Understanding Batteries: Anode, Cathode, Electrolyte

[Alex Hughes]

So I understand in a battery that an anode (such as zinc) and a cathode (such as carbon) are separated by an electrolyte. I also understand that the electrons want to flow into the cathode, but can't get to them, so as soon as a conductor connects the two terminals, current can flow. However, there are several questions I'm confused about and I was hoping somebody could help me. First, why zinc and carbon. Second, what is the purpose of the electrolyte? Why can't charges just flow through the battery to the other electrode, does the electrolyte stop that from happening? Lastly, what actually makes the electrons in the anode want to move to the cathode. Everybody just says that they want to but not why. Sorry for rambling I'm just frustrated that nobody can give me a clear answer.

 

[davenn]

So I understand in a battery that an anode (such as zinc) and a cathode (such as carbon) are separated by an electrolyte. I also understand that the electrons want to flow into the cathode, but can't get to them, so as soon as a conductor connects the two terminals, current can flow. However, there are several questions I'm confused about and I was hoping somebody could help me. First, why zinc and carbon. Second, what is the purpose of the electrolyte? Why can't charges just flow through the battery to the other electrode, does the electrolyte stop that from happening? Lastly, what actually makes the electrons in the anode want to move to the cathode. Everybody just says that they want to but not why. Sorry for rambling I'm just frustrated that nobody can give me a clear answer.

all your questions are answered here ...

http://batteryuniversity.com/index.php/learn/

 

[Dale]

First, why zinc and carbon.

It doesn’t need to be zinc and copper. Those are just easy to find materials that have different potentials. Any pair will do, the further apart the better.

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/electpot.html

Second, what is the purpose of the electrolyte?

The electrolyte is a fluid for the metals to go into solution at one electrode and out of solution at the other electrode. It also conducts electrical current between the electrodes.

Why can't charges just flow through the battery to the other electrode, does the electrolyte stop that from happening?

I don’t understand this question. Charges flowing through the battery is the same as charges flowing through the electrolyte.

Lastly, what actually makes the electrons in the anode want to move to the cathode.

The chemical reactions and the energy they provide.

 

[willem2]

First, why zinc and carbon. [\QUOTE]
Carbon is only used as a conductor, which doesn't corrode. Zinc is a metal is a metal that loses electrons rather easily, and then dissolves, so it can be used for the negative pole of the battery. Metals that are even more reactive can be dangerous (see all lithium battery fires) and will be more expensive to produce in pure form. Using a less reactive metal like Iron would drastically reduce the battery voltage and the reaction rate and therefore the current that could be produced.

Second, what is the purpose of the electrolyte? Why can't charges just flow through the battery to the other electrode, does the electrolyte stop that from happening? Lastly, what actually makes the electrons in the anode want to move to the cathode. Everybody just says that they want to but not why. Sorry for rambling I'm just frustrated that nobody can give me a clear answer.

The important thing with batteries is that the electrons can't move through the electrolyte. The current in the battery comes from the ions in the electrolyte, This current goes against the electric field, and it is caused by diffusion. At the negative pole, Zn²⁺ is produced. If there is lots of Zn²⁺ close to the negative pole, and less of it further away from it, it will diffuse away. Negative Ions will move towards where the Zn²⁺ is .to keep the electrolyte electrically neutral.

 

[Alex Hughes]

Thanks for your answers but I'm still a little confused. Refer to this picture I have of a diagram of a battery.

Can you walk through all the processes that happen when the two terminals of the battery are connected with a conductor. What I was asking earlier is why can't electrons just flow through the electrolyte (which I'm assuming is the paste of MnO2, Nh4Cl and Carbon right?), and flow directly into the cathode without having their terminals connected via a conductor. How does the electrolyte stop electrons from flowing inside the battery?

 

[DrClaude]

Electrons don't flow through the electrolyte because it is not conductive enough.

The basic mechanism in a battery is an redox reaction. One substance gets oxidized (loses electrons) while the other gets reduced (gains electrons). Now, if you put the two substances together, they will simply react and you won't gain much of it. So the idea is to separate the two substances, and allow them to perform the redox reaction only by exchanging electrons through a conductor, such as a wire, allowing you to use the energy of the reaction to produce some electrical work.

As with everything related to electricity, you need a closed circuit to keep the electrons flowing. This is achieved by an electrolytic bridge, where ions can be exchanged. In the dry battery illustrated, there is a thin layer of paper separating the zinc from the paste, such that ions can slowly diffuse but MnO₂ can't react directly with the zinc.

