Ferrous ion $(Fe^{2+})$ can be oxidised by which of the following ions?

The standard reduction potentials of three metals $A, B,$ and $C$ are $+0.5 \, V, -3.0 \, V,$ and $-1.2 \, V$ respectively. What is the order of their reducing power?

$Zn^{2+} + 2e^- \to Zn_{(s)}$; $E^o = -0.76 \ V$
$Fe^{3+} + e^- \to Fe^{2+}$; $E^o = +0.77 \ V$
$Cr^{3+} + 3e^- \to Cr_{(s)}$; $E^o = -0.74 \ V$
$H^{+} + e^- \to 1/2 H_2$; $E^o = 0.00 \ V$
Which of the following is the strongest reducing agent?

For the reactions $Ag^{+} (aq) + e^{-} \rightarrow Ag_{(s)}$ and $Sn^{2+} (aq) + 2e^{-} \rightarrow Sn_{(s)}$,the standard electrode potentials at $25^{\circ}C$ are $0.80 \ V$ and $-0.14 \ V$ respectively. What is the standard $emf$ of the cell $Sn_{(s)} | Sn^{2+}_{(aq)} (1 \ M) || Ag^{+}_{(aq)} (1 \ M) | Ag_{(s)}$ in $volts$?

What is the standard electrode potential $(E^o)$ for the electrode represented by $Pt, O_2(1 \, atm) | 2H^+(1 \, M)$?

What is denoted by a negative $E^o$ and a positive $E^o$ value in electrochemistry?