Redox and Equivalent ConceptHard
Question
At 298 K, the standard reduction potentials are 1.51 V for MnO4- |Mn2+, 1.36 V for Cl2|Cl- , 1.07 V for Br2|Br, and 0.54 V for I2|I-. At pH = 3, permanganate is expected to oxidize :(RT/F = 0.059 V)
Options
A.Cl-, Br- and I-
B.Br- and I-
C.Cl- and Br-
D.I- Only
Solution
MnO + 8H+ + 5e- → Mn2+ + 4H2O
E = 1.51 -
Taking Mn2+ and MnO4- in standard state i.e. 1 M,
E = 1.51 -
× 8 log 
= 1.51 -
× 8 × 3 = 1.2268 V
Hence at this pH, MnO4- will oxidise only Br- and I- as SRP of Cl2/Cl- is 1.36 V which is greater than that for MnO4- /Mn2+.
E = 1.51 -
Taking Mn2+ and MnO4- in standard state i.e. 1 M,
E = 1.51 -
× 8 log = 1.51 -
× 8 × 3 = 1.2268 VHence at this pH, MnO4- will oxidise only Br- and I- as SRP of Cl2/Cl- is 1.36 V which is greater than that for MnO4- /Mn2+.
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