Question

Calculate the standard electrode potential $\left(E^{o}\right)$ for
$Fe ^{+++}+e^{-} \rightarrow Fe ^{++}$
From the given standard electrode potentials value of two half cells
$Fe ^{++}+2 e^{-} \rightarrow Fe ; E^{o}=-0.440 \,V$
$Fe ^{+++}+3 e^{-} \rightarrow F e ; E^{o}=-0.036\, V$

• A. $ + 0.772\, V $
• B. $- 0.404 \,V$
• C. $+ 0.404 \,V$
• D. $ - 0.476 \,V$

Answer 1

Answer: (A)

$\Delta G^{o}=-n E^{o} F$
$F e^{2+}+2 e^{-} \rightarrow F e \ldots .$ (i)
$\Delta G^{o}=-2 \times F \times(-0.440\, V)=0.880\, F$
$F e^{3+}+3 e^{-} \rightarrow F e . . .$ (ii)
$\Delta G ^{\circ}=-3 \times F \times(-0.036)=0.108 \,F$
On subtracting equation (i) from (ii)
$F e^{3+}+e^{-} \rightarrow F e^{2+}$
$\Delta G ^{\circ}=0.108 \,F -0.880\, F =-0.772\, F$
$E^{o}$ for the reaction $=-\frac{\Delta G^{o}}{n F}=-\frac{(-0.772\, F)}{1 \times F}=+0.772\, V$