Chemistry · Electrochemistry · chapter 2 of 10

Class 12 Chemistry Chapter 2: Electrochemistry case study question with answers

One case-based question of the kind the board asks: a passage, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks
Anika sets up a Daniell cell, a zinc rod in 1 M ZnSO₄ and a copper rod in 1 M CuSO₄ joined by a salt bridge, and connects it to an external source whose voltage opposes the cell. A galvanometer in the circuit shows the direction of current. She raises the opposing voltage in three steps, 0.8 V, 1.1 V and 1.4 V, and after each step watches the galvanometer and, over some minutes, the two rods.
Part 1 · 1 mark
With an opposing voltage of 0.8 V, electrons in the external circuit flow
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Answer: (a) from the zinc rod to the copper rod

Below 1.1 V the cell reaction Zn + Cu²⁺ → Zn²⁺ + Cu continues; electrons leave the zinc and flow to the copper.

Part 2 · 1 mark
At exactly 1.1 V the galvanometer reads zero. This shows that
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Answer: (b) the cell reaction has stopped and no chemical change occurs

When the opposing potential equals the cell's 1.1 V, no electrons or current flow and the reaction stops.

Part 3 · 2 marks
At 1.4 V, write what happens at each rod and name the kind of cell the device has become.
Answer and marking scheme

Above 1.1 V the reaction runs in reverse: Zn²⁺ + 2e⁻ → Zn, so zinc is deposited on the zinc rod, and Cu → Cu²⁺ + 2e⁻, so the copper rod dissolves. Electrical energy now drives a non-spontaneous reaction: the device is an electrolytic cell.

  • zinc deposited on the zinc rod and copper dissolving from the copper rod (1)
  • electrolytic cell: electrical energy drives a non-spontaneous reaction (1)
Or
At 1.4 V, which rod is the cathode and which the anode, and what are their signs? Give the direction of the current in the external circuit.
Answer and marking scheme

The roles reverse: the zinc rod, where Zn²⁺ is reduced, is the cathode (+ve) and the copper rod, where Cu is oxidised, is the anode (−ve). Current now flows from zinc to copper in the external circuit.

  • zinc rod cathode (+ve), copper rod anode (−ve) (1)
  • current flows from Zn to Cu (1)
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