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Here is something that no video in youtube about electrochemistry can explain me about galvanic cells.

Suppose we have $\ce{Zn}$ metal immerse in $\ce{Zn^{2+}}$ in one side, then $\ce{Cu}$ metal immerse in $\ce{Cu^{2+}}$ in the other side. Then, if we link the $\ce{Zn}$ metal to $\ce{Cu}$ metal with some conductor wire, electrons will flow from the $\ce{Zn}$ metal to the copper wire, but this will stop quickly without a salt bridge.

My question is why do the electrons move from $\ce{Zn}$ metal through the wire to the $\ce{Cu}$ metal in the first place? What causes that?

Martin - マーチン
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nerdy
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    possible duplicate of Why is it important to use a salt bridge in a voltaic cell? can a wire be used? – Michael DM Dryden Jan 07 '15 at 04:16
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    Your question is answered quite well in that duplicate question, but I should add that Zn should ideally never be in contact with Cu2+ in a useful cell (i.e. Cu2+ should not cross the salt bridge) because the Cu2+ will be directly reduced at the Zn electrode instead of the electrons passing through the external circuit. – Michael DM Dryden Jan 07 '15 at 04:18
  • I had read that answer. The problem is that it starts with "The electrons flow from the anode to the cathode. So, the oxidation ...", but it doesn't answer why the electrons flow from the anode to the cathode with the salt bridge, but not withouth it. – nerdy Jan 07 '15 at 04:20
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    As it says, "The electrons move thrown the wire (and your device, which I haven't included in the diagram), leave unbalanced positive charge in this vessel." So it will work vanishingly briefly, but because one electrode is releasing cations into solution and the other is removing cations from solution, a charge imbalance arises in each half-cell that opposes further electron transfer. Redox reactions will occur until this imbalance matches the cell voltage. (it doesn't take much—I don't know if it's even possible to measure it) – Michael DM Dryden Jan 07 '15 at 04:26
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    This is also pretty much the same thing. – Michael DM Dryden Jan 07 '15 at 04:34
  • Yes, but why will the electrons move thrown the wire in the first place ? It doesnt answer it. – nerdy Jan 07 '15 at 04:46
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    Because it's more energetically favourable for the zinc electrode to be reduced, basically. Another relevant link. Spontaneous chemical reactions are driven by moving a system from a higher energy state to a lower energy state. You're probably familiar with this as ∆G's for reactions at constant pressure and redox reactions are the same, only expressed as a voltage: ∆G=-nFE because voltages are more convenient when dealing with redox reactions. – Michael DM Dryden Jan 07 '15 at 05:05
  • Hmm. Let's forget about the Zn solution , the Cu solution and the salt bridge. If we simply take Zn metal and connect it to Cu metal through a wire, i think there will be a voltage potential difference between them generated ( like LDC3 mentioned ) . Correct ? So, eletrons will flow from the Zn to the Cu ( even if this flow will stop very briefly ) ? – nerdy Jan 07 '15 at 05:09
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    Yes, it's known as the Volta potential, though it's pretty complicated stuff if you haven't studied materials chemistry. – Michael DM Dryden Jan 07 '15 at 05:12
  • Omg, it's really clear now ! I only have two last questions . What is happening in the salt bridge after the initial flow of electrons is that the cations of the salt are being attracted to the Cu2+ solution with extra electrons and are being neutralized by it ( receiving their extra electrons ) , and the anions are being attracted to the Zn2+ solution with lack electrons and also being neutralized by it ( giving their extra electrons to the Zn2+ solution ) ? – nerdy Jan 07 '15 at 05:24
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    No redox reactions happen with the salts in the salt bridge, the ions just migrate to maintain charge balance. The side where the reduction occurs has more negative ions (since the cations are being reduced), so cations migrate from the salt bridge to replace them. Likewise, the other side has more positive ions since the electrode is oxidizing to produce cations, so anions from the salt bridge migrate to keep balance. Nothing is being "neutralized" as such. The only place where electron transfer occurs is at the electrode-solution interfaces. – Michael DM Dryden Jan 07 '15 at 05:37
  • I see. So, i'm guessing that the ammount of extra electrons or missing electrons in the solutions of Zn2+ and Cu2+ are directly related to the Fermi level of the Zn metal and the Cu metal, respectively. If we don't have a salt bridge, the Fermi level of the metals will become equal soon enough and the flow of current will stop. – nerdy Jan 07 '15 at 05:59
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    The Fermi levels are always equal because the two electrodes are in equilibrium (they're connected to each other). Probably should have done this in chat, but last one: the charge imbalance in a galvanic cell is in the solution, not the electrodes. If we pretend the reduction half-cell has 100 Cu^2+ and 100 SO4^2-, if we reduce one copper ion to neutral Cu, there is one extra SO4^2- left in the solution, so it has a net negative charge. This charge makes it harder to reduce a second Cu^2+, eventually stopping the reduction, if a couple of K+ are added from the salt bridge, the charge goes away – Michael DM Dryden Jan 07 '15 at 06:09
  • What i mean is that if we don't have a salt bridge, the fermi level of the Zn will become equal to the fermi level of the Cu soon enough, so current will stop flowing . Anyways, you were very very very helpful. Thanks a bunch from a guy really newbie in Chemistry . – nerdy Jan 07 '15 at 06:12

