Which of the following devices function on redox reaction? l. Dry cell ll. Car battery III. Electric generator

Analyze the question: We need to identify which devices work through a redox (oxidation-reduction) reaction, meaning a chemical reaction where electrons are transferred.

Concept: Redox reactions involve oxidation (loss of electrons) and reduction (gain of electrons) happening together. Electrochemical cells convert chemical energy into electrical energy using redox reactions.

Checking each device:

1. Dry cell (Leclanché cell): Zinc is oxidized at the negative terminal: Zn → Zn²⁺ + 2e⁻. Manganese(IV) oxide is reduced at the positive terminal. This is a redox reaction. ✔

2. Car battery (lead-acid accumulator): On discharge, lead is oxidized: Pb + SO₄²⁻ → PbSO₄ + 2e⁻. Lead(IV) oxide is reduced: PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O. This is a redox reaction. ✔

3. Electric generator: A generator produces electricity by moving a coil of wire through a magnetic field (electromagnetic induction). This is a physical process, converting mechanical energy to electrical energy. No chemical change or electron transfer between species takes place. ✘

Conclusion: Only the dry cell (I) and the car battery (II) work by redox reactions.

Answer: C. I and II only

Exam tip: Remember that anything with a chemical "cell" or "battery" uses redox. Generators, dynamos and turbines are mechanical or magnetic devices (physics, not chemistry). When you see "generator" in a redox question, it's usually the distractor.