Anode Signs in Galvanic vs. Electrolytic cells

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SaintJude

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Can someone please explain/or at least give me a mnemonic to remember that in

Galvanic cells: anode is negative
Electrolytic cells: anode is positive

Also, what's the salt bridge got to do with potential difference in a galvanic cells?

Edit: Actually it looks like both electrolytic and galvanic cells need salt bridges. The pictures I saw were coincidences.

Thank you!!

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I don't have a mnemonic but the way I remember is this:
Oxidation always happens in the anode (both vowels)
Reduction always happens in the cathode (both consonants)
that is independent if the cell is galvanic or electrolytic. The electrons are always going to be moving from the anode to the cathode. Then, you just have to think about the spontaneity of the cell. If the cell is spontaneous, as galvanic is, it makes sense that the electrons will be moving towards a positive electrode, the cathode is positive. While for electrolytic, which is not spontaneous, an electron is actually being forced to move towards a negative pole, the cathode is negative.

In a Galvanic cell, the electrons flow from the anode to the cathode, as mentioned. This means that cations are being released from the electrode into the anode side as the electrons flow away from the anode. In order to balance the increasing amounts of cations at the anode, anions will flow from the salt bridge to the anode in order to maintain an overall neutral charge in the half-cell. Referring to the anode as "negative" in a Galvanic cell is simply a convention and is based on the fact that negative ions flow from the salt bridge into the anode half-cell.

I hope this helps.
However, I would be curious if someone has a quicker way to answer this and not have to go through all through this rationale to get to the answer.
 
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About the sign designations, that makes a lot of sense. Since galvanic cells always have chemical energy to convert to work, they are thus spontaneous. And b/c they are spontaneous it makes sense, that the electrons will move towards the positively charged cathode. Thanks thais!!

About the salt-bridge. Still confused. Edit: Can one say that the purpose of the salt bridge is to "complete the electrical circuit"? Edit: Saw a video. Answer to this is yes.

And EK talks about the salt bridge in relation to potential difference... b/w what?
 
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About the sign designations, that makes a lot of sense. Since galvanic cells always have chemical energy to convert to work, they are thus spontaneous. And b/c they are spontaneous it makes sense, that the electrons will move towards the positively charged cathode. Thanks thais!!

About the salt-bridge. Still confused. Edit: Can one say that the purpose of the salt bridge is to "complete the circuit"? And EK talks about the salt bridge in relation to potential difference... b/w what?

The salt bridge is there to keep charge from building up on one side of the cell. If there is no way for negative charges (anions) to leave the cathode, eventually the reaction will cease. The salt bridge allows anions to leave the cathode (where stuff gets reduced = turned into anions) and move to the anode (where things get oxidized). If all the negative charge built up in the cathode, there would be no flow of electrons.
 
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I understand that when you put in that terminology, but I don't think I understand it completely b/c once EK switches up the wording I'm confused.

EK says
When a cell contains 2 different solutions, a liquid junction is required to separate the solutions. Because ions can move across a liquid junction, any liquid junction creates an additional small potential difference that the affects the potential of the galvanic cell. A salt bridge is a type of liquid junction that minimizes this potential difference

Can someone explain?
 
I understand that when you put in that terminology, but I don't think I understand it completely b/c once EK switches up the wording I'm confused.

EK says

Can someone explain?

It's the same thing that I explained above (but you already know that ;)).

So if you have no salt bridge, eventually all of the electrons from the anode are going to move into the cathode and your will have 0 electron flow. The potential difference will be huge because the anode will be at 0 and the cathode will be at whatever the maximum is.

The salt bridge acts as a way to alleviate the buildup of charge on the cathode side. It's like a fancy billiards table that has a trough from all of the pockets so that one pocket doesn't fill up with balls as the game progresses. The trough is the salt bridge that is preventing the pocket (cathode) from filling as you shoot balls (electrons) into it.
 
It's the same thing that I explained above (but you already know that ;)).

So if you have no salt bridge, eventually all of the electrons from the anode are going to move into the cathode and your will have 0 electron flow. The potential difference will be huge because the anode will be at 0 and the cathode will be at whatever the maximum is.

Ohhhh,:idea:

Thank you!
 
I don't have a mnemonic but the way I remember is this:
Oxidation always happens in the anode (both vowels)
Reduction always happens in the cathode (both consonants)
that is independent if the cell is galvanic or electrolytic. The electrons are always going to be moving from the anode to the cathode. Then, you just have to think about the spontaneity of the cell. If the cell is spontaneous, as galvanic is, it makes sense that the electrons will be moving towards a positive electrode, the cathode is positive. While for electrolytic, which is not spontaneous, an electron is actually being forced to move towards a negative pole, the cathode is negative.

