Cell voltage

[chiddler]

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I understand this question now. I typed it up already so....

maybe it'll help somebody.

"In a galvanic cell, the Ereduction + Eoxidation must be greater than 0 volts. If Eoxidation is greater than 0 volts, the overall voltage is positive so choices B and D are galvanic cells an are eliminated. The only way that Ecell can be less than 0 when Ereduction is positive is when Eoxidation is a negative value with a magnitude greater absolute value than Ereduction. The best answer choice is C."

 

[pfaction]

E must be positive for the reaction to be spontaneous:

dG = -nFE<-must be +

So B and D would be spontaneous on the first virtue. Now A and C.

I'm stuck here, will get back to it later unless someone wants to chime in?

 

[MedPR]

E must be positive for the reaction to be spontaneous:

dG = -nFE<-must be +

So B and D would be spontaneous on the first virtue. Now A and C.

I'm stuck here, will get back to it later unless someone wants to chime in?

The explanation is pretty clear. Ecell = Ered + Eox. The only way Ecell is negative when Ered is positive is when abs(Eox) > abs (Ered).

 

[chiddler]

yes. i tend to skip details in the question. the moment i noticed the "greater than 0" part, I understood it. it just took me a long time to figure this out.

 

[MedPR]

yes. i tend to skip details in the question. the moment i noticed the "greater than 0" part, I understood it. it just took me a long time to figure this out.

These types of problems and answer choices are why I think TBR is so far ahead of the other prep companies. It's not a difficult question, but the answers are presented in such a weird way with multiple parts to each answer that it can be very time consuming if you aren't used to it.

 

[pfaction]

Don't get it but will look at tomorrow.

 

[Buttafuoco]

Don't get it but will look at tomorrow.

Just flip everything in your mind. For you to have a positive E and thus negative deltaG, you need the reduced species to have a positive value that's higher in absolute value than the oxidized species- otherwise the large negative value of the Eox would make Ecell negative. When you turn a reduction potential into an oxidation potential, you flip it. So for a negative deltaG, you need a negative Ecell and for this you need your oxidized species to have a negative value and for the absolute value to be greater than the reduced species, otherwise it's positive Ered will make Ecell positive.