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The general set up, In an alkaline battery, the negative electrode is zinc (Zn) and the positive electrode is manganese dioxide ($\ce{MnO2}$). The alkaline electrolyte of potassium hydroxide (KOH)

At the anode:

$$\ce{Zn(s) + 2OH−(aq) → ZnO(s) + H2O(l) + 2e−}$$

At the Cathode:

$$\ce{2MnO2(s) + 2H2O(l) + 2e− → 2 MnO(OH) (s) + 2OH−(aq)}$$

My question is that why does the reaction "pause" when the battery is not connected to a circuit?

For an example, If I were to have a container holding the electrolyte and I were to place the 2 metals in to the solution, I would expect sometime later that the reaction to be complete, meaning that all zinc and $\ce{MnO2}$ used up. I believe that the electrons would beable to find themselves in the position to be involved in the reaction. But for an unconnected battery, the reaction seems to stop? Why is this?

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  • $\begingroup$ How could the electrons "find themselves in the position to be involved in the reaction" ? The electrons cannot go through the KOH solution. They must go to the cathode through a metallic wire. $\endgroup$
    – Maurice
    Dec 1, 2023 at 16:57
  • $\begingroup$ @Maurice, Thanks for your reply. So I am not really understanding what happens. Suppose no wire again: I guess the reaction takes place at the anode as OH- ions float towards it and react with Zn. Then does the electrons cling to Zn and not float into the solution? $\endgroup$
    – Reuben
    Dec 1, 2023 at 18:20

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If no wire is joining the zinc plate and the $\ce{MnO2}$ plate, the whole setup would not be a cell or a battery. It wouldn't produce any anode or any cathode. The whole setup would simply be a chemical solution of $\ce{KOH}$ in contact with a zinc plate somewhere, and with a block of $\ce{MnO2}$ somewhere else. The only chemical effect that could happen is the possible reaction (not the half-reaction) between Zn and $\ce{KOH}$ somewhere, and independently also the possible reaction between $\ce{KOH}$ and $\ce{MnO2}$ elsewhere. Fortunately no such reaction happens between $\ce{KOH}$ and either zinc or $\ce{MnO2}$. If such a reaction would exist between zinc and $\ce{KOH}$, this reaction will soon consume the zinc in a couple of minutes before the battery would work. And the battery could not be sold: It would be used in a couple of minutes after construction. The zinc metal reacts (and produce electrons) only if there is an electric contact between the zinc and $\ce{MnO2}$. Without any contact, the zinc cannot emit electrons, because these electrons have nowhere to go.

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  • $\begingroup$ Thanks again for the reply. (Sorry for my questions as I am not to good with chemistry) As metals don't know know the situation they are in so I guess the following happens: There are some reactions that take place at each metal but it stops(maybe from the presence of electrons). But when the wire is connected the electrons can flow to the other metal solid, relieving the presence of electrons(or something) and restarting the reaction? $\endgroup$
    – Reuben
    Dec 3, 2023 at 3:07
  • $\begingroup$ @Reuben At the atomic level, the reactions take place at each electrode, but the reverse reactions take place at the same rate. Once there is a wire connection, the anode has a way to get rid of electrons and the cathode has a source of electrons, so oxidation at the anode and reduction at the cathode are the net reactions. So the electrode half-reactions don't have a master switch, they keep on doing what they always do, but the availability of species drives the direction of reaction. $\endgroup$
    – Karsten
    Dec 3, 2023 at 18:33
  • $\begingroup$ Also, if you swap out a half-reaction for another one (on the other side of the electrochemical series), sometimes the direction of electron flow will switch, and suddenly the old anode is the new cathode. Again, nobody has to tell the electrode to switch, it is simply driven by the availability of the species of the half-reaction. $\endgroup$
    – Karsten
    Dec 3, 2023 at 18:37
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In a battery, the zinc and manganese reagents are kept separate, and so the electrons can only flow when both parts of the battery are connected via an electric circuit.

If you take a look at the Construction section of the Wikipedia Alkaline battery article, you’ll see that a thin membrane divides each half-cell of the reaction, but that the electrons will be able to flow so long as a circuit connects both ends of the battery.

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