Question

Given $:E_{F{e}^{3+}/Fe}^o=-0.036V,E_{F{e}^{2+}/Fe}^o=-0.439V$. The value of standard electrode potential for the change, $F{e}_{\left(aq\right)}^{3+}+e^{-}\to F{e}^{2+}\left(aq\right)$ will be :

• A. $-0.072V$
• B. $0.385V$
• C. $0.770V$
• D. $-0.270V$

Answer 1

Answer: (C)

$\because F{e}^{3+}+3e^{-}\to Fe;E^o=-0.036V$
$\therefore \Delta G_1^o=-nF{E}^o=-3F\left(-0.036\right)$
$=+0.108F$
Also $F{e}^{2+}+2e^{-}\to Fe;E^o=-0.439V$
$\therefore \Delta G_2^o=-nF{E}^o$
$=-2F\left(-0.439\right)$
$=0.878F$
To find $E^o$ for $F{e}_{\left(aq\right)}^{3+}+e^{-}\to F{e}^{2+}\left(aq\right)$
$\Delta G^o=-nF{E}^o=-1F{E}^o$
$\because G^o=G_1^o-G_2^o$
$\because G^o=G_1^o-G_2^o$
$\therefore G^o=0.108F-0.878F$
$\therefore -F{E}^o=+0.108F-0.878F$
$\therefore E^o=0.878-0.108$
$=0.77v$