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Batteries — Primary, secondary, fuel cells

Electrochemistry

9

JEE Qs

8%

Hard

75

min

Memorize the anode, cathode, electrolyte, and overall redox reactions for all specified battery types, distinguishing between primary, secondary, and fuel cells.

🧮 Key Formulas

Zn(s) + 2MnO2(s) + 2NH4^+(aq) -> Zn^2+(aq) + 2MnO(OH)(s) + 2NH3(aq) (Dry Cell)
Zn(Hg)(s) + HgO(s) -> ZnO(s) + Hg(l) (Mercury Cell)
Pb(s) + PbO2(s) + 2H2SO4(aq) -> 2PbSO4(s) + 2H2O(l) (Lead-Acid Battery, Discharge)
2PbSO4(s) + 2H2O(l) -> Pb(s) + PbO2(s) + 2H2SO4(aq) (Lead-Acid Battery, Charge)
Cd(s) + NiO2(s) + 2H2O(l) -> Cd(OH)2(s) + Ni(OH)2(s) (Nickel-Cadmium Cell, Discharge)
2H2(g) + O2(g) -> 2H2O(l) (Hydrogen-Oxygen Fuel Cell)

✅ Key Points for JEE

  • 1Primary batteries are non-rechargeable (e.g., Dry Cell, Mercury Cell) as the redox reactions are irreversible. Focus on their specific anode, cathode, electrolyte, and products.
  • 2Secondary batteries are rechargeable (e.g., Lead-acid, Ni-Cd) because their electrode reactions can be reversed by an external electrical energy source. Understand both discharge and charge processes.
  • 3Fuel cells convert the energy of combustion of fuels (like H2, CH4) directly into electrical energy with high efficiency, are pollution-free, and require continuous supply of reactants.
  • 4For each battery type, remember the materials used for anode, cathode, and electrolyte, along with the overall cell reaction and characteristic voltage. Pay attention to how the electrolyte changes (e.g., H2SO4 concentration in lead-acid battery).
  • 5Compare advantages and disadvantages: e.g., mercury cell gives constant voltage; lead-acid is heavy but widely used in automobiles; fuel cells are highly efficient and eco-friendly but expensive.

⚠️ Common Mistakes

  • ✕Confusing the anode and cathode materials or their respective oxidation/reduction reactions for specific batteries.
  • ✕Incorrectly identifying whether a battery is primary or secondary, or failing to recall the key characteristics of each type.
  • ✕Misinterpreting the role of the electrolyte or its changes during discharge/charge cycles (e.g., density of H2SO4 in lead-acid battery).

📝 Practice Questions

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Q54.The element that does not belong to the same period of the remaining elements (modern periodic table) is: 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) Iridium (2) Platinum (3) Osmium (4) Palladium

2025·NumericalHard

Q67.For the given cell Fe2+ (aq) + Ag+ (aq) →Fe3+(aq) + Ag(s) The standard cell potential of the above reaction is Ag+ + e−→Ag Eθ = xV Given: Fe2+ + 2e−→Fe Eθ = yV Fe3+ + 3e−→Fe Eθ = zV (1) x + y −z (2) x + 2y (3) x + 2y −3z (4) y −2x

2025·MCQMedium

Q58.Which of the following electrolyte can be used to obtain H2 S2O8 by the process of electrolysis? (1) Dilute solution of sodium sulphate. (2) Acidified dilute solution of sodium sulphate. (3) Dilute solution of sulphuric acid (4) Concentrated solution of sulphuric acid

2025·MCQMedium

Q66.A solution of aluminium chloride is electrolysed for 30 minutes using a current of 2 A . The amount of the aluminium deposited at the cathode is [Given : molar mass of aluminium and chlorine are 27 g mol−1 and 35.5 g mol−1 respectively. Faraday constant = 96500Cmol−1] (1) 1.660 g (2) 0.336 g (3) 0.441 g (4) 1.007 g 2025 (22 Jan Shift 1) JEE Main Previous Year Paper

2025·MCQMedium

Q58.Standard electrode potentials for a few half cells are mentioned below : E∘ = 0.34 V, E∘ = −0.76 V Cu2+/Cu Zn2+/Zn Which one of the following cells gives the most negative value of E∘Ag+/Ag = 0.80 V, E∘Mg2+/Mg = −2.37 V ΔG∘ ? (1) Zn Zn2+(1M) Ag+(1M) Ag (2) Zn Zn2+(1M) Mg2+(1M) Mg (3) Ag Ag+(1M) Mg2+(1M) Mg (4) Cu Cu2+(1M)∥Ag+(1M) Ag

2025·MCQMedium

Q57. E∘Cr2O2−7 /Cr3+ = 1.33 V E∘Cl2/Cl(−) = 1.36 V Based on the data given below : the strongest reducing agent is : E∘ = 1.51 V E∘ = −0.74 V MnO−4 /Mn2+ Cr3+/Cr (1) Cr (2) Cl− (3) MnO−4 (4) Mn2+

2025·MCQMedium

NCERT Chapters

  • Class 12 Chemistry Ch 3: Electrochemistry