[Bessie]

Hi there,
I have a question concerning the products formed on the electrodes in an electrolytic cell when the solution is aqeuous.

I have a data booklet of standard electrode potentials going fro the most negative to the most positive. Is this called the electrochemical series/reactivity series?

Now let's go to an example:
Let's say we have the electrolysis of aqueous NaCl.

We have Na+ Cl- H+ and OH- ions.

Na+ or H+ will form on the cathode and Cl- or OH- will form on the anode.

Hydrogen Gas (H) will form on the cathode as it is lower than Sodium in the electrochemical series.


Chlorine Gas (Cl) will form on the anode as it is lower than OH- in the electrochemical series.


Are these assumptions correct? Is it ALWAYS whichever is lower in the electrochemical series will form on the corresponding electrode?


A quick response would be highly appreciated as this is the only thing holding me on finishing redox study.

[Borek]

Are these assumptions correct?

In general - no. But on high school level - most likely yes.

Is it ALWAYS whichever is lower in the electrochemical series will form on the corresponding electrode?

No. Several reasons for that. First - potential is a function of concentration, that is described by the Nernst equation. Thus just the presence of substance is not enough, it must have concentration high enough. It should react at a given potential when concentration equals 1M, this is so called standard state - that's why you have a table of STANDARD electrode potetials, as opposed to the FORMAL ones - observed in the real solution.

Then, some reactions are sluggish and/or depending on the electrode material they don't want to start at the standard potential and they require so called overpotential to be added.