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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?

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    $\begingroup$ possible duplicate of Why is it important to use a salt bridge in a voltaic cell? can a wire be used? $\endgroup$
    – Michael DM Dryden
    Commented Jan 7, 2015 at 4:16
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    $\begingroup$ 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. $\endgroup$
    – Michael DM Dryden
    Commented Jan 7, 2015 at 4:18
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    $\begingroup$ 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) $\endgroup$
    – Michael DM Dryden
    Commented Jan 7, 2015 at 4:26
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    $\begingroup$ This is also pretty much the same thing. $\endgroup$
    – Michael DM Dryden
    Commented Jan 7, 2015 at 4:34
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    $\begingroup$ 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. $\endgroup$
    – Michael DM Dryden
    Commented Jan 7, 2015 at 5:05

2 Answers 2

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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.

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  • $\begingroup$ 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 ? $\endgroup$
    – nerdy
    Commented Jan 7, 2015 at 4:39
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    $\begingroup$ @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. $\endgroup$
    – LDC3
    Commented Jan 7, 2015 at 4:46
  • $\begingroup$ 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 ? $\endgroup$
    – nerdy
    Commented Jan 7, 2015 at 4:48
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    $\begingroup$ @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? $\endgroup$
    – LDC3
    Commented Jan 7, 2015 at 4:55
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    $\begingroup$ @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. $\endgroup$
    – LDC3
    Commented Jan 7, 2015 at 5:47
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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

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