Question

Given standard electrode potentials $Fe ^{2}:+2 e ^{-} \longrightarrow Fe E ^{\circ}=-0 \cdot 440 \,V$ $Fe ^{3+}+3 e ^{-} \longrightarrow Fe E ^{\circ}=-0 \cdot 036\, V$ The standard electrode potential $\left(E^{\circ}\right)$ for $Fe ^{3+}+ e ^{-} \longrightarrow Fe ^{2+}$ is :

• A. + 0.772 V
• B. - 0.772 V
• C. + 0.417 V
• D. -0.414 V

Answer 1

Answer: (A)

+ 0.772 V

Answer 2

We know, $\triangle G ^{\circ}=- nFE ^{\circ}$
$Fe ^{2+}+2 e ^{-} \rightarrow Fe$
$\Delta G ^{\circ}=-2 \times F \times(-0.440 V )=0.880 \,F\,\,\,\,...$ (1)
$Fe ^{3+}+3 e ^{-} \rightarrow Fe$
$\Delta G ^{\circ}=-3 \times F \times(-0.036)=0.108 \,F\,\,\,\,...$ -(2)
On subtracting Eqs. (1) from (2),
$Fe ^{3+}+ e ^{-} \rightarrow Fe ^{2+}$
$\Delta G ^{\circ}=0.108 F -0.880 F =-0.772 F$
$\Delta G ^{\circ}=- nFE ^{\circ}$
$E ^{\circ}=\frac{\Delta G ^{\circ}}{- nF }=\frac{-0.772 F }{-1 \times F }=+0.772\, V$