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Introduction to galvanic/voltaic cells

How to use a redox reaction to construct a galvanic/voltaic cell to produce a flow of current.. Shows the flow of electrons and ions, and explains the role of the salt bridge. Created by Sal Khan.

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  • blobby green style avatar for user Shayan Khan Lodhi
    hey, why doesnt the salt bridge transfer sulphate ions from the copper sulphate to the zinc syulphate and why doesnt it transfer zinc ions from the zinc sulphate solution to the copper sulphate? i mean its meant to transfeer ionsn not give away its owninto the solution :\
    (39 votes)
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    • piceratops tree style avatar for user Jayden Tan
      Here's a more in depth look at what's happening:

      Electrons are flowing from the zinc solid metal strip to the copper strip. Zinc ions in the solution are positive, and hence the solution on the left side becomes more positive. As it becomes more and more positive electrons can no longer travel to the copper because they are attracted to this strong positive charge. What would happen then? No electron flow = no current. Our battery stops working!

      A salt bridge however balances the charges of the solutions on both sides. Once the Zn ion solution becomes too positive the salt bridge allows negative Sulfate ions to flow into the zinc side. Now the electron flow can continue.

      Now however our right side is becoming to negatively charged and starting to repel electron flow. The salt bridge now allows for Na+ ions to flow to the right side to balance out the negative charge and keep the battery working.

      Why don't Zn or Cu ions flow through the salt bridge and end up on each side? Remember ions can only flow in the solution they are in (usually water or some kind of liquid) For a Zn ion to flow across the salt bridge it would have to first defy gravity to get up the first part of the bridge, resist bonding with sulfate ions, and continue all the way across the other side to the copper solution. I also just read on Wikipedia that often agar gel is used with a glass U shaped tube for the salt bridge which minimizes the amount of intermixing that the two solutions can do.

      Hope that helps! :)
      (160 votes)
  • spunky sam blue style avatar for user Manav Rajvanshi
    what if when the 2 solutions gets too positve and too negative and i do not use a salt bridge at that time, and dip another wire with both ends in each solution instead...... will it constitute current as one end of the wire is at a higher potential (say in the positive solution ) and the other in the negative solution(low potential).... will this potential difference make a flow of current?
    (24 votes)
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    • aqualine ultimate style avatar for user Dominika Lacala
      Interesting question. Hypothetically If you had two solutions with different potentials I think that even with one wire there would be a flow of current - electrons would just move to the more 'positive' one.
      But I think the main problem is how to create such a situation. I mean that if you use a salt bridge to make a redox reaction happen, it causes a flow of electrons, but potentials equalize thanks to the bridge. On the other hand if you only use two wires (with equal solutions) nothing will happen, because the anions will not be able to flow and the electrons will be 'kept' (causing such a difference in potentials of solutions would 'coast' more energy that would be gained by moving electrons form Zn to Cu).
      So in other words without salt bridge nothing happens as the flow of current requires a closed 'cycle' and the potential of solutions doesn't differ without additional energy (Sal simplified it a bit).
      (13 votes)
  • blobby green style avatar for user Vann Dosque
    I have two questions.
    Firstly, from what Sal says I understand the copper... "stick" is used only as a conductor of current so that they can reach the Cu2+ ions in water. So, if it is used only as a conductor, can it be any other conductive metal or does it have to be copper for the battery to work?
    Secondly, if the battery were actually being used, and so the flow of electrons was being redirected somewhere else before it got to the Cu2+ in the water, wouldn't it mean that the Na+ in the salt bridge wouldn't be necessary, because the Cu2+ never gets into neutral state and therefore there is no major charge difference caused by the SO4-2?
    The answer to both of these questions seems to be yes from what I understand and can deduct logically, but I'm just checking probably to make sure that I have understood this correctly. Thank you!
    (10 votes)
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  • leaf green style avatar for user Charlie Lee
    How does Zinc and Copper get oxidised and reduced when they are not in direct contact?
    Also, can I please have an explanation of sulfate's charge in Copper sulfate solution? I thought in Copper Sulfate solution, Sulfate would have it's charge as 2-
    (11 votes)
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  • blobby green style avatar for user Tal Robbins
    What causes the initial 2e- to move through the wire from the anode to the cathode? I understand the entire process once it has already commenced. However, what is not clear is how the process initiates.
    (8 votes)
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    • female robot grace style avatar for user noreen.randhawa
      This is because the copper is more electronegative than the zinc. The cell potential for this reaction is +1.10, since copper is being reduced and zinc is being oxidized, and if you plug this into the free energy equation for a cell ,ΔG= −n× F × EMF, you get ΔG= −2e-× (9.64 x 10^4 F) × (+1.10V) = -212 kJ. Since E>0 (+1.10) and ΔG< 0 (-212 kJ) the reaction is spontaneous, and that explains why the process is initiated since this reaction is thermodynamically and electrochemically favoured. Hope this helps!
      (0 votes)
  • leaf green style avatar for user terry taylor
    What eventually stops the flow of electrons from the zinc to the copper ? Zinc is used up ? Copper gets used up ? Too much sodium gets into solution ?
    (4 votes)
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    • leaf blue style avatar for user Kevin George
      In this case, the Zinc electrode would slowly dissolve into the solution adopting a pencil like form as it "melts" becomming smaller and smaller, until it runs out. If the salt bridge runs out before the zinc electrode, electron flow might become interrupted due to a large positive charge on the zinc´s side. The copper, from what I understand, will acquire mass as a form of rust that will start appearing on the electrode´s surface.
      (8 votes)
  • blobby green style avatar for user jomwit22
    What oxidises the Zinc bar? (Unlike the previous setup which the Zinc is in direct contact with Cu2+) To put it another way, if I connect a Zinc bar and a Copper Bar together via a metal strip, will the electrons start to flow just like that? (I think not because we won't need anymore batteries) So what makes the solid zinc bar loses it's electrons? The so4 2-?
    (5 votes)
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  • male robot johnny style avatar for user Sahil Acharya
    how do u know which electrolytes to use when constructing a galvanic cell?
    (3 votes)
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  • mr pants teal style avatar for user Amun Lie
    Won't sodium be attracted to the copper stick and take electrons as well - so we get solid sodium?
    (3 votes)
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  • female robot ada style avatar for user salomech96
    Hey, what I don't get is in the picture of the galvanic fuel cell, the left solution is supposed to have the SO4 already reduced... How did this happen? From the previous video, I thought that the SO4 was reduced by the Cu instead of the Zn and if the Zn in this case is the one who is responsible for the reduction of SO4 then the electrons would never flow through the wire if the SO4 already have them and also because SO4 is more electronegative than Cu. Thank you for any help.
    (2 votes)
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    • leaf red style avatar for user Richard
      Ok so firstly, sulfate (SO4) is not being reduced at any time during this reaction. The only reduction occurring here are the copper ions being reduced to neutral copper atoms. Simultaneously as the neutral zinc atoms are being oxidized to zinc ions.

