Question

E.M.F. diagram for some ions is given as:

$Fe{O}_4^{2-}\stackrel{E^o=+2.20V}{\to }F{e}^{3+}\stackrel{E^o=0.77V}{\to }F{e}^{2+}F{e}^{2+}\stackrel{E^o=0.445V}{\to }F{e}^0$

The value of $E_{Fe{O}_4^{2-}/F{e}^{2+}}^o$ is _________(as nearest integer).

Answer 1

Given : $F{e}^{6+}+3e\to F{e}^{3+};$ $-\Delta G_1^o=3\times 2.20\times F$ ...... (i)

$F{e}^{3+}+e\to F{e}^{2+};$ $-\Delta G_2^o=1\times 0.77\times F$ ...... (ii)

$F{e}^{2+}+2e\to F{e}^0;$ $-\Delta G_3^o=2\times (-0.445)\times F$ ...... (iii)

By equation (i) and (ii)

$F{e}^{6+}+4e\to F{e}^{2+};$ $-\Delta G_4^o=-\Delta G_1^o-\Delta G_2^o$

$\because 4\times E_4^o\times F=3\times 2.20\times F+(1\times 0.77\times F)$

$\therefore E_4^o=1.84V$

or $F{e}^{6+}+4e\to F{e}^{2+};$ $E^o=1.84\approx 2V$