Conditions when anode/cathode are positive vs negative /Kaplan FL 3 B/B 44

Question

Can someone list all the places anode or cathode are used and when each are + vs -? I am struggling on a way to remember when the anode/cathode is + or -. I have watched too many videos explaining the difference between galvanic vs. electrolytic cells and I start to confuse all the tips like 'the cathode attracts cations..so it is negative'. Does anyone have way to remember the conditions well? Below are tips someone mentioned referring to electrolytic cells that seem to help because it's clear what is firm vs variable. ​ I thought I was starting to get the hang of identifying when anode is +/-, but then on a practice full length (Kaplan FL 3 B/B) it asked about electrophoresis migration to anode or cathode and I didn't equate that it would be similar to electrolytic cell. ​ OIL RIG: ALWAYS An Ox/Red Cat: ALWAYS Electrons flow anode to cathode: ALWAYS Which is positive/which is negative: Depends ​ Thank you!!

Answer 1

Hey Carol!

The positive/negative designations of electrochemical cells can definitely be a confusing topic. You are completely right in saying that reduction always happens at the cathode, and oxidation always happens at the anode! So definitely continue to rely on that fact as your firmest characterization of electrodes in a cell.

As for positive and negative, the designations change based on whether the cell is spontaneous (emf>0) or non-spontaneous (emf<0). In spontaneous cells (galvanic cells, concentration cells, anything that does not require outside energy), the cathode is positive and the anode is negative. In non-spontaneous cells (electrolytic cells, anything that requires a battery, electrophoresis is a prime example), the reverse happens: the cathode is negative and the anode is positive.

As for remembering this, I like to think of the spontaneous cells simply as regular cells! On a regular basis, we like to think of the cathode as positive and the anode as negative, so spontaneous cells simply follow that pattern. On the other hand, non-spontaneous cells are "backwards" (they use a battery to force the electrons to go in the opposite direction that they do normally). Thus, the positive and negative designations are "backwards" as well. Hope this helps!