JEE Advanced 2014 · Paper 1, Chemistry question 35

Oxidise I⁻ to I₂:

• Acidified $\ce{K2Cr2O7}$: $\ce{Cr2O7^{2-} + 6I- + 14H+ -> 2Cr^{3+} + 3I2 + 7H2O}$. Yes.
• $\ce{CuSO4}$: $\ce{2Cu^{2+} + 4I- -> 2CuI + I2}$. Yes.
• $\ce{H2O2}$: $\ce{H2O2 + 2I- + 2H+ -> I2 + 2H2O}$. Yes.
• $\ce{Cl2}$, $\ce{O3}$: both give $\ce{I2}$. Yes.
• $\ce{FeCl3}$: $\ce{2Fe^{3+} + 2I- -> 2Fe^{2+} + I2}$. Yes.
• $\ce{HNO3}$: oxidises iodide to iodine. Yes.

Do not:

• Alkaline $\ce{KMnO4}$ oxidises iodide to iodate, $\ce{I- + 2MnO4- + H2O -> IO3- + 2MnO2 + 2OH-}$, not to iodine.
• $\ce{Na2S2O3}$ is a reducing agent.

Answer: 7