The last part of General Chemistry course in the Faculties of Engineering and Agricultural Sciences at my University is electrochemistry
(covered by Chapter 8 of the Physical chemistry
I believe the Nernst equation (below) to be one of the most elegant results of thermodynamics, as well as a really powerful tool for a society which heavily relies on electricity – and, thus, desperately needs it, both for big factories and for personal mobile phones!
However, I usually find that students believe this equation to be difficult to understand
(my main problem, instead, is to pronounce
it, mainly because I’m Italian, and my native language uses far more vowels than the Nobel prize-winner Walther Nernst
cares to use in his surname ).
Nernst equation contains every dream of scientists: work, equilibrium constants, logarithms, and predictive ability
. Usually chemists prefer to use a log10
instead of a ln
(for ease of calculation), but this is a minor issue, since the difference between using them is just a factor of 2.30.
The major issue comes from what it’s inside the logarithm; to correctly use it, one should use activities
of the involved species. But activity is a difficult guest to handle (and fugacity
doesn’t help simplifying the equation for gases, too!). For this reason, it is usual to use molar concentration for diluted species, and pressure for gases. It has to be noted that this works only if the values themselves are small enough to let us substitute the latter terms to the former ones.
Another problem usually arises when considering the difference between halfcell
equation and cell
equation. One can obtain electromotive force for a cell in two ways, either solving the cell
equation or solving the two halfcell
for any electrode and then subtracting the anode potential from the cathode potential.
A difficulty (and usual question) is the following: if I have to
use the same number of electrons when balancing the cell
reaction, why can I solve each semireaction
by its own, and then merely subtract the values? Is not the number of electrons needed anymore?
The truth is: of course, the number of electron involved in each semireaction DOES count, but you already take it into account when calculating the equation… if you double the number of electrons, also exponents in the logarithm double, and so the global effect is null. For this reason, you can use both methods to calculate the emf in your cell (how nice!).
Lastly, the answer to the title question: Nernst equation is simple enough to be used, and so I deem the pronounciation more difficult. Right? Well, unless you have help