# Reaction of an electrode

Why is the cathodic reaction of $I^-/AgI/Ag$ is $$\ce{AgI + e- -> Ag + I-}$$ but not, $$\ce{Ag+ + e- -> Ag }$$

whereas, in $Ag/AgCl(s)$, $KCl$(0.2M); it is $$\ce{Ag+ + e- -> Ag}$$

The real reaction is $\ce{Ag+ + e- -> Ag}$ for both cases. Equation written in this way, ${E^0}$ will be equal to the reduction potential of Ag(I) in its 1 M solution.
If it is written as $\ce{AgI + e- -> Ag + I-}$, then ${E^0}$ will be equal to the reduction potential of Ag(I) in 1 M $\ce{I-}$ solution -or in other terms in ${K_{sp}(AgI)}$ Molar Ag(I). solution
Hence, technically, $\ce{Ag+ + e- -> Ag}$ is same with $\ce{AgI + e- -> Ag + I-}$. Latter is more descriptive and practically more relevant, however, because the presence of $\ce{I- /Cl-}$ ions in the medium limits the amount of $\ce{Ag+}$ cations via ${K_{sp}}$ and the notation readily implies this.