In the electrochemical cell $H_{2(g)}, 1 \ atm \ | \ H^+_{(aq)}, 1 \ M \ || \ Cu^{2+}_{(aq)}, 1 \ M \ | \ Cu_{(s)}$,which of the following statements is true?

The standard reduction potentials at $298 \ K$ for the following half-cells are given below:
$Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$,$E^{\circ} = 1.33 \ V$
$Fe^{3+} + 3e^- \rightarrow Fe$,$E^{\circ} = -0.04 \ V$
$Ni^{2+} + 2e^- \rightarrow Ni$,$E^{\circ} = -0.25 \ V$
$Ag^+ + e^- \rightarrow Ag$,$E^{\circ} = 0.80 \ V$
$Au^{3+} + 3e^- \rightarrow Au$,$E^{\circ} = 1.40 \ V$
Consider the given electrochemical reactions. The number of metal$(s)$ which will be oxidized by $Cr_2O_7^{2-}$ in aqueous solution is $.....$

The following four solutions are kept in separate beakers and copper metal is put in each of them. Which solution will become blue after some time?

What is the change in oxidation number of $Pb$ at the positive electrode of a lead accumulator acting as a galvanic cell?

Which statement is correct for a fuel cell?

Write a note on mercury cell $(Zn-Hg)$.