Question

It is commonly taught that in electrochemical cells where there are 2 different chemical species that can be oxidised/reduced, species with more positive actual reduction potentials (concentration being considered as well as standard reduction potential) are preferentially reduced at cathodes, and species with more negative actual reduction potentials are preferentially oxidised at anodes.

It appears to me that, theoretically speaking, this need not always be the case, since reduction potentials predict thermodynamics, not kinetics. Therefore, it seems plausible that a case could arise where: 2 species are present at the anode of an electrolytic cell; one species has a more negative reduction potential, but for some reason (charge, hydration shell etc) has much slower oxidation kinetics compared to the other species, which has a more positive reduction potential but has much more rapid kinetics, causing the species with a more positive reduction potential to be oxidised preferentially.

Is this reasoning flawed, and if not, is there such a case in real life experiments?

Answer 1

The preference for reduction or oxidation of species in an electrochemical cell can be attributed to both thermodynamics and kinetics. While standard reduction potentials guide us generally, kinetic factors like reaction speed can override these preferences leading to unexpected reactions based on kinetics rather than just on electrode potential.

Step-by-step explanation:

The reasoning behind the preferential reduction or oxidation of species in an electrochemical cell is indeed largely based on thermodynamics, specifically the standard reduction potential. However, it's absolutely correct that kinetics can also play a significant role, and cases can and do arise where the species with the more positive reduction potential is oxidized preferentially due to faster kinetics.

An example of this can be found in chlorine production, where ideally, water would be reduced to hydrogen at the cathode because it has a more positive standard electrode potential than sodium. Still, the kinetics of chlorine evolution are much faster, leading to preferential oxidation of chloride ions.