# How do the two half cells of Daniel cell know when they are connected?

I'm currently studying electrochemistry in school, and there is something I don't can't quite understand about Daniel cell.

When you put $$\ce{Zn}$$ solid into $$\ce{ZnSO4}$$ solution, does the electrode become negatively charged even before connecting it with the $$\ce{Cu}$$ half cell?

I searched but I wasn't able to form a clear understanding, I read that the answer to the previous question is no, the the two cells need to be connected for the redox reaction to happen. If that was the case, how do the half cells communicate, how do they know when they are connected even when there is distance between them?

If the electrodes were charged before connecting them that would make perfect sense for me, the electrons repel each other and when connecting the two half cells with a wire, the electrons move because of the negatively charged electrode, but from what I've read that is not the case?

I'm really sorry if that is a stupid question but I really can't find an answer.

No, electrodes do not know about each other.

When an electrode is inserted to an electrolyte, the electrochemical reaction is ongoing in both directions. If reduction direction overruns oxidation, the potential of the electrode is increasing ( or vice versa ) until the rate of both reaction gets equal, the net reaction rate is zero and the electrode reaches the equilibrium potential.

That may be quick or slow, depending on the electrode and respective reaction kinetics.

When both electrodes are galvanically connected, the electron exchange due the potential difference disbalances the equilibrium potentials of both electrodes and the net electrochemical reactions get the steady non zero rate.

Note that the electrode potential is conventionally referred to the Standard hydrogen electrode (SHE), which itself has potential $$\pu{+4.44 \pm 0.02 V}$$ ( Wikipedia ) wrt the potential of a free electron in vacuum. So saying $$\ce{Zn}$$ electrode potential is negative rather means it is less positive.

You may want to try searching various terms in wikipedia and following relevant internal/external links. The great source of compactly explaned topics is hyperphysics, but with limited scope. It is a kind of hyperlinked quick cards/cheat sheets, very handy, if you want to understand the basics. This link is particularly for Electrochemistry

• Does that mean that each half cell have a potential even before connecting the two cells? And when the two are connected with a wire electrons move because of the potential difference? Also is it possible that you direct me to where I can read more about this? – Mathew Hany Mar 26 '20 at 11:43
• Yes, I have tried to say exactly that. See the answer update for more. – Poutnik Mar 26 '20 at 11:50

Another way of describing the electric behavior of the electrode is to state that when dipped in water, one Zn atom quickly looses two electrons and gets dissolved in the solution. The electron remains in the metal and prevents any second Zn atom from bringing more electrons in the metal. Simultaneously the solution gets positively charged and the first $$\ce{Zn^{2+}}$$ ion repels any second $$\ce{Zn^{2±}}$$ ion from trying to enter the solution.

• 1 atom does not make much difference. Imagine the electrode has capacity just 1 pF, ( in fact much more) so 1 electron would make potential difference 0.16 microV.( in fact much less ). 1 Extra electron just very very slightly speeds up reduction and slows down oxidation. – Poutnik Mar 26 '20 at 16:01
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