Electrolytic cell anodes & cathodes

[Addallat]

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I'm having a hard time understanding why the anode is considered positive in electrolytic cells and negative in galvanic cells.


I understand that Anodes are where oxidation take place
Cathodes are where reductions take place
the flow of electrons is always anode to cathode

but for some reason I have a hard time connecting the above to why an anode is negative in a galvanic cell, and why it's considered positive in an electrolytic cell. Can someone please help me connect the dots so I don't have to rely on rote memorization.

 

[MJS4878]

I like to think of it as a matter of Delta G and spontaneity. Galvanic cells are spontaneous and the electrons flow from anode (-) to cathode (+), which makes sense logically.

In electrolytic cells it is non-spontaneous and the Delta G is positive so I like to think that because of this the electrons are forced in a direction that they may not want to go ( + to -).

Hope that helps!

 

[Addallat]

thanks for the memory trick appreciate it, but can anyone out there explain to me why anodes are considered positive in an eletrolytic cell and negative in a galvanic cell

 

[dcamkl1]

Okay, picture a circuit with an EMF source (ex: battery) connected to two capacitor plates.

The positive charges leave the battery at the (+) terminal and build up on one of the capacitor plates, causing the plate to have a net positive charge. Similarly, the negative charges leave the battery at the (-) terminal and build up on the other capacitor, causing it to have a net negative charge.

So you have two plates of opposite charges. If you put an anion (negative charge) it will go toward the positively charged plate. This positively charge plate is called the "anode," despite our intuition. Similarly, cations (positive charge) flow toward the negatively charged plate that is called the "cathode."

So, anions flow to the anode, and cations flow to the cathode based on electrical attraction.

For an electrolytic cell, you need an EMF source to allow unfavorable redox reactions to proceed. It follows the same principle as above.

 

[QrtrLifeCrisis]

Don't sweat it. I had the same issue when I was studying, but it's not worth wracking yourself over. It's just a convention. Just know it and you'll be fine.