Hey meredith did you ever get this cleared up?
I looked back at a few old threads and found conflicting details and also am confused regarding this
this thread has like 5 different answers
http://forums.studentdoctor.net/threads/anode-and-cathode.403273/
http://forums.studentdoctor.net/threads/berkeley-reveiws-unique-approach.980939/ @BerkReviewTeach did good job explaining it here, but didn't go into electrolytic cells..so am still wondering about those
for galvanic cell: I get that electrons from anode to cathode as BerkReviewTeach defines current as flow of positive charge opposite to that of electron flow. Cations flow to
negatively charged surface cathode and Anions flow to
positively charged surface anode. But would MCAT ever ask us which is the positively or negatively charged electrode? Is this a feasible question? Do we answer with the "surface answer" or the "core answer'?
And for electrolytic cells, how does the "Core/surface" charges work out? as per BerkReviewTeach's explanation?
Any clarifications would be super appreciated!
@milski @inasensegone @Meredith92
MCATreview dosn't even have the +s/-s on their diagrams
View attachment 179884
I highly doubt the MCAT would ever ask you about the convention of sign usage for the charge on an anode or cathode, given that the convention is different among different scientific disciplines. For example, as per TBR, a biologist would say that the anode is positively charged because anions (negatively charged) flow to the anode. A physicist on the other hand would say that the anode is negatively charged because electrons flow from the anode (remember oxidation happens at the anode and thus electrons are leaving the anode).
The two above definitions clearly conflict with each other. This is why MCAT is very highly unlikely to ask something along these lines.
As for electrolytic cells, the definition that BerkReviewTeach provides in the above holds true universally:
**Oxidation happens at the anode, and reduction at the cathode.
An
Ox,
Red
Cat.
Anions flow to the anode and cations flow to the cathode.
Electrons flow from the Anode toward the cathode.**
So how does this apply to the electrolytic cell? Well the only thing that has changed is that now you have supplied a voltage source that will cause a current to counteract the natural EMF of the cell. So as the pictures from MCAT-review show, in the absence of a voltage source (galvanic cell), silver would preferentially be reduced and would plate out at the cathode. Once you supply a voltage source (electrolytic cell) the current is reversed,
and the electrode labels have been reversed. The silver electrode is now at the anode where oxidation occurs.
It makes sense that MCAT-review doesn't show a + or - charge associated with the electrodes, because these are just conventions. Both are right, and both are wrong.
What TBR says about the cores and surface is more of an attempt to create a multi-discipline approach. The conclusion is that the electrons leave the anode so the surface of the anode is positive because it has a bunch of electrons leaving the surface and this makes the surface positive. Similarly, all the electrons that are leaving the anode must be coming from the core of the anode, so the anode has a negative core. By this logic, the surface of the cathode is gaining negative charge because all the electrons are arriving there from the anode. Also, it must have a positive core if it is to be attracting the negative electrons from the anode. Again, this is just TBR's way of trying to rationalize the seeming contradiction of the two definitions between physicists and biochemists.
I HIGHLY doubt MCAT is going to test this point somehow without a passage for perspective. They're MUCH more likely to ask you something along the lines of "where do the negatively charged proteins migrate to when you run a gel in gel electrophoresis?"... although the question will likely be a little more convoluted than that, it is going to be asking something that can be answered based off of the universal definition that TBRteach has provided in the above link.