Nernst equation: half cell vs overall reaction

Question

Can the Nernst equation always be applied to the overall redox reaction as opposed to only half cell reactions?

For example, in the following silver-silver chloride electrode: $$\ce{Ag (s)|AgCl (s)|Cl- (aq)}$$ rather than applying the Nernst eqution to the half cell reaction $$\ce{AgCl + e- -> Ag + Cl-}$$ can I apply it to $$\ce{AgCl -> Ag+ + Cl-}$$

The answer is the same for the question I was trying to solve (whether I use the overall reaction or the half-cell reaction), but I was wondering if this was just a coincidence or if it was the same.

If they are the same, are there any other factors I should consider such as whether or not the electrolyte fully dissociates in the given conditions?

Answer 1

It is coincidence, since $$\ce{AgCl -> Ag+ + Cl-}$$ is not a half cell reaction.

The silver chloride electrode functions as a redox electrode and the reaction is between the silver metal (Ag) and its salt — silver chloride (AgCl). There are two ways to represent the processes (half reactions) happening on the electrode: $$\ce{AgCl + e- -> Ag + Cl-}$$ or $$\ce{Ag+ + e- -> Ag}$$ $$\ce{AgCl -> Ag+ + Cl-}$$

And you should be applying Nernst equation to one of the two half cell reactions shown above.

The reason why you got the same result/answer is because the Nernst equation for silver chloride electrode has the dependence on the activity or effective concentration of only chloride-ions.

$$E = E^0 - \frac{\mathrm RT}{\mathrm F}\ln(a(\ce{Cl^-}))$$