 

[Alex Hughes]

Electrons don't flow through the electrolyte because it is not conductive enough.

The basic mechanism in a battery is an redox reaction. One substance gets oxidized (loses electrons) while the other gets reduced (gains electrons). Now, if you put the two substances together, they will simply react and you won't gain much of it. So the idea is to separate the two substances, and allow them to perform the redox reaction only by exchanging electrons through a conductor, such as a wire, allowing you to use the energy of the reaction to produce some electrical work.

As with everything related to electricity, you need a closed circuit to keep the electrons flowing. This is achieved by an electrolytic bridge, where ions can be exchanged. In the dry battery illustrated, there is a thin layer of paper separating the zinc from the paste, such that ions can slowly diffuse but MnO₂ can't react directly with the zinc.

Thank you very much that helped a ton. The only thing I was confused about is your last paragraph. You said a closed circuit is needed to keep electrons from flowing. Isn't there already a closed circuit though when you connect the two terminals of the battery with a wire? How is separating the zinc and the electrolyte contributing to anything. Is it because since electrons leave the zinc and leave behind positively charged zinc ions, these ions need to diffuse to keep from a charge building up?

 

[DrClaude]

How is separating the zinc and the electrolyte contributing to anything.

You have to separate it to avoid a direct reaction of the zinc and the MnO₂.

Is it because since electrons leave the zinc and leave behind positively charged zinc ions, these ions need to diffuse to keep from a charge building up?

If there is an accumulation of ions, the reaction will stop. (The electrochemical potential will change as ions accumulate, to the point where there is no longer any potential difference between the anode and the cathode.) It is usually counter ions that diffuse, not the ions produced at the anode.

 

[Alex Hughes]

You have to separate it to avoid a direct reaction of the zinc and the MnO₂.If there is an accumulation of ions, the reaction will stop. (The electrochemical potential will change as ions accumulate, to the point where there is no longer any potential difference between the anode and the cathode.) It is usually counter ions that diffuse, not the ions produced at the anode.

What are counter ions? Also, if the paper serves as a bridge between the zinc to allow the ions to diffuse, which stops the build up of ions, how do batteries finally go dead? I always thought, once there is no longer a potential difference between the cathode and anode, the battery dies, but what you described seems like there will always be a potential difference?

 

[Dale]

why can't electrons just flow through the electrolyte

The charge carriers in the electrolyte are the ions. There are no free electrons in the electrolyte. The electrons are bound to the metal, not to a specific atom in the metal, but to the metal as a whole. If you want to remove an electron from a metal you have to overcome the so called work function.

 

[Alex Hughes]

The charge carriers in the electrolyte are the ions. There are no free electrons in the electrolyte. The electrons are bound to the metal, not to a specific atom in the metal, but to the metal as a whole. If you want to remove an electron from a metal you have to overcome the so called work function.

And you overcome this work function by connecting the two terminals of the battery with a conductor giving a path for the electrons to flow to into the cathode?

 

[Dale]

And you overcome this work function by connecting the two terminals of the battery with a conductor giving a path for the electrons to flow to into the cathode?

No, the work function is not overcome in a battery. The electrons do not leave the conductor.

The work function is overcome, for example, in a cathode ray tube where the cathode is heated so that the electrons have enough thermal energy to overcome the work function and leave the metal.

 

[davenn]

how do batteries finally go dead? I always thought, once there is no longer a potential difference between the cathode and anode, the battery dies

Once the battery chemistry can no longer sustain a chemical reaction, a potential difference cannot be generated = dead battery

did you actually go and read the www page I posted in post #2 ? it seems not

 

[Alex Hughes]

Once the battery chemistry can no longer sustain a chemical reaction, a potential difference cannot be generated = dead battery

did you actually go and read the www page I posted in post #2 ? it seems not

yes I did, it was a bunch of information and I couldn't seem to find where my question was answered. I'll go back and reread it again though.

 

[Alex Hughes]

Once the battery chemistry can no longer sustain a chemical reaction, a potential difference cannot be generated = dead battery

did you actually go and read the www page I posted in post #2 ? it seems not

Going over the link again, I actually did find it very helpful. Thanks

 

[davenn]

Going over the link again, I actually did find it very helpful. Thanks

you mite also find this helpful concerning the electrolyte and the chemical reaction

from
http://www.explainthatstuff.com/batteries.html

Chemical reactions
Now back to our battery. The positive and negative electrodes are separated by the chemical electrolyte. It can be a liquid, but in an ordinary battery it is more likely to be a dry powder.

