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Cell potential in a non-redox reaction?

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rentj:
I saw these reactions in a chemistry textbook (attached in this post). I understand that reaction 1 and 2 both are redox reactions and thus they have their own standard potential. But, I do not really understand why reaction 3 (coordination complex) and reaction 4 (AgCl) both have a standard potential since they are non-redox reactions. Thank you.

Borek:
Nothing looks correct here, Ag+/Ag is +0.8, not -0.8V, AgCl/Ag is around 0.22V, Cu+/Cu looks OK, but potential for the cyanide complex reaction is again off.

My first thought was that they mean comparison between Ag+/Ag and AgCl/Ag - the latter potential can be predicted using Ksp (also potential of the redox reactions of complexes can be predicted using stability constants). But the numbers listed don't add up.

mjc123:
They write the first reaction as Ag/Ag+ (oxidation) so reverse the sign (reduction favoured).
For the last two reactions, the "potential" is the difference between the electrode potentials of the reactions with and without complexation. For example
Cu+ + e-  :rarrow: Cu         ΔG°1 = -nFE°1
Cu(CN)2-  :rarrow: Cu+ +2CN-        ΔG°2
Cu(CN)2- + e- :rarrow: Cu +2CN-        ΔG°3 = -nFE°3
Now ΔG°2 = ΔG°3 - ΔG°1 = -nF(E°3 - E°1)
But to say E°2 = E°3 - E°1 assumes E°2 = -ΔG°2/nF
But n = 0 for reaction 2, which is the conceptual problem. However, if you use the n value for the overall reaction, you get a number that can be regarded as "the difference that complexation makes to the electrode potential", which may be a useful thing. But not an actual redox potential.

Borek:

--- Quote from: mjc123 on July 15, 2024, 09:24:41 AM ---They write the first reaction as Ag/Ag+ (oxidation) so reverse the sign (reduction favoured).
--- End quote ---

Good point.


--- Quote ---you get a number that can be regarded as "the difference that complexation makes to the electrode potential", which may be a useful thing. But not an actual redox potential.
--- End quote ---

Interesting, I have never seen it done this way.

Somehow I find it more confusing than helpful though, I don't like introducing artificial concepts when the logic behind reasonably easy follows from the basic principles. In my experience way too often they just muddy the water and do more harm than good to students understanding. KISS principle :)

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