Hey Sukham,
1.One of the differences between electrolytic and galvanic cells is that for galvanic cells the cathode is positive and the anode is negative and this is the opposite for electrolytic cells.
I was wondering what what makes a cathode positive or negative and what makes an anode negative or positive.
This is a great question. I will answer it after I have addressed your second point.
Also, does difference in reduction potential drive reactions in a galvanic cell and the external energy source drive reactions in a electrolytic cell?
Yes exactly, and this is what is significant to the positive and negative charge of the anode and cathode. The following explanation is quite beyond the scope of the MCAT, but interesting nonetheless.
In a galvanic cell, redox reactions happen spontaneously.
As a result, if you are having reactions:
1.Cu --> Cu2+ + 2e- at the anode
2.Ag2+ + 2e- --> Ag at the cathode
We should consider what the order of these reactions is, and what drives the flow of these electrons.
We would logically say that the electron origin event must occur first, that is, this process would need to initiate with one Cu --> Cu2+ +2e-. If we take a snapshot of this initial moment there is no electric field to compel these electrons to go through the circuit.
It is only when you get a certain amount of electrons that you get a negatively charged anode, and positively charged Cu2+ ions in solution, which would then attract Cl- anions from the salt bridge. To prevent polarization, the solution on the cathode side would also become slightly positive to match this side of solution, which will then start drawing the electrons through the circuit. However, to maintain electron flow spontaneously through the circuit, this negative anode and positive cathode must be maintained.
This all happens on such a short timescale that it looks instantaneous, but this is why for a galvanic cell the anode is negative and the cathode positive. I imagine it like electrons moving out of their homes when they know it is the right time.
In contrast, an electrolytic cell is almost like evicting electrons when they aren't ready to leave their homes.
An electrolytic cell uses an external electric field to force reactions to happen.
Here the reactions are unchanged:
1.Cu --> Cu2+ + 2e- at the anode
2.Ag2+ + 2e- --> Ag at the cathode
However, the difference here is that the first event is an electron being forced from the anode to the circuit immediately by the external electric field. This causes it to shoot into the cathode under the influence of the external electric field.
In this way, the cathode becomes negative and has to "catch up" delivering cations to this spot to be reduced, making it negatively charged. Similarly, the anode has had electrons snatched from under its nose, and needs some time to "catch up" and lose cations to try to become less positive.
I hope this helps!
Katt