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

Mar 8, 2016

See below:

Explanation:

Forget that the Nernst equation

$E = {E}^{0} - \frac{59.15}{n} \log \left(\frac{B}{A}\right)$
(with units of potential in $m V$, 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

$C {r}_{2} {O}_{7}^{- 2} + 14 {H}^{+} + 6 {e}^{-} \rightarrow 2 C {r}^{+ 3} + 7 {H}_{2} O$

The concentration of $C {r}^{+ 3}$ 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 ${K}_{2} C {r}_{2} {O}_{7}$ was added to a solution of 25 mL of 0.1 N of $N a N {O}_{2}$, 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)