Under what condition is $E_{cell} = 0$ or $\Delta_r G = 0$?

The Gibbs energy for the decomposition of $Al_2O_3$ at $500\,^oC$ is as follows:
$\frac{2}{3}Al_2O_3 \to \frac{4}{3}Al + O_2, \Delta_rG = +940\,kJ\,mol^{-1}$
The potential difference needed for the electrolytic reduction of aluminium oxide is ............... $V$.

While charging the lead storage battery,

An electrochemical cell is constructed using half-cells in the direction of spontaneous change: $Fe(OH)_{2}(s) + 2e^{-} \rightarrow Fe(s) + 2OH^{-}(aq)$ $(E^{0} = -0.88 \text{ V})$ and $AgBr(s) + e^{-} \rightarrow Ag(s) + Br^{-}(aq)$ $(E^{0} = +0.07 \text{ V})$. Which of the following options is correct?

Write the redox reaction that occurs in a Daniell cell.

Which of the following metals will not react with a solution of $CuSO_4$?