## Determining n

$\Delta G^{\circ} = -nFE_{cell}^{\circ}$

He whose name cannot be spoken
Posts: 20
Joined: Sat Apr 28, 2018 3:00 am

### Determining n

We use the equation $\Delta G=-n*F*E$ to determine the change in Gibbs free energy in a cell reaction, but I'm having some trouble making sure what the value of n is.

Taking this exercise, for example (14.9 in 6e):
14.9 Calculate the standard reaction Gibbs free energy for the following cell reactions:
(a)$2Ce^{4+}_{(aq)}+3I^{-}_{(aq)}\rightarrow 2Ce^{3+}_{(aq)}+I^{-}_{3}_{(aq)}$
Ecellº=+1.08 V
(b)$6Fe^{3+}_{(aq)}+2Cr^{3+}_{(aq)}+7H_{2}O_{(l)}\rightarrow 6Fe^{2+}_{(aq)}+Cr_{2}O_{7}^{2-}_{(aq)}+14H^{+}_{(aq)}$
Ecellº=-1.29 V

For (a), I can reach the right answer by multiplying the coefficient by the charge (2*4+=8+,2*3+=6+; charge difference is 2=n).
For (b), however, the 14H+ is throwing me off. I can see that that using it with the Cr2O72- would get me the right answer (n=6), but how do I know where to apply the positive charges?

I kinda just need a basic explanation on determining n, please.

Xuan Kuang 2L
Posts: 31
Joined: Wed Nov 14, 2018 12:23 am

### Re: Determining n

The easiest way to figure out n is to split the the reaction into its reducing and oxidizing half-reactions, and then balance out those reactions os they both have the same number of e- in each rxn.
For part b), we know to put the 14H+ for Cr because we see that its product has an O in it, meaning that the H2O is needed to balance it out, and hence, H+ to balance the addition of H2O. Hence, you know to put the positive charge (H+) on whichever side is missing a certain number of H.

Elle_Mendelson_2K
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Joined: Fri Sep 28, 2018 12:28 am

### Re: Determining n

how do u find delta n as in moles not electrons

eden tefera 2B
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Joined: Fri Sep 28, 2018 12:21 am

### Re: Determining n

N is 2 because 1 electron is being added to Ce, but there are two moles of it so you multiply the number of electrons added/removed by the number of moles of the reduced/oxidized compound.

josephperez_2C
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Joined: Wed Nov 15, 2017 3:04 am

### Re: Determining n

"Delta n as in moles" would be the change in moles from there reactant to product side. For the combustion reaction CH4 + 2O2 > 2H2O + CO2, delta n would be negative 2 because water is a liquid.

chrisavalos-2L
Posts: 63
Joined: Fri Sep 28, 2018 12:18 am

### Re: Determining n

The easiest way to determine n is to take the 2 half reactions and balance them normally and then balance them with their charges. This means to add a number of electrons to the side that contains the more positive value, and then after this, you need to assure that this value of electrons is equal for both of the half reaction in order for the entire redox reaction to be able to be balanced. Once you have done these steps, the n value is equal to the amount of electrons needed in the half reaction to balance the charge.

704913468
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Joined: Wed Nov 15, 2017 3:02 am

### Re: Determining n

n is the number of electrons being transferred in the reaction. When you balance both half reactions you can find the value of n.