The standard reduction potential data at 25 ° C is given below....

P.)
$\begin{cases} & {\text{Fe}}^{3 +} + {\text{e}}^{-} ⟶ {\text{Fe}}^{2 +} & \text{;} & \Delta {\text{G}}_{\text{I}}^{\text{o}} = - 1 \text{F} \left(0 \text{.} 7 7\right) \\ & {\text{Fe}}^{2 +} + 2 {\text{e}}^{-} ⟶ \text{Fe} & \text{;} & \Delta {\text{G}}_{\text{II}}^{\text{o}} = - 2 \text{F} \left(- 0 \text{.} 4 4\right) \\ & \text{----------------------------------} & & \\ \text{Add} & {\text{Fe}}^{3 +} + 3 {\text{e}}^{-} ⟶ \text{Fe} & \text{;} & \Delta {\text{G}}_{\text{III}}^{\text{o}} = - 3 \text{F} {\left({\text{E}}^{\text{o}}\right)}_{{\text{Fe}}^{3 +} / \text{Fe}}\end{cases}$
$- 3 \text{F} {\left({\text{E}}^{\text{o}}\right)}_{{\text{Fe}}^{3 +} / \text{Fe}} = - 0 \text{.} 7 7 \text{F} + 0 \text{.} 8 8 \text{F}$
=$0 . 11F$
${\text{E}}_{{\text{Fe}}^{3 +} / \text{Fe}}^{\text{o}} = - \dfrac{0 \text{.} 1 1}{3} = - 0 \text{.} 0 3 7 ≅ - 0 \text{.} 0 4 \text{V}$
Q.) $2 {\text{H}}_{2} \text{O} ⟶ {\text{O}}_{2 \left(\text{g}\right)} + 4 {\text{H}}_{\text{(aq)}}^{+} + 4 {\text{e}}^{-}$ (Oxidation half reaction)
${E}^{°} = - 1 . 23V$
${\text{O}}_{2 \left(\text{g}\right)} + 2 {\text{H}}_{2} \text{O} + 4 {\text{e}}^{-} ⟶ 4 {\text{OH}}_{\text{(aq)}}^{-}$ (reduction half reaction)
${E}^{°} = + 0 . 40V$
It's an electrochemical cell, adding
$4 {\text{H}}_{2} \text{O} ⇌ 4 {\text{H}}^{+} + 4 0 {\text{H}}^{-}$ at equilibrium
${\text{E}}_{\text{cell}}^{\text{o}} = - 0 \text{.} 8 3 \text{V}$
R.)
\begin{cases}2 {\text{Cu}}_{\left(\text{s}\right)} ⟶ 2 {\text{Cu}}^{+} + 2 {\text{e}}^{-} & {\text{E}}^{0} = - 0 \text{.} 5 2 \text{V} \\ {\text{Cu}}^{2 +} + 2 {\text{e}}^{-} ⟶ {\text{Cu}}_{\left(\text{s}\right)} & {\text{E}}^{0} = 0 \text{.} 3 4 \text{V}\end{cases}$$<br />Adding$\begin{cases}{\text{Cu}}^{2 +} + {\text{Cu}}_{\left(\text{s}\right)} ⟶ 2 {\text{Cu}}^{+} & {\text{E}}_{\text{cell}}^{0} = - 0 \text{.} 1 8 \text{V}\end{cases}$<br />S<br />$\begin{cases}{\text{Cr}}^{3 +} + 3 {\text{e}}^{-} ⟶ \text{Cr} & \text{;} & \Delta {\text{G}}_{\text{I}}^{0} = - 3 \left(\text{F}\right) \left(- 0 \text{.} 7 4\right) \\ {\text{Cr}}^{2 +} + 2 {\text{e}}^{-} ⟶ \text{Cr} & \text{;} & \Delta {\text{G}}_{\text{II}}^{0} = - 2 \left(\text{F}\right) \left(- 0 \text{.} 9 1\right)\end{cases}$<br /><br />Subtracting<br />$\begin{cases}{\text{Cr}}^{3 +} + {\text{e}}^{-} - {\text{Cr}}^{2 +} ⟶ 0 & \text{;} & \\ {\text{Cr}}^{3 +} + {\text{e}}^{-} ⟶ {\text{Cr}}^{2 +} & \text{;} & \Delta {\text{G}}^{0} = - 1 \left(\text{F}\right) \left({\text{E}}_{{\text{Cr}}^{3 +} / {\text{Cr}}^{2 +}}^{0}\right)\end{cases}$<br />$- {\text{F E}}_{\left({\text{Cr}}^{3 +} / {\text{Cr}}^{2 +}\right)}^{0} = \text{F} \left(3 \times 0 \text{.} 7 4 - 2 \times 0 \text{.} 9 1\right)$<br />$- {\text{E}}_{{\text{Cr}}^{3 +} / {\text{Cr}}^{2 +}}^{0} = \text{F} \left(2 \text{.} 2 2 - 1 \text{.} 8 2\right)$<br />${\text{E}}_{{\text{Cr}}^{3 +} / {\text{Cr}}^{2 +}}^{0} = - 0 \text{.} 4 0 \text{V}$