Determining n in G=-nFE


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BrandonCooper2C
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Joined: Fri Sep 25, 2015 3:00 am

Determining n in G=-nFE

Postby BrandonCooper2C » Mon Feb 08, 2016 4:35 pm

Hey Chem Community,

How can you tell what the correct number of moles is to use in the G=-nFE equation if you are given a balanced redox reaction and E of the cell only? Does it have to do with the moles of e- being transferred in the rxn?

Thanks.

Chem_Mod
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Re: Determining n in G=-nFE

Postby Chem_Mod » Mon Feb 08, 2016 8:04 pm

Yes, you are correct, n essentially refers to the number of electrons (mol) being transferred in the redox reaction.

004643111
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Re: Determining n in G=-nFE

Postby 004643111 » Tue Feb 09, 2016 4:36 am

I'm still having trouble finding n. I understood it to as being the number of moles in a chemical equation, but that seems to not be right.. What do you mean by "the number of electrons being transferred"? Could you please elaborate? For example, problem 14.9a, for the equation
2Ce^(4+)+3I^(-) -> 2Ce^(2+)+I3^(-), "n" is said to be -2. How is that? Also, with 14.9b, for the equation
6Fe^(3+)+2Cr^(3+)+7H2O -> 6Fe^(2+)+Cr2O7^(2-)+14H^(+), "n" is said to be -6.

Chem_Mod
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Re: Determining n in G=-nFE

Postby Chem_Mod » Tue Feb 09, 2016 5:57 am

Transferred meaning the number of electrons going from the oxidized species to the reduced. To find this, you need to balance the half reactions and then balance the half reactions against one another in order to determine the number of moving electrons.

In the first example, Ce4+ is reduced to Ce2+
Ce4++2e----->Ce2+
I3----->3I-+2e-
When we balance the half reactions we set the number of electrons equal to one another. So in this case, the half reactions are already balanced against one another and we get the full redox equation. We can see that we balanced the reaction so that 2 electrons are being transferred from I3- to Ce4+ which is how we get n=2.

Erik Khong 2E
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Joined: Fri Sep 29, 2017 7:07 am

Re: Determining n in G=-nFE

Postby Erik Khong 2E » Wed Feb 21, 2018 5:46 am

As for part b, it is much simpler than it looks. All you need to do is make: 6 Fe(3+) --> 6 Fe(2+)
From here, the charges on both sides are +18 vs +12. Just add 6 electrons to the left side, and you'll have equal charges. This means that n for this part is 6. There is no real need to focus on the other parts of the equation.

(Super old question, but it's still useful information (also i need posts lol))


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