What is the electrode potential for following reaction ?
`E_(Fe^(+3)|Fe)^(o)=-0.036V,E_(Fe^(2+)|Fe)^(o)=-0.439V`
`Fe^(3+)+e^(-) to Fe^(2+)`.

Options

A. `-0.072V`
B. `0.385` V
C. 0.770 V
D. `-0.270V`

Answer

C

Explanation

(i) `Fe^(3+)+3e^(-) to Fe,`
`E_(1)^(o)=-0.036V,DeltaG_(1)^(@)=-3xxFxx(-0.036)`
(ii) `Fe^(2+)+2e^(-) to Fe,`
`E_(2)^(@)=-0.439V,DeltaG_(2)^(@)=-2xxFxx(-0.439)`
Required reaction :
`Fe^(3+)+e^(-) to Fe^(2+)` Reduction, `DeltaG_(3)^(@)=-1xxFxxE_(3)^(@)`
Subtract (ii) from (i),
`DeltaG_(3)^(@)=DeltaG_(1)^(@)-DeltaG_(2)^(@)`
`(-1)xxFxxE_(3)^(@)=[-3xxFxx(-0.036)]-[(-2xxFxx-0.439)]`
`therefore -E_(3)^(@)=(0.108-0.878)`
`therefore E_(3)^(@)=(-0.108+0.878)`
`therefore E_(3)^(@)=0.77V`.