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Which of the following chemical reactions is correct?

1. Br⁻ + H⁺ -> HBr; then 2HBr + 2H2SO4 -> Br2 (reddish-brown) + SO2 + SO4²- + H2O
2. 4NO3⁻ + 2H2SO4 -> 4NO2 (reddish-brown) + O2 + 2SO4²- + 2H2O
3. SO3²- + 2H⁺ -> SO2 (colourless gas) + H2O
4. 3I⁻ + 2H2SO4 -> I3⁻ (violet) + SO4²- + 2H2O + SO2

Correct answer: SO3²- + 2H⁺ -> SO2 (colourless gas) + H2O

Solution

Option C (SO3²- + 2H⁺ -> SO2 + H2O) is a correctly balanced reaction: sulphite ions reacting with acid produce sulphur dioxide gas (colourless, pungent) and water. Option A's second step is unbalanced and incorrect. Option B is missing electrons/charges — the actual reaction of NO3⁻ with H2SO4 does not produce O2. Option D's product I3⁻ is wrong (should be I2).

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