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Topic: electrochemistry problem  (Read 326 times)

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Offline LucaZombini

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electrochemistry problem
« on: February 10, 2021, 06:25:36 PM »
determinate the direction of the reaction 2Fe3+ + 2I-  ::equil:: I2 + 2Fe2+ when every specie is 1 mol/dm-3 and find the concentration of every ionic specie when the equilibrium is reached. T = 25 °C

EFe3+/Fe2+ = 0,771V
EI2/I- = 0,536V

given the Nernst's formula i find the Keq
E = RT/zF * logKeq
Keq = 10(zE/0,05916) = 8,8*107

[Fe3+] = [I-] = 1-2x
[Fe2+] = 1+2x

then i don't know how to proceed. i've tried to substitute the results above in the Keq equation but i wasn't able to find the results.
 
for example

Keq = 1+2x/(1-2x)2




Offline mjc123

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Re: electrochemistry problem
« Reply #1 on: February 11, 2021, 06:25:58 AM »
What is the expression for Keq for the reaction
2Fe3+ + 2I-  ::equil:: I2 + 2Fe2+ ?

Offline LucaZombini

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Re: electrochemistry problem
« Reply #2 on: February 11, 2021, 06:40:08 AM »
Keq = [Fe2+]2 [I2]/ [Fe3+ ]2 [I-]2

I2 should be solid, i think.
 
EDIT:

I forgot to write the solutions

Fe3+ = I- = 1,44E-2
Fe2+ = 2-1,44E-2

Offline mjc123

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Re: electrochemistry problem
« Reply #3 on: February 11, 2021, 06:52:55 AM »
Yes (leaving out [I2] which is a solid). That is not (1+2x)/(1-2x)2, is it?

You could simplify it by writing
Fe3+ + I-   ::equil:: 1/2 I2 + Fe2+
for which z = 1 in the Nernst equation. Then
Keq = [Fe2+]/[Fe3+][I-]
but this Keq has a different value from the one you calculated.

Offline LucaZombini

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Re: electrochemistry problem
« Reply #4 on: February 11, 2021, 07:02:16 AM »
I've managed to solve it by making an approximation which eliminated some terms!!  :)

i counted z as 2, that's how i found that value for Keq

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