What are some common mistakes students make with the Nernst equation?

1 Answer
Mar 8, 2016

See below:

Explanation:

Forget that the Nernst equation

E = E^0 - 59.15/n log([B]/[A])E=E059.15nlog(BA)
(with units of potential in mVmV, for convenience's sake, as when used in VV some students may end up confusing the amounts of zeros in 0.059150.05915 or 0.05920.0592)

Only works for the standard temperature and pressure, having to change that for different temperatures.

Forget that the compounds in the log must be in mol/L or one of its derivates (like mmol/L or mol/mL, but not g/L or eqg/L)

Forget/confuse that the compounds in the log must be in product / reagent order according to the REDUCTION equation, and not the oxidation, even if the species is being oxidated.

Forget that in semirreactions like this

Cr_2O_7^(-2) + 14H^+ + 6e^(-) rarr 2Cr^(+3) + 7H_2OCr2O27+14H++6e2Cr+3+7H2O

The concentration of Cr^(+3)Cr+3 needs to be squared because two moles of that are made.

Which is the anode and which is the cathode; how to calculate the potential after an incomplete reaction, which shows up when calculating titration curves, e.g.:

20 mL of 0.1 N K_2Cr_2O_7K2Cr2O7 was added to a solution of 25 mL of 0.1 N of NaNO_2NaNO2, what is the potential of the system then?

(Although the biggest problem here is figuring out the molar ratios and not necesarily one of the Nernst equation)