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

Efficiency of a cell with cell reaction under standard conditions is $70\%$. Cell reaction:
$A_{(s)} + B^{\oplus} \longrightarrow A^{\oplus} + B_{(s)}$ ; $\Delta H^o = -551.5 \ kJ$. Calculate standard electrode potential of cell (in $volt$).

Give the chemical reaction when a $Cu$ rod is kept in an $AgNO_{3}$ solution.

What is the approximate standard free energy change per mole of $Zn$ (in $kJ \ mol^{-1}$) for a Daniell cell at $298 \ K$?

The reaction $\frac{1}{2} H_{2(g)} + AgCl_{(s)} \rightarrow H_{(aq)}^{+} + Cl_{(aq)}^{-} + Ag_{(s)}$ occurs in which of the following galvanic cells?

Consider the galvanic cell: $A_{(s)} | A_{(aq)}^{+2} || B_{(aq)}^{+} | B_{(s)}$. If the $EMF$ of the cell is positive,identify the correct cell reaction from the following.