2
$\begingroup$

I was thinking about how Daniell cell works and got into a serious doubt.

Let's consider a half cell of Zn. In standard conditions, it'll have an electrode potential of -0.76V with respect to SHE. If I now increase the zinc ion concentration, I know from Nernst equation that it's electrode potential (still with respect to SHE) should become less negative. Intuitively also, it's tendency to get reduced will be more due to Le Chatelier's principle, so more reduction potential as predicted by Nernst equation. So far so good.

But, if I see it like this, then the results seem to get flawed: When I increase zinc ion concentration, in the new equilibrium state that will be attained, the absolute potential of the electrolyte will be greater (due to more positive (zinc) ions), and a little bit (safe to neglect it's consequences) of negative charge on Zn electrode will be decreased, with a little deposition of Zn. Zn2+ + 2e- <---> Zn So the net effect would be of electrolyte solution becoming more positive. Now, the definition of "reduction" potential is the (absolute) potential difference between the reduced and the oxidised species (along with some additive constant that makes SHE's electrode potential 0V, but it's of no concern here as we're seeing where the electrode potential (of Zn) increases or decreases, NOT its absolute value); hence, E = V(Zn electrode) - V(Zn2+ in electrolyte). So this should get more negative as the potential of electrolyte increases. (Potential of the electrode also becomes more negative, but its effect is marginal.)

So what is the flaw in this perspective?

$\endgroup$

closed as unclear what you're asking by Mithoron, Tyberius, airhuff, Todd Minehardt, Jon Custer Aug 31 '17 at 0:52

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

Browse other questions tagged or ask your own question.