Electrolysis and concentration cells

[AnathemaXI]

What exactly is the difference between a concentration cell and galvanic/voltaic cell? How do you know what are the half reactions? Also, is the ion and electron flow the same found in voltaic cells or is it the opposite?

My biggest question of electrolysis has to do with Cl species.

Here's an example:

What appears at the cathode and anode after the electrolysis of SnCl2

I've got this part down:

Cathode:
Sn2+ +2e =>Sn E: -.14
Water: E:-.83

Anode:
2Cl-=>Cl2 + 2e : 1.36
Water: 1.229

I understand how Sn appears, but why does Cl appear? it has a larger reduction potential meaning it would stand a lesser chance of being oxidized yet for whatever reason every book says that it's more reactive but I don't understand why. Is this the case every time?

 

[Borek]

Overpotential and kinetics. Most electrode reactions involving water/oxygen are just notoriously slow. To speed them up you have to apply higher potential, when you apply higher potential, you are in the realm of Cl^- oxidation.

So - your thinking based on potentials is right, just the reality is more complex than that.

--

 

[AnathemaXI]

Oh ok, so it's just because the Cl and hydrogen potentials are so close, that going a little over will create the Cl in the end. So if the reduction potentials are fairly far apart, the water products will form instead eve if it is slow?

Such as Fe3+ + e => Fe2+?

 

[Borek]

Huh? If there is reduction going on, something must be oxidized as well. In water solution of tin chloride, if there are no other things present, it is either water of chlorides that WILL be oxidized. If you add something else - it may change the situation.

 

[AnathemaXI]

I'm just saying, that just because the oxidation potentials of Cl- ad H2O are close, the Cl will appear b/c of it's high reactivity.

But if we're electrolyzing FeF2, the products of the H2O reaction will appear at the anode since there is a big difference between it and the Fe products. Am I on the right track by judging off the reduction table potentials?