Question

At $298\, K$, the standard reduction potentials are $1.51\, V$ for $MnO _{4}^{-} \mid Mn ^{2+} 1.36\, V$ for $Cl _{2} \mid Cl ^{-} 1.07\, V$ for $Br _{2} \mid Br ^{-}$, and $0.54\, V$ for $\left. H _{2}\right|^{-}$. At $pH =3$, permanganate is expected to oxidize: $\left(\frac{R T}{F}=0.059\, V \right)$

• A. $Cl ^{-}, Br ^{-}$ and $I ^{-}$
• B. $Cl ^{-}$ and $Br ^{-}$
• C. $Br ^{-}$ and $I ^{-}$
• D. $I^-$ only

Answer 1

The Correct Option is C

To determine which species permanganate (\( \text{MnO}_4^- \)) is expected to oxidize at \( pH = 3 \), we should analyze the standard reduction potentials provided and the conditions affecting these potentials.

1. Standard reduction potentials are given for various half reactions:
   • \( \text{MnO}_4^- \mid \text{Mn}^{2+} \) : \( 1.51\, \text{V} \)
   • \( \text{Cl}_2 \mid \text{Cl}^- \) : \( 1.36\, \text{V} \)
   • \( \text{Br}_2 \mid \text{Br}^- \) : \( 1.07\, \text{V} \)
   • \( \text{H}_2 \mid \text{H}^+ \) : \( 0.00\, \text{V} \) (standard hydrogen electrode)
2. Species with lower reduction potentials are more likely to be oxidized. Therefore, we compare the reduction potentials of chloride (\( \text{Cl}^- \)), bromide (\( \text{Br}^- \)), and iodide (\( \text{I}^- \)) ions.
3. \( \text{Br}^- \mid \text{Br}_2 \), with a reduction potential of \( 1.07\, \text{V} \), and iodide ions (\( \text{I}^- \)) are expected to get oxidized in the presence of permanganate.
4. Given the permanganate reduction potential (\( 1.51\, \text{V} \)), permanganate can oxidize substances with lower reduction potentials. Here, bromide (\( 1.07\, \text{V} \)) and iodide (assumed lower than bromide, commonly around \( 0.54\, \text{V} \)) fit this criteria.
5. \( \text{Cl}^- \) is not mentioned with a specific potential lower than \( 1.07\, \text{V} \) in this case, and traditionally, its potential is typically high, making it less likely to be oxidized compared to bromide and iodide.

At \( pH = 3 \), the potential may slightly alter for all reactions due to the Nernst equation effects, but the relative comparison remains consistent.

Conclusion: Based on the standard reduction potentials and in line with the given options, permanganate \(( \text{MnO}_4^- )\) is expected to oxidize \( \text{Br}^- \) and \( \text{I}^- \) at \( pH = 3 \).