Question

E.M.F. diagram for some ions is given as
$Fe{O}_4^{2-}\stackrel{E^o=+2.20V}{\to }F{e}^{3+}\stackrel{E^o=+0.77V}{\to }F{e}^{2+}\stackrel{E^o=-0.445V}{\to }F{e}^o$
Determine the value of $E_{Fe{O}_4^{2-}/F{e}^{2+}}^o$

• A. $1.84V$
• B. $1.42V$
• C. $1.3V$
• D. $2.0V$

Answer 1

Answer: (A)

$F{e}^{6+}+3e\to F{e}^{3+};$ $-\Delta G_1^o=3\times 2.20\times F$ …(i)
$F{e}^{3+}+e\to F{e}^{2+};$ $-\Delta G_2^o=1\times 0.77\times F$ …(ii)
$F{e}^{2+}+2e\to F{e}^o;$ $-\Delta G_3^o=2\times (-0.445)\times F$ …(iii)
Adding equation (i) and (ii)
$F{e}^{6+}+4e\to F{e}^{2+};$ $-\Delta G_4^o=-\Delta G_1^o-\Delta G_2^o$
Hence $4\times E_4^o\times F=3\times 2.20\times F+\left(1\times 0.77\times F\right)$
So $E_4^o=1.84V$
or $F{e}^{6+}+4e\to F{e}^{2+};$ $E^o=1.84V$