Question

Standard electrode potentials of
$F{e}^{2+}+2e^{–}\to Fe$ and $F{e}^{3+}+3e^{–}\to Fe$
are $–0.440V$ and $–0.036V$ respectively.
The standard electrode potential ($E^0$) for
$F{e}^{3+}+e^{–}\to F{e}^{2+}$ is -

• A. $–0.476V$
• B. $–0.404V$
  
• C. $+0.404V$
• D. $+0.772V$

Answer 1

The correct option is D $+0.772V$

$F{e}^{2+}+2e^{-}\to Fe$ $E_1^0$ ​= −0.440V ......1

$F{e}^{3+}+3e^{-}\to Fe$ $E_2^0$ ​= −0.036V ......2

$\Delta G^0=-nF{E}_{cell}^0$​

It is given in the question that
$F{e}^{3+}+e^{-}\to F{e}^{2+}$......3
to obtain the equation 3,
We must subtract 1 from 2

$\Delta G_{cell}^0=\Delta G_2-\Delta G_1$​


$-nF{E}_{cell}^0=-n_{2}F{E}_2^0-(-n_{1}F{E}_1^0)$

$n_2=3$

$n_1=2$

For the equation 3, $n=1$
$-E_{cell}^0=-3\times E_2^0+2\times E_1^0$

$-E_{cell}^0=-3\times -0.0036+2\times -0.44$

$-E_{cell}^0$ ​= 0.108-0.88

$E_{cell}^0$ ​= −0.108+0.88

=+0.772