Question
On the basis of the following data, explain why $Co - \ (III)$ is not stable in aqueous solution? 
$\ce{Co^{3+}+e^{-} \rightarrow Co^2+, E^0=+1.82 V}$
$\ce{2 H_2 O \rightarrow O_2+4 H^{+}+4 e^{-}, E^0=1.23 V}$

Answer

Adding the two half reactions $, \text{e.m.f}$. comes out to positive.
$\text{emf} =E_{\text {cathode }}-E_{\text {anode }}$
$\text{emf } =1.82-1.23=+0.59$
This means that $Co \ (III)$ in aqueous solution has the tendency to change to $Co\ (II)$.
Hence, $Co \ (III)$ is not stable in aqueous solution.

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