## Winter 2013 Midterm #7: Which Nernst Equation to Use?

$E_{cell} = E_{cell}^{\circ}-\frac{RT}{nF}\ln Q$

Alyssa Chan 3B
Posts: 16
Joined: Wed Sep 21, 2016 2:57 pm

### Winter 2013 Midterm #7: Which Nernst Equation to Use?

#7 of the Winter 2013 practice midterm uses the Nernst equation E = E˚ - $\frac{0.05916V}{n}$logQ to solve for K. Why isn't the other equation E = E˚ - $\frac{RT}{nF}$lnQ used? When I used the second equation, the answer wasn't the same as when I used the first. How do you know when to use which equation?

Thanks!
Last edited by Alyssa Chan 3B on Tue Feb 14, 2017 10:48 pm, edited 1 time in total.

Matt Goff 1F
Posts: 10
Joined: Wed Nov 18, 2015 3:00 am

### Re: Winter 2013 Midterm #7: Which Nernst Equation to Use?

I just did it your way and got K = 9.2 x 10^-72, which is the same as when I used

$\Delta G=-RTlnK$ , which is just a different way to do it.

It says at the bottom of the page though they accepted 9.2 x 10^-72 as well as 1.0 x 10^-71 for the answer for K, so you should be good as long as you put the numbers in correctly.

The equation you used is more general, and can be used to calculate E under any nonstandard conditions.
The equation used in the key is just a convenient way to use the Nernst equation when the reaction is at 25 Celsius, which it is in this question.

Hope this helps!

Drake_Everlove_1K
Posts: 19
Joined: Sat Sep 24, 2016 3:02 am

### Re: Winter 2013 Midterm #7: Which Nernst Equation to Use?

The above poster is probably correct, but in lecture he said that log has to do with reactions where pH is a relevant value, as it is measured in terms of log.

Return to “Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)”

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