When you connect the battery to a lamp and switch on, chemical reactions start happening. One of the reactions generates positive ions (shown here as big yellow blobs) and electrons (smaller brown blobs) at the negative electrode. The positive ions flow into the electrolyte, while the electrons (smaller brown blobs) flow around the outside circuit (blue line) to the positive electrode and make the lamp light up on the way. There's a separate chemical reaction happening at the positive electrode, where incoming electrons recombine with ions taken out of the electrolyte, so completing the circuit.

The electrons and ions flow because of the chemical reactions happening inside the battery—usually two of them going on simultaneously. The exact reactions depend on the materials from which the electrodes and electrolyte are made. (Some examples are given further on in this article where we compare different types of batteries. If you want to know more about the reactions for a particular battery, enter the type of the battery you're interested in followed by the words "anode cathode reactions" in your favorite search engine.) Whatever chemical reactions take place, the general principle of electrons going around the outer circuit, and ions reacting with the electrolyte (moving into it or out of it), applies to all batteries. As a battery generates power, the chemicals inside it are gradually converted into different chemicals. Their ability to generate power dwindles, the battery's voltage slowly falls, and the battery eventually runs flat. In other words, if the battery cannot produce positive ions because the chemicals inside it have become depleted, it can't produce electrons for the outer circuit either.

Now you may be thinking: "Hang on, this doesn't make any sense! Why don't the electrons just take a short cut and hop straight from the negative electrode through the electrolyte to the positive electrode? It turns out that, because of the chemistry of the electrolyte, electrons can't flow through it in this simple way. In fact, so far as the electrons are concerned, the electrolyte is pretty much an insulator: a barrier they cannot cross. Their easiest path to the positive electrode is actually by flowing through the outer circuit.Dave

 

[Alex Hughes]

you mite also find this helpful concerning the electrolyte and the chemical reaction

from
http://www.explainthatstuff.com/batteries.html

View attachment 220372

Chemical reactions
Now back to our battery. The positive and negative electrodes are separated by the chemical electrolyte. It can be a liquid, but in an ordinary battery it is more likely to be a dry powder.

When you connect the battery to a lamp and switch on, chemical reactions start happening. One of the reactions generates positive ions (shown here as big yellow blobs) and electrons (smaller brown blobs) at the negative electrode. The positive ions flow into the electrolyte, while the electrons (smaller brown blobs) flow around the outside circuit (blue line) to the positive electrode and make the lamp light up on the way. There's a separate chemical reaction happening at the positive electrode, where incoming electrons recombine with ions taken out of the electrolyte, so completing the circuit.

The electrons and ions flow because of the chemical reactions happening inside the battery—usually two of them going on simultaneously. The exact reactions depend on the materials from which the electrodes and electrolyte are made. (Some examples are given further on in this article where we compare different types of batteries. If you want to know more about the reactions for a particular battery, enter the type of the battery you're interested in followed by the words "anode cathode reactions" in your favorite search engine.) Whatever chemical reactions take place, the general principle of electrons going around the outer circuit, and ions reacting with the electrolyte (moving into it or out of it), applies to all batteries. As a battery generates power, the chemicals inside it are gradually converted into different chemicals. Their ability to generate power dwindles, the battery's voltage slowly falls, and the battery eventually runs flat. In other words, if the battery cannot produce positive ions because the chemicals inside it have become depleted, it can't produce electrons for the outer circuit either.

Now you may be thinking: "Hang on, this doesn't make any sense! Why don't the electrons just take a short cut and hop straight from the negative electrode through the electrolyte to the positive electrode? It turns out that, because of the chemistry of the electrolyte, electrons can't flow through it in this simple way. In fact, so far as the electrons are concerned, the electrolyte is pretty much an insulator: a barrier they cannot cross. Their easiest path to the positive electrode is actually by flowing through the outer circuit.Dave

Wow thanks a ton, that cleared a lot of things up. I think I finally understand it. So a battery consists of 2 electrodes, an anode and a cathode. If you just connected two metals (with different electronegativities) through a wire WITHOUT an electrolyte nothing would happen because electrons don't want to just leave their stable state and cause a difference of positive and negative charge. However, if the two metals are exposed to an electrolyte, A chemical reaction occurs between the anode and cathode at the same time. The electrolyte oxidizes the anion therefore removing the electrons from it leaving behind a positive ion. This ion combines in solution with the electrolyte. When a wire is connected between the two terminals, the leftover electrons from the reaction now have a place to go and move towards the cathode since it has a stronger electronegativity than the anode. Once the electrons reach the cathode they combine with the ions out of the electrolyte and reduce them. Eventually a battery will die when the reaction between the anode and the electrolyte cannot happen due to the anode not having anymore electrons to give up. Is what I said correct?

