The reduction potential of hydrogen half cell will be negative if :

The correct order of $$E_{{M^{2 + }}/M}^o$$ values with negative sign for the four successive elements Cr, Mn, Fe and Co is :

The Gibbs energy for the decomposition of Al₂O₃ at 500^oC is as follows :

$${2 \over 3}A{l_2}{O_3}$$ $$\to$$ $${4 \over 3}Al + {O_2}$$, $${\Delta _r}G$$ = + 966 kJ mol^–1

The potential difference needed for electrolytic reduction of Al₂O₃ at 500^oC is at least :

Given : $$E_{F{e^{3 + }}/Fe}^o$$ = -0.036V; $$E_{F{e^{2 + }}/Fe}^o$$ = -0.439 V
The value of standard electrode potential for the change,
Fe³⁺ (aq) + e^- $$\to$$ Fe²⁺ (aq) will be