We know for a spontaneous reaction
$\Delta G \,<\,0 \text {. }$
$\therefore- RT \ln K \,<\,0$
$\therefore \ln K\,>\,0$
$\therefore K\,>\,1$
and $- nF E ^{\circ}<0$
$\therefore E _{\text {cell }}^{\circ}\,>\,0$
$(i)$ $A + e \rightarrow A^{-} ; E^o= -0.24 V$
$(ii)$ $B^{-} + e \rightarrow B^{2-}; E^o = + 1.25 V$
$(iii)$ $C^{-} + 2e \rightarrow C^{3-}; E^o = -1.25 V $
$(iv)$ $D + 2e \rightarrow D^{2-}; E^o = + 0.68 V$
$Z{n^{2 + }}\,(aq)\, + \,2e\, \rightleftharpoons \,Zn\,(s)\,;\, - \,0.762\,V$
$C{r^{3 + }}\,(aq)\, + \,3e\, \rightleftharpoons \,Cr(s)\,;\, - \,0.740\,\,V$
$2{H^ + }\,(aq)\, + \,2e\, \rightleftharpoons \,{H_2}(g)\,;\,\,\,0.00\,\,\,V$
$F{e^{3 + }}\,(aq)\, + \,e\, \rightleftharpoons \,F{e^{2 + }}(aq)\,;\,\,\,0.770\,\,\,V$
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