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KilaTsunami
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Anybody good at college chem?

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Posts: 2566 - Karma: 1,026
Basically can't figure this one out and haven't found shit for examples....

Yes I've searched google first but I'd like to think there are some chemistry knowledge inclined people out there. I'd like to know how to solve for the thing cause I've hit a brick wall.

The E°cell= +0.135 V for the reaction
3I2(s) + 5Cr2O72-(aq) + 34H+(aq) Click for larger image 6IO3-(aq) + 10Cr3+(aq) + 17H2O
What is Ecell if [Cr2O72-] = 0.027 M, [H+] = 0.25 M, [IO3-] = 0.00014 M, and [Cr3+] = 0.0045 M?


Apr 11 2011 1:01AM
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3I2(s) + 5Cr2O72-(aq) + 34H+(aq) <-> 6IO3-(aq) + 10Cr3+(aq) + 17H2O
What is Ecell if [Cr2O72-] = 0.027 M, [H+] = 0.25 M, [IO3-] = 0.00014 M, and [Cr3+] = 0.0045 M?

had an image for that
Apr 11 2011 1:02AM
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wow that got butchered in copy paste...

3I2(s) + 5Cr2O7(2-)(aq) + 34H+(aq) <-> 6IO3(-)(aq) + 10Cr(3+)(aq) + 17H2O
What is Ecell if [Cr2O7(2-)] = 0.027 M, [H+] = 0.25 M, [IO3(-)] = 0.00014 M, and [Cr(3+)] = 0.0045 M?
Apr 11 2011 1:04AM
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Colin.W
Posts: 592 - Karma: 155
well for standard potentials of reactions the equation is

E = E* - (R*T/nF)ln(Q) where Q is the reaction constant of the concentrations of your products and reactants... so Q = (Concentration Products)/(concentration reactants) remember to raise the concentrations to any powers like,

3Fe(aq) would be (concentration of Fe)^3 because of the number moles.

*** Q doesnt include solids.

T is your temp in kelvin

n is number of electrons that are used in the oxidation/reduction half reactions

F is a constant.. (Faraday's i believe) = 96,500

that should about do it
Apr 11 2011 1:09AM
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zerospinskier
Posts: 4867 - Karma: 350
I haven't done inorganic stuff in a long time and forgot a bunch, but don't you have to go and look up a bunch of numbers for each of the compounds?
Apr 11 2011 1:09AM
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Colin.W
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thats for Gibbs Free energy change and stuff like that, this is oxidation-redux shits
Apr 11 2011 1:12AM
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well the hint is
First evaluate the net reaction in order to determine the number of electrons, n, involved. Next, write the equilibrium constant expression for the reaction. Finally, use the given information in the Nernst equation to calculate Ecell.
so I've worked it like 4 ways and haven't got the right answer yet
I've figured n is 30 and F is given for 96485 and R is 8.314 and the equation is

Click for larger image
so yeah like i said I've hit a wall
Apr 11 2011 1:15AM
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Well I know how to use the equation but with all this shit involved is where I hit a dead end. I think you have to use +0.135 V for Ecell or E* but can't figure this shit out
Apr 11 2011 1:17AM
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mainly the What is Ecell if [Cr2O7(2-)] = 0.027 M, [H+] = 0.25 M, [IO3(-)] = 0.00014 M, and [Cr(3+)] = 0.0045 M? part
Not sure how that implements with the equation with the given information.

Sorry for so many posts but I have a test tomorrow and we learned this shit on friday and I've been going over it all weekend.
Apr 11 2011 1:19AM
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Colin.W
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the molar concentrations are used in the 'ln(Q)' part where the Q is just a ratio of the concentration of the products to the reactants...


go to 'Nernst Equation' and it explains Q pretty clearly

and the +.135V is your E* or w/e not the E
Apr 11 2011 1:28AM
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Colin.W
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is there a temp involved with this too?
Apr 11 2011 1:29AM
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Colin.W
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hope all that helped man, im off to bed, 8 am classes... good luck on the test. i got a 97 on that test, took it two weeks ago
Apr 11 2011 1:37AM
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so here's how my equation basically looks for it then

E= .135- (8.314*298/(30*96,485)xln(( 0.00014^6 x 0.0045^10)/(0.027^5 x 0.25^34))
I got .151215
the answer is actually 0.171017980910 V

WTF!!!!

Apr 11 2011 1:38AM
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I must've fucked up when I assumed the temperature was 298 since thats standard temp since it wasn't otherwise stated......
Fuck me right?
Apr 11 2011 1:39AM
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