AgI and AgCl reduction potentials

The reduction potential for $\ce{AgI}$ is $-0.15\ \mathrm{V}$ and for $\ce{AgCl}$ it’s $0.22\ \mathrm{V}$. As $nFE =-\Delta{G} = T\Delta{S} - \Delta{H}$ this must imply that $\ce{AgI}$ is a stronger bond in order for it to be negative. 


I would think $\ce{AgI}$ is weaker due to having a much larger ionic radii for $\ce{I-}$ than $\ce{Cl-}$ and the lack of covalence due to a smaller $\Delta\chi$


I think that it must be something to do with the size of the $\ce{I-}$ mean that it causes significant ordering of the water molecules in the solution and it's this entropic effect that is actually the key idea as otherwize I can't rationalise it at all.




The answer is [hard-soft acid/base](http://en.wikipedia.org/wiki/HSAB_theory) theory (or HSAB theory). Essentially, some Lewis acids and bases are **hard** and some are **soft**, with a few **borderline** cases.



> 
> **Hard** acids and bases tend to have
> 
> 
> 


* Small atomic/ionic radius
* Highly charges (or highly positive/negative oxidation state)
* High electronegativity for bases and low electronegativity for acids
* Low polarizability



> 
> **Soft** acids and bases tend to have
> 
> 
> 


* Large atomic/ionic radius
* Low or zero charge / oxidation state
* Moderate electronegativity
* High polarizability


According to HSAB theory, matches that are **hard-hard** and **soft-soft** form strong interactions, where **hard-soft** matches are weaker interactions.


$\ce{Ag+}$ is a soft Lewis acid. $\ce{Cl-}$ is a hard Lewis base, while $\ce{I-}$ is a soft Lewis base. Therefore, we would predict that $\ce{Ag+}$ and $\ce{I-}$ will have have a stronger attraction than than $\ce{Ag+}$ and $\ce{Cl-}$.


We can see this difference in attraction in the solubilities of $\ce{AgCl}$ and $\ce{AgI}$ in water. $\ce{AgCl}$ is much more soluble in water than $\ce{AgI}$ (realizing that both are nearly insoluble). See the [solubility constant data](http://bilbo.chm.uri.edu/CHM112/tables/KspTable.htm):


* $\ce{AgCl}\ \ \ k\_{\text{sp}}=1.8\times 10^{-10}$
* $\ce{AgI}\ \ \ \ \ \ k\_{\text{sp}}=8.5\times 10^{-17}$


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