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Which one of the following reactions shows sulfuric acid acting as an oxidizing agent?

1. NaCl + H2SO4 → NaHSO4 + HCl
2. 2PCl5 + H2SO4 → 2POCl3 + 2HCl + SO2Cl2
3. 2HI + H2SO4 → I2 + SO2 + 2H2O
4. Ca(OH)2 + H2SO4 → CaSO4 + 2H2O

Correct answer: 2HI + H2SO4 → I2 + SO2 + 2H2O

Solution

In 2HI + H2SO4 -> I2 + SO2 + 2H2O, sulfur goes +6 to +4 (reduced) while I- is oxidised to I2, so H2SO4 acts as the oxidising agent. The stored reaction (with PCl5) involves no change in sulfur's oxidation state.

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