Polarity of electrodes in electrolytic cells?? (1 Viewer)

[Jake1892]

Hi . I had always thought that the electrodes have opposite polarity for electrolytic cells (i.e. anode is +). This was confirmed in the diagrams of a few textbooks that I've looked in as well. However:

1) I can't understand why this would be the case - as reduction always occurs at the cathode in any cell so why would the fact that the cell is non-spontaneous changed the electrode polarity?

2) My teacher taught the opposite in class. He said the cathode is positive in both electrolytic and voltaic cells. But on further interrogation he said the top of the cathode is positive (where the wire attaches) as it is an electron sink, wheres the bottom of the cathode plate is negative. (this confuses me further, but makes sense in a way.) He is usually an excellent chemist but not the best at explaining.

So, can anybody please explain to me what the polarity of the electrodes in electrolytic cells are and why?

P.S perhaps someone could define polarity and what constitutes it. Also, when we say 'the polarity of the anode', do we generally mean the top of the anode, or the bottom, or either? (this would be critical if my teacher's explanation is correct?)

My current understanding is that if i had to label an electrolytic cell in the hsc exam, i would put a fat plus sign near the anode and a fat - near the cathode and be done with it...

Any help would be appreciated

 

[YoloStudent]

You can simply look at electron flow. Electrons flow to the positive from the negative terminal. Oxidation occurs at the anode and reduction at the cathode. Follow the electron flow.

Polarity: For simplicity's sake, the "charge" of an electrode. Electrons are attracted to positive charges so if an electron is attracted to an electrode, it's positive.
I reckon polarity in these cells are in the region where the electrode is in contact with the solution since that is where the electrons "originate" from. I haven't seen a question asking to show where the exact region of polarity is, its usually just the entire electrode.

Now lets answer your first question. Consider an galvanic cell with iron and zinc and a galvanic cell with iron and zinc with the appropriate applied voltage so the reaction occurs (of course the electrolyte and salt bridge etc are there). In a galvanic cell, zinc oxidises, iron reduces. We know that oxidation occurs at anode and reduction occurs at cathode. Since zinc is oxidising, it is losing electrons while iron ions gain the electrons. The flow of electrons here is from Zinc electrode to Iron electrode so zinc is negative (electrons are repelled) and iron is positive (electrons being attracted to it).

Now in a electrolytic cell, we know that the reactions are reversed. Iron oxidises and zinc reduces so iron loses electrons and transfers it to zinc ions. Iron electrode is now negative and zinc is positive.

See how the polarity has changed?

You're probably assuming polarity is constant with the anode and cathode. Its not. Think about electron flow.

You should ignore the top part of an electron being positive and bottom negative or something like that. It doesn't need to be known in the hsc. If you want to understand, sure go for it but don't if it'll just confuse you more.