Galvanic cells and Electrolytic cells question

[Ckvo77]

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Dear SDN,

Can you guys please check my knowledge on galvanic and electrolytic cells? The following is modeled off a question from TBR CBT 6 question 46.

Zn2+ + 2e- = -.76 V
Cu2+ + 2e-= .34 V

If we have a galvanic cell that has Zn(s) as the reducing agent (in our anode) and Cu2+ as our oxidizing agent (in our cathode), t
hen electrons go from anode to cathode. Zn(s) dissociates and Cu2+ plates out. Cathode is positive and anode is negative.

However, if we were to apply a battery voltage, the cell becomes an electrolytic cell and the unfavorable rxn occurs:

Zn2+ is our oxidizing agent (in our cathode) and Cu(s) is now our reducing agent (in anode), e- still goes from anode to cathode.
Zn2+ plates out now and Cu(s) dissociates. So basically their roles change.According to definition, the anode of an electrolytic cell is positive and
the cathode is negative. Why is this so? Shouldn't the anode still be negative and the cathode still positive? Am I missing anything?

 

[LianaStan]

Your understanding of galvanic cells in the first paragraph is correct.

As for the electrolytic cells, an easy way to think about it is that instead of the plates (cathode and anode) being connected to each other through which electrons can transfer spontaneously, they are now each connected to the separate ends of a battery instead of to each other (this must be so because an external voltage must do work in the nonspontaneous rxn of the electrolytic cell).

Now, the important point here is that the anode would be connected to the positive terminal of the battery (oxidation by 'receiving' positive charges from the positive end) while the cathode would be connected to the negative terminal of the battery (reduction by receiving electrons from negative end). Their designation as positive and negative corresponds to the ends of the battery that they are connected to, hence the sign switch from galvanic to electrolytic.

 

[383]

So a positive terminal (anode or cathode) always loses electrons, while a negative terminal gains them?

Does this mean a cathode is always negatively charged because it is the site of reduction with electrons present? What is the point of designating the cathode as + or - ?

 

[dontbeanegaton]

The designation of + or - is confusing and not all that useful. Reduction always occurs at the cathode. In a galvanic cell, that means that electrons reduce ions in solution at the cathode and cause them to "plate out" onto the cathode. Because electrons flow from the anode to the cathode, and electrons always flow from low to high potential (a + test charge flows from high to low), you therefore call the cathode positive.
In an electrolytic cell, the anion is connected to the positive terminal, and anions from solution migrate to the anode where they lose their electrons.
The reason this is confusing is that the +/- designation in galvanic cells is basically just convention. There is no actual charge separation if you have a salt bridge, since anions flow into the anode chamber to balance the new cations, and cations flow into the cathode to balance the loss of cations.

 

[LianaStan]

The designation of + and -, while I agree may not seem all that useful, is a tested concept on the MCAT. Therefore, it would be wise to understand why the designations are different in electrolytic vs galvanic cells.

In the electrolytic cell case, the designation is based on the anode's connection to the positive end of a battery and the cathode's connection to the negative end.

In the galvanic cell, the electrons flow spontaneously from anode to cathode, thus the anode and cathode are the battery. Since electrons will flow spontaneously to the higher potential, the cathode is designated as the positive potential.

In both scenarios the cathode and anode are the sites of reduction and oxidation respectively. This does not change. The +/- designation changes in accordance to what is supplying the voltage, either an external battery for nonspontaneous processes or the anode & cathode itself acting a chemical battery.