2 Answers2

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There is no reaction since the electrons will only flow one way in the wire. When you connect the zinc metal to the copper metal with a wire, there is a voltage potential generated between them, just like in a thermocouple.

From Wikipedia:

Any junction of dissimilar metals will produce an electric potential related to temperature.

In this case, we don't care about the temperature effects.

When a salt bridge is placed between the solution, then the electrons are carried by the anions in the solution from one electrode to the other electrode.

So the zinc metal releases electrons when it dissolves, which travel through the wire easily, to the copper. The $\ce {Cu^{2+}}$ in solution grabs the electrons from the copper electrode, adding copper metal to the electrode. Since there is now more negative ions in this solution, the anions move across the bridge to the zinc electrode. The anions attack the zinc metal, causing the zinc to loose electrons when it dissolves.

I'm not sure what starts the reaction, nor why the zinc dissolves easily.

LDC3
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  • I think you just mentioned what really starts the reaction. Whenever we connect two dissimilar metals with wire, electrons will flow from the least electronegative one to the most electronegative one. Correct ? – nerdy Jan 07 '15 at 04:39
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    @nerdy There is a potential, the electrons won't flow unless there is a circuit for them. In thermocouples, the metals are usually fused together and the potential from this junction is monitored. – LDC3 Jan 07 '15 at 04:46
  • Also, i didn't understand 100%. The potential is based on the temperature difference ? So, if they are with the same temperature, no potential difference betweem them is guaranteed ? – nerdy Jan 07 '15 at 04:48
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    @nerdy No, when using thermocouples to measure temperature, you have 2 thermocouples, one where you know what the temperature is. You measure the difference in activity to determine the temperature since it is nearly linear. There is always a potential difference until you cool it way down. For example, if there is a potential of 1.27V at 30°C and you know that the potential changes 0.07V for each degree, what temperature would the junction need to be at to give a potential of 2.75V? – LDC3 Jan 07 '15 at 04:55
  • I see. There is a potential between the Zn metal and the Cu metal . So, if we connect them through a wire, electrons will flow from Zn metal to the Cu metal, right ? What do you mean by a circuit ? Isn't a wire enough ? – nerdy Jan 07 '15 at 05:07
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    @nerdy Because of the potential, the electrons can only flow one way. Zinc will give up electrons, but it must go into solution. Without a salt bridge, the solution get positively charged and will stop the reaction. The salt bridge allows anions to balance the charge and to take electrons away from the copper electrode. – LDC3 Jan 07 '15 at 05:47
  • Could you say that electrons start moving from zinc to copper because copper has higher electronegativity than zinc? – Petr L. Mar 12 '23 at 17:01
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Electron move from zinc to copper as zinc is more reactive metal than copper and thus more readily loses the electron to turn into zn2+ ion . Thus electrons move from zinc to copper

user56496
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