Electrolytic Cells

[Dr Gerrard]

I don't understand why in electrolytic cells, the anode is positive and the cathode is negative.

Are these just arbitrarily assigned? The same thing happens in both electrolytic cells and galvanic cells.

That is, in both cells, oxidation occurs at the anode, thus creating electrons. These electrons in turn flow towards the cathode for reduction of another species.

Why do we say the anode is negative in one (galvanic cells) while positive in the other (electrolytic cells)?

I understand that the anode should be negative in the galvanic cells, because that is where electrons are produced. As more electrons flow to the cathode, more electrons are produced keeping a negative charge.



I guess I just don't see any reason to call the anode positive in an electrolytic cell besides convention. But WHY is this the case???

EDIT --- Also, question 2, lets say Zn is being oxidized while Cu2+ is being reduced. I understand why you must have a Cu2+ solution on the side of the cathode, so there is a source for the copper ions, but why do you need a Zn solution on the anode side? What is the purpose of the electrolyte solution there?

Question 3 --- The salt bridge is for movement of ions to maintain a charge balance to prevent the cell from reaching zero potential, correct? Well what are the ions moving across? Not the Zn or the Cu, right? What anions flow towards the anode, and what cations flow towards the cathode? Say it is a KCl salt bridge. All of the Cl will move towards the anode, and the K will move towards the cathode. But the cathode is positive, while the anode is negative. Why does this still happen? Also, what happens once all of the K ions are at the cathode and Cl ions are at the anode? What replenishes this supply?

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[loveoforganic]

1) It's my opinion that the positive/negative terminology is completely arbitrary and only the way it is due to convention. There isn't actually any charge or lack thereof present in an electrode - there's only a tendency to gain electrons relative to something else.

2) If you're asking why you need an electrolyte solution on the anode side at all... I'm not really sure =/ If you're asking why it should be a zinc salt solution, it's to prevent undesireable side reactions from occurring.

3) What a salt bridge is is an unreactive hollow plastic U-tube. It's filled with an agar gel that was made with salt water (usually KCl). Picture your galvanic cell - electrons flow from the anode to cathode. This means you're gaining a net postive charge on the anode and a net negative charge on the cathode. The electrostatic force is very, very powerful, probably 10^10 orders of magnitude or more powerful than the forces driving the redox reaction. Without a salt bridge, enough repulsive charge would accumulate very quickly to halt the reaction. When you have your salt bridge place in solution, the K+ and Cl- ions can keep the charge gradient from forming to too large of an extent. As electrons (negative charge) leaves the anode, Cl- anions gravitate toward the anode end of the salt bridge, to compensate for that lost charge. Likewise for K+ moving toward the cathode.

 

[Cjc555777]

1. okay so the reason the anode is positive and the cathode is negative in an electrolytic cell is because of the battery you have attached. In an electrolytic cell you have a battery set up and the negative side faces the cathode and the positive side faces the anode.

2. If you use the Nernst equation and you don't have the amount of solution there, you will change the potential of the cell.

3. The salt bridge is a way to maintain charge balance, that is correct. The anode is the site of oxidation and the cathode is the site of reduction. When an compound/element undergoes oxidation, losing an electron and it becomes more positive. So in order to keep the charge balanced on that side the anion of the salt bridge will go to that side to negate the increase in positive charge. Once the salt bridge runs out, well you need a new salt bridge because the charge can no longer remain balanced.

 

[loveoforganic]

1) doesn't sound right to me. Shouldn't the battery be the other way?

2) Could you give a conceptual reason that you need an electrolyte solution?

 

[Cjc555777]

1. This one is correct. The negative side faces the cathode because the negative side produces the electrons, these electrons then flow down to the cathode where reduction happens. This is why the battery is set up this way, and this is why it's named this way.

2. You really don't need an electrolytic solution, it's perfectly fine without it. In lab with my professor we've done experiments where one solution was Zn2+ and had a Zn wire and the other solution was Cu2+ and the wire was just Cu. When you look at the Nernst equation you get E=Eo-(RT/nF)(ln(reduced/oxidized)) The activity of both "reduced" forms will be 1 because the activity of any metal in it's solid form is 1. So it is unnecessary to have the electrolytic solution, but it does affect the electric potential if you don't have one and just have the other.

 

[loveoforganic]

1) That makes sense, sorry, was thinking anode/cathode as in a galvanic cell.

2) Good! I was having trouble coming up with a reason you'd have to have that solution for it to run.

 

[Dr Gerrard]

But that would still mean the positive/negative notation for a galvanic cell is arbitrary, correct?

Because say we had a cell without a salt bridge, then the anode side would become positive while the cathode side would become negative. However, the anode is what is called negative, while the cathode is called positive, arbitrarily to simply signify the direction of electron flow.

 

[Dr Gerrard]

Additionally, for your answer to question 2, so do you have to have the ions present along with the metals, but usually its just that the ions are presented as salts in an electrolyte solution?

I basically am not sure I understand your use of the Nernst equation.
The only use of the Nernst equation I am familiar with is to find the cell potential for a concentration cell.

I do not really see how it relates to an electrolyte solution.

Other than that, THANKS!! Both of your responses helped.

 

[Dr Gerrard]

Especially if we have standard conditions inside the cell, then E = Eo and so the extra aspects of the nernst equation don't even matter.

 

[loveoforganic]

For your last question, standard conditions include the solution concentration the electrode is in. I think standard conditions is 1M.

 

[Cjc555777]

Nothing is arbitrary. In a galvanic cell the electrons are produces on the anode there fore is it negative. And the Nernst equation will help you out if you just use some examples. Because changing the concentration changes the electrical potential. That's why you use the Nernst equation in the first place. Also, I said that the solution doesn't even need to contain the ions, the experiment can be done without it.

 

[loveoforganic]

I still say it's arbitrary. You could just as easily say, "the electrons are leaving the anode, therefore it's positive." No electrons are being produced, they're just being moved.

 

[bkz]

i think it goes back to the Gibbs free energy equation. Electric potential is related to free energy...the universe likes low potential energy and high entropy...so spontaneous reactions should be negative delta G.. If you try to derive the Nernst equation thinking about energy change, you get cell potential = E= E^0 - kT/ne InQ...n is the number of electrons transferred, Q is rxn quotient, T is absolute temperature, k=boltzmann constant

deriving equations always helps!!

edit: see http://hyperphysics.phy-astr.gsu.edu/HBASE/CHEMICAL/electrode.html

 

[bkz]

so did I do that right? I'd really like someone to correct it if I am thinking about it wrong...thanks 🙂

 

[Cjc555777]

Yeah, you derived it right, and if it helps you remember it then that's great, I just don't think it has anything to do with the naming. Maybe i'm wrong. But you did derive it correctly which in my opinion is the most important part of the PS section

 

[enemi]

Here's what the exam crackers say about the naming convention, althought I don't quite understand it fully.

"The assignment of positive and negative to electrodes in galvanic and electrolytic cells is based upon perspective. Galvanic cells are used to provide energy to an external load, so the electrodes re labeled so that the negative electrons will flow toward the positive electrode. Electrons flow from the load to the cathode so the cathode is labeled positive in the galvanic cell. The focus of electrolytic cells is within the cell itself. For instance, electrophoresis uses an electrolytic cell. It is important that negatively charged amino acids within the electrolytic cell flow toward the positive electrode, so the anode is labeled positive in the electrolytic cell."

CAN SOMEONE PUT THIS IN PLAIN ENGLISH? please. thanks. 🙂