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Dry cell, lead accumulator and fuel cells: NEET MCQs with answers

89 questions on this topic of Redox Reactions and Electrochemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. In a Leclanché dry cell, which coordination complex is formed between zinc ions and ammonia to prevent the buildup of free ammonia gas around the electrode?

  1. [Zn(NH₃)₄]²⁺
  2. [Zn(NH₃)₂]²⁺
  3. [Zn(NH₃)₆]²⁺
  4. [Zn(NH₃)₃Cl]⁺
Show answer and explanation

Answer: (A) [Zn(NH₃)₄]²⁺

The NH₃ gas liberated in the cathodic reaction forms a complex with Zn²⁺ ions from the anode reaction to produce the diamagnetic coordination cation [Zn(NH₃)₄]²⁺. This prevents internal gas pressure buildup in the sealed cell.

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2. An alkaline H₂-O₂ fuel cell delivers a steady current of 48.25 A for a duration of 1000 seconds. Assuming 100% Faraday efficiency, what is the total amount of water (H₂O) produced by the cell?

  1. 0.50 mol
  2. 0.25 mol
  3. 1.00 mol
  4. 0.125 mol
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Answer: (B) 0.25 mol

Total charge passed Q = I × t = 48.25 A × 1000 s = 48250 C. Moles of electrons transferred = 48250 / 96500 = 0.50 mol. The overall cell reaction is 2H₂ + O₂ → 2H₂O, involving 4 moles of electrons per 2 moles of H₂O formed (i.e., 1 mol H₂O per 2 mol e⁻). Therefore, moles of H₂O produced = 0.50 / 2 = 0.25 mol.

hard

3. In a commercial lead storage battery consisting of 6 cells connected in series, how many total moles of electrons are transferred through the external circuit when 0.50 mol of PbO₂ is consumed in each individual cell during discharge?

  1. 0.50 mol
  2. 1.00 mol
  3. 3.00 mol
  4. 6.00 mol
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Answer: (B) 1.00 mol

In series connection, the same quantity of charge (and moles of electrons) passes through every cell and the external circuit. For each cell, the reduction is PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O, which requires 2 moles of electrons per mole of PbO₂. Therefore, consuming 0.50 mol of PbO₂ per cell corresponds to (0.50 × 2) = 1.00 mol of electrons transferred through the external circuit.

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4. Consider the following two statements regarding electrochemical cells: Statement I: A primary battery like the dry cell cannot be recharged efficiently because the electrode reactions cannot be reversed by passing an external electric current. Statement II: In a Leclanché cell, NH₃ gas released at the cathode is absorbed by Zn²⁺ to form a soluble complex ion. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Primary cells undergo practically irreversible redox reactions, meaning they cannot be recharged. In Leclanché cells, the gaseous NH₃ produced at the cathode combines with Zn²⁺ ions from the anode to form the complex [Zn(NH₃)₄]²⁺, preventing pressure buildup.

easy

5. A cell that can be recharged by passing current in the opposite direction is a:

  1. Concentration cell only
  2. Secondary cell (e.g. lead storage battery)
  3. Primary cell (e.g. dry cell)
  4. Fuel cell only
Show answer and explanation

Answer: (B) Secondary cell (e.g. lead storage battery)

Secondary cells such as the lead storage battery and nickel-cadmium cell can be recharged; primary cells like the dry cell and mercury cell cannot.

NCERT Class 11

hard

6. An alkaline H₂-O₂ fuel cell consumes hydrogen gas at standard conditions (STP, where 1 mol = 22.4 L) to generate a constant current of 9.65 A for 2000 seconds. What volume of H₂ gas at STP was consumed?

