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ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 4 marks · Case-based

CBSE Class 12 Chemistry sample paper 2023-24: question 30

From the official CBSE sample question paper for 2023-24, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedElectrochemistry

The lead-acid battery represents the oldest rechargeable battery technology. Lead acid batteries can be found in a wide variety of applications including small-scale power storage such as UPS systems, ignition power sources for automobiles, along with large, grid-scale power systems. The spongy lead act as the anode and lead dioxide as the cathode. Aqueous sulphuric acid is used as an electrolyte. The half-reactions during discharging of lead storage cells are:
Anode:
Cathode:
There is no safe way of disposal and these batteries end-up in landfills. Lead and sulphuric acid are extremely hazardous and pollute soil, water as well as air. Irrespective of the environmental challenges it poses, lead-acid batteries have remained an important source of energy.
Designing green and sustainable battery systems as alternatives to conventional means remains relevant. Fuel cells are seen as the future source of energy. Hydrogen is considered a green fuel. Problem with fuel cells at present is the storage of hydrogen. Currently, ammonia and methanol are being used as a source of hydrogen for fuel cell. These are obtained industrially, so add to the environmental issues.
If the problem of storage of hydrogen is overcome, is it still a "green fuel?" Despite being the most abundant element in the Universe, hydrogen does not exist on its own so needs to be extracted from the water using electrolysis or separated from carbon fossil fuels. Both of these processes require a significant amount of energy which is currently more than that gained from the hydrogen itself. In addition, this extraction typically requires the use of fossil fuels. More research is being conducted in this field to solve these problems. Despite the problem of no good means to extract Hydrogen, it is a uniquely abundant and renewable source of energy, perfect for our future zero-carbon needs.
Answer the following questions:

  1. (a)

    How many coulombs have been transferred from anode to cathode in order to consume one mole of sulphuric acid during the discharging of lead storage cell?

    1 mark
  2. (b)

    How much work can be extracted by using lead storage cell if each cell delivers about 2.0 V of voltage? (1 F = 96500 C)

    1 mark
  3. (c)

    Do you agree with the statement – "Hydrogen is a green fuel." Give your comments for and against this statement and justify your views.

    2 marks
  4. OR

    (c)

    Imagine you are a member of an agency funding scientific research. Which of the following projects will you fund and why?
    (i) safe recycling of lead batteries
    (ii) extraction of hydrogen

    2 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Add the two half-reactions and count moles of H₂SO₄ against moles of electrons.

Hint 2

Maximum work equals –ΔG = nFE for the cell reaction.

Hint 3

For (c), consider both how hydrogen is used and how it is produced at present.

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) Overall reaction: . 2 mol e⁻ (2 F) are transferred while 2 mol H₂SO₄ are consumed, so 1 mol H₂SO₄ needs 1 F = 96500 C.

    1 mark
  2. (b) (per mole of cell reaction).

    1 mark
  3. (c) For: Hydrogen burns (or reacts in a fuel cell) to give only water, with no CO₂ or other pollutants, and it is abundant and renewable. Against: today it is obtained by electrolysis using electricity from fossil fuels or by separation from fossil fuels (via ammonia/methanol). These steps use more energy than the hydrogen gives and add carbon emissions. View: hydrogen is green at the point of use, but not yet green overall. It becomes truly green only when extracted by clean methods such as solar-powered electrolysis, so research in this area is needed.

    2 marks
  4. (c) OR (either choice with justification earns credit): (i) Safe recycling of lead batteries — they are the most widely used rechargeable batteries (UPS, vehicles, grid storage), but lead and H₂SO₄ dumped in landfills pollute soil, water and air. Research into safe recycling would cut these hazards and recover lead. OR (ii) Extraction of hydrogen — a fuel cell is a clean energy source giving only water. Its main barrier is clean, low-energy extraction of hydrogen, and research here would give a sustainable zero-carbon fuel. (Alternative part: 1 mark importance + 1 mark need for research)

Answer: (a) 96500 C (1 F); (b) 386000 J (386 kJ); (c) Hydrogen is clean in use, but its present extraction is not green — justified view

Where the marks are

  • (a) 2 F for 2 mol H₂SO₄, so 96500 C per mole (1 mark)
  • (b) W = nFE = 2 × 96500 × 2.0 = 386000 J (1 mark)
  • (c) Argument for (clean combustion product water) and against (energy-intensive, fossil-fuel-based extraction) with a justified conclusion (2 marks)
  • (c) OR: importance of the chosen project (1 mark) and need for the research (1 mark)

Revise the chapter

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.