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The standard redox potentials for the re...

The standard redox potentials for the reactions `Mn^(2+) + 2e^(-) rarr Mn and Mn^(3+) + e^(-) rarr Mn^(2+)` are -1.18 V and 1.51 V respectively. What is the redox potential for the reaction `Mn^(3+) + 3e^(-) rarr Mn` ?

A

0.33 V

B

1.69 V

C

`-0.28 V`

D

`-0.85 V`

Text Solution

Verified by Experts

The correct Answer is:
C
Given :
(i) `Mn^(2+) + 2e^(-)rarr Mn, E^(@) = - 1.18, Delta G_(1)^(@) = - 2 xx F xx - 1.18 J`
(ii) `Mn^(3+) + e^(-) rarr Mn^(2+), E^(@) = 1.51 V, Delta G_(2)^(@) = - 1 xx F xx 1.51 J`
Aim : `Mn^(3+) + 3e^(-) rarr Mn, Delta G_(3)^(@) = ?`
(i) + (ii) gives the required results,
i.e., `Delta G_(3)^(@) = Delta G_(1)^(@) + Delta G_(2)^(@)`
`= 2.36 F + (-1.51 F) = 0.85 F`
`therefore -nFE_(Mn^(3+)//Mn)^(@) = 0.85 F`
or `E_(Mn^(3+)//Mn)^(@) = - (0.85)/(3)V" "(because n = 3)`
= - 0.28 V
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