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Batteries and fuel cells… (Class 12 Chemistry 2024, Q29)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2024: question 29

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedElectrochemistry

Batteries and fuel cells are very useful forms of galvanic cell. Any battery or cell that we use as a source of electrical energy is basically a galvanic cell. However, for a battery to be of practical use it should be reasonably light, compact and its voltage should not vary appreciably during its use. There are mainly two types of batteries — primary batteries and secondary batteries.
In the primary batteries, the reaction occurs only once and after use over a period of time the battery becomes dead and cannot be reused again, whereas the secondary batteries are rechargeable.
Production of electricity by thermal plants is not a very efficient method and is a major source of pollution. To solve this problem, galvanic cells are designed in such a way that energy of combustion of fuels is directly converted into electrical energy, and these are known as fuel cells. One such fuel cell was used in the Apollo space programme.
Answer the following questions :

  1. (a)

    How do primary batteries differ from secondary batteries ?

    1 mark
  2. (b)

    The cell potential of Mercury cell is 1·35 V, and remains constant during its life. Give reason.

    1 mark
  3. (c)

    Write the reactions involved in the recharging of the lead storage battery.

    2 marks

  4. OR

    (c)

    Write two advantages of fuel cells over other galvanic cells.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Recall the Nernst equation — the potential changes only if concentrations of dissolved species change.

Hint 2

Recharging is simply the discharge reactions reversed: PbSO₄ is converted back to Pb and PbO₂.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) In primary batteries the reaction occurs only once and they cannot be recharged (e.g. dry cell); secondary batteries can be recharged by passing current in the opposite direction and reused (e.g. lead storage battery).

    1 mark

  2. (b) The overall reaction involves no ion in solution whose concentration changes during use, so the cell potential stays constant at 1.35 V.

    1 mark

  3. (c) On recharging, the cell works as an electrolytic cell. At one electrode: ; at the other:

    1 mark

  4. Overall recharging reaction:

    1 mark

  5. (c) OR: Advantages of fuel cells: (i) high efficiency (about 70%) since chemical energy of fuel is directly converted to electricity; (ii) pollution-free — the H₂–O₂ cell produces only water. (They also give continuous supply as long as reactants are fed.)

Answer: (a) Primary – not rechargeable; secondary – rechargeable; (b) no ions in solution whose concentration changes; (c) 2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄ / OR high efficiency and pollution-free

Where the marks are

  • Correct difference: rechargeability (1 mark)
  • Overall reaction of mercury cell has no ion in solution whose concentration changes (1 mark)
  • Electrode reactions on recharging (1 mark) and overall reaction (1 mark), OR two advantages of fuel cells (1 + 1 mark)

Revise the chapter

  • Chapter: Electrochemistry (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Electrochemistry, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 56/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.