      If you asking along the lines of why does the sulfate have a negative charge already, then this is different from being reduced. The sulfate anions began with a negative charge at the start of the reaction and will remain with the same charge at the conclusion of the reaction. The reason they already have negative charges is that the sulfates were introduced to the cell solutions as salts (or ionic compounds) which already possessed a negative charge which includes compounds like: zinc sulfate, copper sulfate, and sodium sulfate.

      Hope that helps.
      (3 votes)

Video transcript

In the last video, we saw that if you took some solid zinc and stuck it in a solution of copper sulfate, that the zinc will essentially give electrons to the copper. So then you have zinc cations that are in the solution. So essentially, it'll become a solution of zinc sulfate. And the copper, once it gets those two electrons is going to go into it's solid state, and it's going to precipitate out of the solution. And we saw the reaction right over here-- solid zinc plus copper sulfate in solution and water. It's an aqueous solution. You have the solid copper precipitating out. And now it's a solution of zinc sulfate, that the zinc has essentially been oxidized. It lost two electrons. It went from neutral to positive. And the copper went from positive to neutral. So the copper took those two electrons. Zinc was oxidized by copper. It lost electrons to the copper. Copper was reduced by zinc. Its charge was reduced by zinc. It gained electrons from zinc. Now, this by itself is interesting. It's an interesting redox reaction. Something was oxidized, something was reduced. But wouldn't it be interesting is if we could somewhat separate these two half reactions and make these electrons travel over a wire. Now, why would that be interesting to make electrons travel over a wire? Well, electrons traveling over a wire, that's a current. And you could make current do useful things, like power a motor or a light or whatever it might be. And so essentially, if we can do that, we would have constructed something of a battery. And if we can keep that going, if we can keep the current flowing, we would have constructed something like a battery. And what I have here, this is a picture of a galvanic-- sometimes called a voltaic-- cell. And this is doing exactly that. It's separating these two half reactions and separating them with a wire. So zinc can gave copper its electrons, but it forces the electrons to go along this wire and produce an actual current. So let's think about why this is working. So you have solid zinc right over here. We've already said that look, the solid zinc wouldn't mind giving its electrons to copper. Copper wouldn't mind taking it. Copper is more electronegative. And so you have a reality where the solid zinc could give away its two electrons and become the cation zinc, so a positive charge, and then it dissolves in the water. Once it has a positive charge, it's easy to dissolve into a polar solvent like water. And then you have those two electrons. Where are those two electrons going to go? Those two electrons can then go and be given to the copper. And both zinc and copper are great conductors of electricity. They're transition metals. They have these seas of electrons. So electrons can travel within them fairly easily. And so you have your two electrons. So those are your two electrons that I showed traveling in green. And they can come all the way to the bottom of where this copper bar is in contact with the copper with the copper sulfate solution. And now you're going to have a cation, an ion of copper, that when it comes into contact with those electrons, it's going to nab them up and become neutral. And when it becomes neutral, it's going to precipitate out of the solution. It's going to precipitate onto that bar. Now, you might be saying, look, if more and more positive things, if more and more of this positive zinc is flowing in this, wouldn't this make this an imbalance? And if this solution becomes too positive, then the electrons wouldn't want to leave as much anymore. So if this starts becoming very, very, very, very positive, and similarly, if all the positive stuff, all the copper cations are capturing the electrons, the solution is going to become more and more negative. It's going to have more sulfate and less of the positively charged copper ions. So what can we do to make sure that doesn't happen too quickly? Well, what we do is we use something called a salt bridge. And the salt bridge right over here, this helps neutralize that effect that we just talked about. And with a salt bridge, you can view it. It's not going to be liquid, because then everything inside of it would just fall out. You can view it as a goo of a salt. In this diagram, we picked sodium sulfate as our salt. So for every sulfate molecule, you have sulfate anion. You have two sodium cations. And so what's going to naturally happen here? Well, as this becomes more and more positively charged, as more and more positive zinc ions go into the solution, the negative sulfate ions are going to want to come out of here. So the negative sulfate ions are going to want to leave all of their negative friends right over here, go into the salt bridge, and then the ones that are already in the salt bridge are going to want to come out here. Similarly, the sodium right over here will be tempted to help neutralize. The sodium-- let me do it this way-- could go in this direction and help neutralize any negativity that's happening there. And so that will keep each of these solutions from becoming too positive or too negative and allow this current to continue to flow and do useful things.