Two half cell reactions are given below : $\mathrm{CO}^{3+}+\mathrm{e}^{-} \rightarrow \mathrm{Co}^{2+},…
Two half cell reactions are given below :
$\mathrm{CO}^{3+}+\mathrm{e}^{-} \rightarrow \mathrm{Co}^{2+}, \mathrm{E}^{\circ} \mathrm{Co}^{2+} / \mathrm{Co}^{3+}=-1.81 \mathrm{~V}$
$2 \mathrm{Al}^{3+}+6 \mathrm{e}^{-} \rightarrow 2 \mathrm{Al}(\mathrm{s}), \mathrm{E}^{\mathrm{o}} \mathrm{Al} / \mathrm{Al}^{3+}=+1.66 \mathrm{~V}$
The standard EMF of a cell with feasible redox reaction will be :
$+7.09 \mathrm{~V}$
$+0.15 \mathrm{~V}$
$+3.47 \mathrm{~V}$
$-3.47 \mathrm{~V}$
Solution
The cell reaction is written as :
$\begin{aligned}
& \mathrm{E}^{\circ} \mathrm{Co}^{2+} / \mathrm{Co}^{3+}=-1.81 \mathrm{~V}(\mathrm{SOP}) \\
& \therefore \quad \mathrm{E}^{\circ} \mathrm{Co}^{3+} / \mathrm{Co}^{2+}=+1.81 \mathrm{~V} \text { (SRP) } \\
& \mathrm{E}_{\mathrm{A} / / \mathrm{Al}^{3+}}^{\circ}=+1.66 \mathrm{~V}(\mathrm{SOP}) \\
& \therefore \mathrm{E}_{\mathrm{Al}^{3+} / \mathrm{Al}}^{\circ}=-1.66 \mathrm{~V}(\mathrm{SRP}) \\
&
\end{aligned}$
Here $\mathrm{E}^{\circ}$ is given in standard Reduction Potential
$\therefore \mathrm{Co}^{3+} / \mathrm{Co}$ will undergo reduction acting as cathode and $\mathrm{Al}^{3+} / \mathrm{Al}$ will undergo oxidation acting as anode.
$\begin{aligned}
\therefore \quad \mathrm{E}_{\text {cell }}^{\circ} & =\mathrm{E}_{\text {cathode (Red.) }}^{\circ}-\mathrm{E}_{\text {Anode (oxide) }}^{\circ} \\
\mathrm{E}_{\text {cell }}^{\circ} & =1.81-(-1.66) \\
\mathrm{E}_{\text {cell }}^{\circ} & =+3.47 \mathrm{~V}
\end{aligned}$