Which of the following equations represents a redox reaction?

Analyze the question: A redox reaction is one in which oxidation numbers change: one species is oxidized (loses electrons) and another is reduced (gains electrons).

Check each option:

• A. HNO₃ + NaOH → NaNO₃ + H₂O is an acid-base neutralisation. Oxidation numbers stay the same (N stays +5, Na +1, H +1, O -2). Not redox.
• B. AgNO₃ + KCl → AgCl + KNO₃ is a precipitation (double displacement) reaction. Ag stays +1, Cl stays -1. Not redox.
• C. CaCO₃ → CaO + CO₂ is thermal decomposition. Ca stays +2, C stays +4, O stays -2. Not redox.
• D. H₂S + SO₂ → H₂O + S. This is the classic redox reaction. • Sulfur in H₂S: -2 → 0 in S (oxidation, loses electrons) • Sulfur in SO₂: +4 → 0 in S (reduction, gains electrons) • H₂S is the reducing agent and SO₂ is the oxidising agent.

Note on the typing of option D: As printed, D has errors ("HO₂" and "3S₍g₎", and H₂S written as solid). The correctly balanced equation is:

2H₂S + SO₂ → 2H₂O + 3S

The intended product is water (H₂O), not "HO₂", and sulfur is deposited as a solid.

Answer: D

Tip: In a redox question, look for an element that changes oxidation number, especially a reaction where the same element is produced in its free (elemental) form, such as S, Cl₂, or a metal. Neutralisation, precipitation, and simple decomposition of carbonates are almost never redox.