead storage battery?
    A: It increases.
71. Q71: What is the general term for the current-carrying ability of a metallic wire?
    A: Metallic (or Electronic) Conductance.
72. Q72: How is Λm related to C (concentration) for a strong electrolyte? (Debye-Hückel-Onsager equation)
    A: Λm = Λm° - A√C.
73. Q73: Why does the molar conductivity of strong electrolytes not increase sharply on dilution?
    A: They are already completely dissociated at all concentrations.
74. Q74: What is the role of the Pt electrode in SHE?
    A: It provides a surface for the reaction and for the transfer of electrons.
75. Q75: What is the major disadvantage of a dry cell?
    A: It is an acidic cell, which corrodes the Zn casing.
76. Q76: What determines the extent of electronic conductance in a metal?
    A: The number of valence electrons per atom.
77. Q77: How does temperature affect electronic conductance?
    A: It decreases with an increase in temperature.
78. Q78: Give one example of a weak electrolyte.
    A: CH₃COOH (Acetic acid).
79. Q79: Give one example of a strong electrolyte.
    A: NaCl (Sodium chloride).
80. Q80: Which process is faster: rusting in moist air or rusting in saltwater?
    A: Rusting in saltwater (due to higher electrolyte content).
81. Q81: Write the cell reaction for the charging of a lead storage battery.
    A: 2PbSO₄(s) + 2H₂O(l) → Pb(s) + PbO₂(s) + 2H₂SO₄(aq).
82. Q82: What is the approximate voltage of a single lead storage cell?
    A: 2 V (A typical car battery has 6 cells for 12 V).
83. Q83: What is the maximum work obtained from a Galvanic cell?
    A: The decrease in Gibbs free energy (ΔrG).
84. Q84: Define electrochemical series.
    A: The arrangement of various half-cells in order of their increasing standard reduction potentials.
85. Q85: What is the property of elements that have a high negative E°?
    A: They are strong reducing agents.
86. Q86: What is the property of elements that have a high positive E°?
    A: They are strong oxidizing agents.
87. Q87: Can you store AgNO₃ solution in a Cu vessel? (E°Ag+/Ag > E°Cu2+/Cu)
    A: No, Cu will spontaneously displace Ag (positive Ecell).
88. Q88: What is the role of a solid electrolyte in a fuel cell?
    A: To facilitate the migration of ions between the electrodes.
89. Q89: What is the electrode material of the cathode in a dry cell?
    A: Carbon rod (C) surrounded by MnO₂ and carbon powder.
90. Q90: What is the reaction quotient (Q)?
    A: The ratio of product concentrations to reactant concentrations raised to their stoichiometric powers, at any point.
91. Q91: At what temperature is the simplified Nernst equation (with 0.0591) valid?
    A: 298 K (or 25 °C).
92. Q92: Name the force that drives electrons in the external circuit.
    A: Electromotive Force (EMF).
93. Q93: Which electrode is the site of Cl₂ evolution during the electrolysis of molten NaCl?
    A: Anode (Cl^- → ½Cl₂ + e^-).
94. Q94: What is the common name for the reaction Fe²⁺ → Fe³⁺ + e^-?
    A: Oxidation of ferrous to ferric ion.
95. Q95: Why are strong electrolytes like KCl used in salt bridges?
    A: Because the mobility of the cation (K⁺) and anion (Cl^-) are nearly the same.
96. Q96: If ΔrG° = 0, what is the value of E°cell?
    A: Zero (E°cell = 0).
97. Q97: How much electricity is needed to obtain 1 mol of Al from Al₂O₃ by electrolysis?
    A: 3 Faradays (3F).
98. Q98: Define Concentration Cell.
    A: A Galvanic cell in which the two half-cells have the same electrodes but different concentrations of the electrolyte.
99. Q99: What technique is used to prevent corrosion by connecting the metal to a more easily oxidizable metal?
    A: Cathodic protection (or Sacrificial protection).
100. Q100: What is the term for the decrease in EMF due to the formation of gas bubbles around the electrode?
     A: Polarization.

Practice Quiz (30 MCQs)

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