Over potential -what and why ?

Question

Value of standard electrode potential for the oxidation of Cl– ions is more positive than that of water, even then in the electrolysis of aqueous sodium chloride, why is Cl– oxidised at anode instead of water?

Standard electrode potentials describe the tendency of getting reduced.Positive value indicates that H2 can reduce that element or reduced form of the element is more stable than proton. The answer to the question above is over potential Please explain how it came into the story.My book explained it under "products of electrolysis",but the explanation is not lucid. They explained it as : Some electrochemical processes although feasible, are so slow that at lower voltage these do not seem to take place and extra potential called over potential has to be applied which makes such process more difficult to occur.

Answer 1

When I took P-Chem, decades ago, one experiment we had was using a dropping mercury electrode, DME. I don't recall the details, but the set-up was fairly elaborate, especially compared to a Ag/AgCl electrode. The DME is fairly common (or was, before we decided that Hg was probably better avoided by the undergrad). The reason WHY we used it was it provided a CLEAN surface. Think about it. We wouldn't go to such great lengths if the problem of contamination wasn't significant.

All materials absorb contaminants, may chemically react with near-by chemicals (solvent, ions, gasses), and will form a charge polarization layer, especially in polar solvents like H2O. In fact some electrochemical methods rely on it. But for getting the "pure" ideal thermodynamic (reversible) numbers the large tendency of electrode surfaces to change is a real problem.

Overpotential is the term we use to describe the additional energy (voltage) we have to put in to these non-ideal systems (either at the anode or the cathode) to overcome these "side reactions". This energy is ultimately converted (mostly) to heat. Although maybe not an overpotential issue, surface area can be important, so smoothness does matter.

I can think of no area in chemistry (short of biochem) where you have to get into the weeds (details) so quickly as you do with electrochemistry. I wonder if its because chemistry is actually about electric interactions between atoms, molecules, ions, etc.? I should also mention that on other nice thing about the DME was that the surface was smooth - although I can't recall whether the surface was smooth at the atomic level, most solids are far from smooth, another problem in reproducibility.

Answer 2

This is a question of kinetics vs thermodynamics. From the perspective of thermodynamics equilibrium of water, HCl, O2 and Cl2 is shifted towards HCl+O2. However kinetics dictates Cl2 to form faster than O2.

Kinetic vs thermodynamic control is a commonplace in chemistry. Thermodynamics suggests that we all should be dead, but thanks to kinetics we are still alive. Why Cl- is kinetically favorable? Because 4 electrons should be taken from 2 H2O molecules to form O2. At the same time intermediates in this oxidation (atomic O, H2O2) can easily oxidize two chloride ions and yield chlorine gas.