 

[Alex Hughes]

Wow thanks a ton, that cleared a lot of things up. I think I finally understand it. So a battery consists of 2 electrodes, an anode and a cathode. If you just connected two metals (with different electronegativities) through a wire WITHOUT an electrolyte nothing would happen because electrons don't want to just leave their stable state and cause a difference of positive and negative charge. However, if the two metals are exposed to an electrolyte, A chemical reaction occurs between the anode and cathode at the same time. The electrolyte oxidizes the anion therefore removing the electrons from it leaving behind a positive ion. This ion combines in solution with the electrolyte. When a wire is connected between the two terminals, the leftover electrons from the reaction now have a place to go and move towards the cathode since it has a stronger electronegativity than the anode. Once the electrons reach the cathode they combine with the ions out of the electrolyte and reduce them. Eventually a battery will die when the reaction between the anode and the electrolyte cannot happen due to the anode not having anymore electrons to give up. Is what I said correct?

Ok sorry for going on and on about this but last question. So does that mean the terminals of a battery, which we refer to as positive and negative, aren't actually positive and negatively charged but instead neutral? The negative and positive terminals are just an indication of the potential at that point on the battery, not the charge of the terminals? Something tells me this isn't right, but I just assumed if the anode is giving up electrons but also having the ion it leftover dissolve into the electrolyte, then the total charge of the anode isn't changing right?

 

[DrClaude]

If you just connected two metals (with different electronegativities) through a wire WITHOUT an electrolyte nothing would happen because electrons don't want to just leave their stable state and cause a difference of positive and negative charge.

The reaction doesn't concern two native metals, but one in its reduced form and the other oxidized. Take for example zinc and copper. If you mix metallic zinc with copper oxide (CuO), they will react together (you need to supply heat if they are in solid form) to form zinc oxide and metallic copper. The idea of the battery is to separate the two so they don't react directly, but have to exchange electrons through a wire.

Rechargeable batteries work by inverting the reaction: you supply electricity to reduce the oxide and oxidize the metal. The battery is then ready to be reused.

However, if the two metals are exposed to an electrolyte, A chemical reaction occurs between the anode and cathode at the same time. The electrolyte oxidizes the anion therefore removing the electrons from it leaving behind a positive ion. This ion combines in solution with the electrolyte. When a wire is connected between the two terminals, the leftover electrons from the reaction now have a place to go and move towards the cathode since it has a stronger electronegativity than the anode. Once the electrons reach the cathode they combine with the ions out of the electrolyte and reduce them. Eventually a battery will die when the reaction between the anode and the electrolyte cannot happen due to the anode not having anymore electrons to give up. Is what I said correct?

That's not correct, see above. Also, things don't happen in steps: the circuit has to be closed for the section to take place.

Ok sorry for going on and on about this but last question. So does that mean the terminals of a battery, which we refer to as positive and negative, aren't actually positive and negatively charged but instead neutral? The negative and positive terminals are just an indication of the potential at that point on the battery, not the charge of the terminals? Something tells me this isn't right, but I just assumed if the anode is giving up electrons but also having the ion it leftover dissolve into the electrolyte, then the total charge of the anode isn't changing right?

Everything is neutral. It is common when dealing with electricity to use + and - to indicate potential differences, not accumulation of charges.

 

[Alex Hughes]

The reaction doesn't concern two native metals, but one in its reduced form and the other oxidized. Take for example zinc and copper. If you mix metallic zinc with copper oxide (CuO), they will react together (you need to supply heat if they are in solid form) to form zinc oxide and metallic copper. The idea of the battery is to separate the two so they don't react directly, but have to exchange electrons through a wire.

Rechargeable batteries work by inverting the reaction: you supply electricity to reduce the oxide and oxidize the metal. The battery is then ready to be reused.That's not correct, see above. Also, things don't happen in steps: the circuit has to be closed for the section to take place.Everything is neutral. It is common when dealing with electricity to use + and - to indicate potential differences, not accumulation of charges.

Ok, so are you saying in a battery, the anode is already oxidized and the cathode is already reduced? I don't understand why this would be. I'm so confused now. Can you explain what exactly was wrong with what I said in the section?

Also, as I thought about it, doesn't something need to be positively charged in order to have a positive potential? If that's the case, how does one terminal of the battery have a positive potential and other a negative potential if they are both neutral?