In a Galvanic cell, the electrons flow from the anode to the cathode, as mentioned. This means that cations are being released from the electrode into the anode side as the electrons flow away from the anode. In order to balance the increasing amounts of cations at the anode, anions will flow from the salt bridge to the anode in order to maintain an overall neutral charge in the half-cell. Referring to the anode as "negative" in a Galvanic cell is simply a convention and is based on the fact that negative ions flow from the salt bridge into the anode half-cell.

I hope this helps.
However, I would be curious if someone has a quicker way to answer this and not have to go through all through this rationale to get to the answer.


mnemonic: AN OX, RED CAT =)
 
Can someone please explain/or at least give me a mnemonic to remember that in

Galvanic cells: anode is negative
Electrolytic cells: anode is positive

Also, what's the salt bridge got to do with potential difference in a galvanic cells?

Edit: Actually it looks like both electrolytic and galvanic cells need salt bridges. The pictures I saw were coincidences.

Thank you!!

I remembered it like this....

Galvanic cells are spontaneous and the direction of electron flow is from Anode to Cathode (An Ox & Red Cat). Since like charges repel and an electron is negative, the Anode must be negative as well.

Electrolytic cells are used for measureing things in biological systems (generalizing a lot) and I associate them with electrophoresis. Running gels requires some sort of potential difference to operate the cell and the direction of electron flow has to be the same as before (An Ox & Red Cat). The focus of these cells is on what's being measured so the determination of the postiive and negative poles is based on the "what." Since 99% of the time the "what" being ran on the gel is negative (DNA, Protein, RNA, etc) and negative things move away from negative things, in this case the build up of negative charge at the cathode....the anode must be positive.

Rather then memorize the latter, I just memorized how galvanic cells work along with their labeling and remember that the signs are the opposite for electrolytic cells. Also, when I read electrolytic, I think SDS Page or regular ol' electrophoresis.
 
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Can someone please explain/or at least give me a mnemonic to remember that in

Galvanic cells: anode is negative
Electrolytic cells: anode is positive

Also, what's the salt bridge got to do with potential difference in a galvanic cells?

Edit: Actually it looks like both electrolytic and galvanic cells need salt bridges. The pictures I saw were coincidences.

Thank you!!

You dont need a salt bridge for electrolysis...
 
I don't have a mnemonic but the way I remember is this:
Oxidation always happens in the anode (both vowels)
Reduction always happens in the cathode (both consonants)
that is independent if the cell is galvanic or electrolytic. The electrons are always going to be moving from the anode to the cathode. Then, you just have to think about the spontaneity of the cell. If the cell is spontaneous, as galvanic is, it makes sense that the electrons will be moving towards a positive electrode, the cathode is positive. While for electrolytic, which is not spontaneous, an electron is actually being forced to move towards a negative pole, the cathode is negative.

In a Galvanic cell, the electrons flow from the anode to the cathode, as mentioned. This means that cations are being released from the electrode into the anode side as the electrons flow away from the anode. In order to balance the increasing amounts of cations at the anode, anions will flow from the salt bridge to the anode in order to maintain an overall neutral charge in the half-cell. Referring to the anode as "negative" in a Galvanic cell is simply a convention and is based on the fact that negative ions flow from the salt bridge into the anode half-cell.

I hope this helps.
However, I would be curious if someone has a quicker way to answer this and not have to go through all through this rationale to get to the answer.
I'm having difficulty grasping why we use different signs for the anode and cathode in the electrolytic vs. galvanic cells. TBR didnt explain it as clearly as I would have liked. they went through a long explanation of how anode and cathode are positive vs. negative depending on whether a biochemist or a physicist is looking at it (biochemist looks at where ions are attracted- like a capacitor, and a physicist looks at where electrons move - like a battery). However, they didnt go into an explanation of how the signs change with galvanic vs. electrolytic cells. Instead, they just focused on the spontaneous vs. nonspontaneous aspect, which i understand.

If anyone could help clarify this a bit more that would be great thank you!
 
ALso, TBR says on page 269 of the chem 2 book "cathodes have a positive core and accumulate neg charge on their surface, as current flows. Anodes have a neg core and accumulate positive charge on their surface as current flows." I'm having difficulty understanding what they mean. Are they just referencing the figure 10-3 where the batter is considered the core and the plates are considered the surface??
 
