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

1 Answer
Mar 8, 2016

See below:


Forget that the Nernst equation

#E = E^0 - 59.15/n log([B]/[A])#
(with units of potential in #mV#, for convenience's sake, as when used in #V# some students may end up confusing the amounts of zeros in #0.05915# or #0.0592#)

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_2O#

The concentration of #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_7# was added to a solution of 25 mL of 0.1 N of #NaNO_2#, 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)