Question

Please check a Nernst equation for me?

Calculate the cell potential for the following reaction as written at 61 °C, given that [Zn2 ] = 0.800 M and [Fe2 ] = 0.0170 M.

Half reactions:
Zn-->Zn^2+ + 2e^- (potential= -0.76)(anode)

Fe^2+ + 2e^- -->Fe (potential= -0.44)
(cathode)

E^nought=cathode-anode
E^nought=(-0.44)-(-0.76)
E^nought=0.32

Q=products/reactants
Q=.8M/.0170
Q=47.0588

(lnQ=3.851)

E=E^nought-(RT/nF)lnQ
E=[.32-(8.3145x334.15)/(2x96485)]3.851
E=1.177

Thank you for your help!

Answer 1

It looks ok until the final step. Isn't that 0.32 - some number. So how can it be larger than 0.32?

Answer 2

Doesn't make sense to me either but I can't figure it out :( The way I arrived at that answer was:

E=[.32-(8.3145x334.15)/(2x96285)]3.851
E=[.32-(2778.29)/192570)]3.851
E=(.32-.014427)3.851
E=(.305573)3.851
E=1.177

It's not right but for the life of me I don't know why. Maybe I'm doing something out of order?

Answer 3

You are. You can't subtract terms like that.
You have
0.32 - [(8.3145*334.15)/(2*96,485)]3.851 =
0.32 - (2778.29)/192,570)*3.851
0.32 - (0.01443)3.851
<b>Next you complete the multiplication you started 3 steps above.</b>
0.32 - (0.05556)
<b>Now you subtract.</b>
0.32 - 0.05556 = ? about 0.26444 which is 0.26 to 2 s.f. (You're allowed only 2 since this is subtraction; we can have hundredths.)

Answer 4

That was it!

Apparently it has been WAY too long since I did order of operations but I have refreshed myself now. I should have known better! Thank you for your help.