  1. 1.12 L
  2. 0.56 L
  3. 4.48 L
  4. 2.24 L
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Answer: (D) 2.24 L

Charge Q = 9.65 A × 2000 s = 19,300 C. Number of moles of electrons transferred = 19,300 / 96,500 = 0.20 mol. Oxidation of H₂: H₂ + 2OH⁻ → 2H₂O + 2e⁻, so 1 mole of H₂ corresponds to 2 moles of electrons. Moles of H₂ consumed = 0.20 / 2 = 0.10 mol. Volume at STP = 0.10 mol × 22.4 L mol⁻¹ = 2.24 L.

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7. Consider the overall cell reaction of an alkaline H₂-O₂ fuel cell: 2H₂(g) + O₂(g) → 2H₂O(l). How many moles of electrons are transferred per mole of O₂ consumed?

  1. 4
  2. 2
  3. 1
  4. 8
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Answer: (A) 4

At the cathode, reduction of oxygen occurs: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq). Thus, for every 1 mole of O₂ consumed, 4 moles of electrons are transferred through the external circuit.

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8. A portable emergency device is powered by a dry cell delivering a constant current of 0.05 A. How long can the cell operate before 0.654 g of the zinc container is consumed? (Atomic mass of Zn = 65.4 u, 1 F = 96500 C mol⁻¹)

  1. 19,300 s
  2. 48,250 s
  3. 9,650 s
  4. 38,600 s
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Answer: (D) 38,600 s

Moles of Zn consumed = 0.654 / 65.4 = 0.01 mol. The reaction is Zn → Zn²⁺ + 2e⁻, so n = 2 mol of electrons per mol of Zn. Total charge Q = 0.01 × 2 × 96500 C = 1930 C. Time t = Q / I = 1930 / 0.05 = 38,600 s.

easy

9. During the discharge of a lead accumulator, which of the following changes occurs in the electrolyte and cell components?

  1. The density of H₂SO₄ increases and water is consumed
  2. H₂SO₄ is regenerated while water is consumed
  3. PbO₂ is formed at both electrodes
  4. The density of H₂SO₄ decreases and water is produced
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Answer: (D) The density of H₂SO₄ decreases and water is produced

The overall discharging reaction is Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l). Sulfuric acid is consumed and water is produced, causing the density and concentration of H₂SO₄ to decrease.

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10. Which of the following catalysts is commonly incorporated into the porous carbon electrodes of an alkaline H₂-O₂ fuel cell to accelerate electrode kinetics?

  1. Finely divided platinum or silver
  2. Nickel-cadmium alloy
  3. Lead sulfate
  4. Manganese heptoxide
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Answer: (A) Finely divided platinum or silver

In an alkaline H₂-O₂ fuel cell, finely divided catalysts like platinum, palladium, or silver are impregnated into the porous carbon electrodes to increase the rate of electrochemical electrode reactions.

hard

11. During the operation of an alkaline H₂-O₂ fuel cell, how many moles of water are formed per Faraday of electricity generated?

  1. 0.25 mol
  2. 1.00 mol
  3. 0.50 mol
  4. 2.00 mol
Show answer and explanation

Answer: (C) 0.50 mol

The overall cell reaction is 2H₂(g) + O₂(g) → 2H₂O(l), where 4 moles of electrons (4 F of charge) produce 2 moles of H₂O. Thus, moles of water formed per Faraday = 2 / 4 = 0.50 mol.

easy

12. What is the physical state and chemical nature of the substance that deposits on both the anode and cathode during the complete discharge of a lead-acid storage battery?

  1. Solid PbSO₄ precipitated on both electrodes
  2. Gaseous SO₂ evolving from both electrodes
  3. Soluble Pb(NO₃)₂ dissolved in the acid solution
  4. Spongy lead metal adhering to both electrodes
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Answer: (A) Solid PbSO₄ precipitated on both electrodes

During discharging of a lead storage cell, oxidation at the anode gives PbSO₄(s) and reduction at the cathode also produces PbSO₄(s). Consequently, insoluble lead sulfate deposits on both electrodes, causing the battery voltage to drop as the active materials and H₂SO₄ are consumed.

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