Remember: RED CAT AN OX
o Reduction at the cathode
o Anode hosts oxidation
Electrons always flow from anode to cathode
• As the redox reaction continues, electrons will continue to flow to the cathode
o But this would induce a build-up of negative charge at the cathode, which is energetically
unfavorable
o The charge build-up would be so unfavorable that it'd halt the reaction
• To prevent the build-up of charge, we introduce a salt bridge
• The salt bridge is a salt containing channel that connects
the 2 solutions to balance the charges.
Galvanic is spon, so e- move from Anode to cathode because they love to do that , whereas in Electrolytic non-spon it consume energy to move an e- from a positive anode to a negative cathode , ( negative e- hate negative cathode, but they are forced )
 
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Thank you for your response. Just to clarify, besides for spontaneous/nonspontaneous difference, all that changes between glavanic and electrolytic is what is defined as negative or positive, however everything else, like reduction at the cathode and oxidation at the anode still remains the same?

Also, could you comment on how this relates to the quote from BR
" TBR says on page 269 of the chem 2 book "cathodes have a positive core and accumulate neg charge on their surface, as current flows. Anodes have a neg core and accumulate positive charge on their surface as current flows." I'm having difficulty understanding what they mean. Are they just referencing the figure 10-3 where the batter is considered the core and the plates are considered the surface?? "
 
The following rules hold true for both electrolytic and galvanic/voltaic cells.
Anode is always the place where oxidation happens.
Cathode is always the place where reduction happens.
electrolyte
Ions = electrolyte.
Electrolytes conduct electricity by the motion of ions.
Without electrolytes, there won't be a circuit because electricity won't be able to travel.
electron flow; oxidation, and reduction at the electrodes
Electrons shoot out of the anode because oxidation occurs there to lose electrons. M → M+ + e-.
Electrons travel into the cathode, where it crashes into the cations on the surface of the cathode. This is because reduction occurs at the cathode to receive electrons. M+ + e- → M.
 
Thank you for your response. Just to clarify, besides for spontaneous/nonspontaneous difference, all that changes between glavanic and electrolytic is what is defined as negative or positive, however everything else, like reduction at the cathode and oxidation at the anode still remains the same?

Also, could you comment on how this relates to the quote from BR
" TBR says on page 269 of the chem 2 book "cathodes have a positive core and accumulate neg charge on their surface, as current flows. Anodes have a neg core and accumulate positive charge on their surface as current flows." I'm having difficulty understanding what they mean. Are they just referencing the figure 10-3 where the batter is considered the core and the plates are considered the surface?? "
electrons flow Anode to Cathode.
so the opposite direction is the flow of current, i.
current is ALWAYS referred to the FLOW OF POSITIVE CHARGES.

ANODE is the source of electrons, electrons flow out from the Anode to the Cathode. thus, anodes have a negative core because it is the source of electrons, but has a positive surface because electrons flow away from anode. and current flows from Cathode to Anode.

CATHODE is the where the electrons end up! so they say cathode has a positive core but a negative surface because the electrons flow from Anode to Cathode!


this might help:
"AN OX. RED CAT"

Anode is where Oxidation occurs.
Reduction happens at the Cathode.


it's more confusing to try & put a (+) or (-) for anode or cathode without knowing the problem/question/cell set-up. you have to recognize if there is a voltage applied to the cell or not, and if the cell is spontaneous on its own or not (aka, if it's electrolytic or galvanic).



hope this helped? :)
 
Thank you for your response. Just to clarify, besides for spontaneous/nonspontaneous difference, all that changes between glavanic and electrolytic is what is defined as negative or positive, however everything else, like reduction at the cathode and oxidation at the anode still remains the same?

Also, could you comment on how this relates to the quote from BR
" TBR says on page 269 of the chem 2 book "cathodes have a positive core and accumulate neg charge on their surface, as current flows. Anodes have a neg core and accumulate positive charge on their surface as current flows." I'm having difficulty understanding what they mean. Are they just referencing the figure 10-3 where the batter is considered the core and the plates are considered the surface?? "

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-reviews-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

upload_2014-3-31_22-52-50.png
 
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.**

sticker,375x360.png

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.
 
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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-reviews-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'll bet you a jar of Nutella that @Meredith92 is done worrying about cathode vs. anode complications. :)

As for me - signs are all relative. I might have even forced myself to memorize at some point which one is which but that is not something that I have ever retained long term. @inasensegone has plenty of good points, so I don't think I have anything productive to add at this point.
 
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I'll bet you a jar of Nutella that @Meredith92 is done worrying about cathode vs. anode complications. :)

As for me - signs are all relative. I might have even forced myself to memorize at some point which one is which but that is not something that I have ever retained long term. @inasensegone has plenty of good points, so I don't think I have anything productive to add at this point.

haha i'm not willing to take that wager.
 
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