deciding which cell is the cathode/anode

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Aoife Galvin 2D
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Joined: Fri Sep 25, 2015 3:00 am

deciding which cell is the cathode/anode

Postby Aoife Galvin 2D » Wed Feb 10, 2016 11:12 am

How do you know which one is the cathode and which one is the anode?
For example in midterm 2014 question 8, the E standard values are given as +3.03 and +2.87. I thought that the smaller positive number was the one where the reaction should be switched and the E standard value changed to a negative in order to end up with a positive E value overall. But in this question, the opposite occurs and the E value in the end is negative. Why is this the case?

Cole Malkoff 3E
Posts: 15
Joined: Fri Sep 25, 2015 3:00 am

Re: deciding which cell is the cathode/anode

Postby Cole Malkoff 3E » Wed Feb 10, 2016 2:10 pm

I was wondering the same thing. You'd think a positive E value would be desired because that would give a negative G value so the reaction would be spontaneous. Somebody please respond to this!

Ariana Bowers 1H
Posts: 9
Joined: Fri Sep 25, 2015 3:00 am

Re: deciding which cell is the cathode/anode

Postby Ariana Bowers 1H » Wed Feb 10, 2016 3:44 pm

I have the same question. How are we supposed to know when to switch the anode?

Megna Patel 1I
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

Re: deciding which cell is the cathode/anode

Postby Megna Patel 1I » Wed Feb 10, 2016 7:50 pm

For a galvanic cell, E has to be positive. This is why you would make the cathode the more positive one and the anode the negative one.

However, if the question does not say that it is a galvanic cell, such as in Question 8, you must look at the balanced equation. HF has to be on the reactant side because Ka= [H+][A-]/[HA] meaning that HF has to be on the bottom and since K=[P]/[R], HF must be a reactant. We want HF--> H+ + F- which is the reaction for the dissociation of hydrofluoric acid. This is why the first half reaction is flipped, making it the anode.


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