Overpotential, OH- oxidation

Over potential is a very advanced concept in electrochemistry, so normal secondary student will probably learn it. Simply speaking, concentration of OH- is always higher than that of Cl- no matter in what kind of solution as the equilibrium positiom of dissociation of water always lies to the RHS during electrolysis. So O2 is always formed at the beginning of electrolysis, this is applicable to 3M NaCl solution as well. As O2 bubble is formed on the surface of electrode while O2 is not electrical conducting, the potential on the surface increase (the underlying mechanism involves very complicated physics and material chemistry, so just forget about it). Normally, OH- has a lower oxidation potential compared with Cl-, so in dilute solution, OH- is discharged first even over potential occur due to probability effect. But in concentrated solution, the overprotential after O2 formed is high enough to oxidize Cl- as well, causing Cl- discharged as well. Overpotential can occurs in cathode as well, but it is not significant as not other cations can be discharged easily.

Why bubbles is formed on its surface instead of forming gas directly is related to properties of that gas, so it is difficult to account this phenomenon simply.

It should be noted that even Cl- is discharged significantly in concentrated NaCl solution, OH- is simultaneously discharged as well! It doesn't mean that Cl- is discharged only in that case.

2013-01-28 09:01:10 補充：
Btw, voltage is not directly equal to electrical energy. Voltage is the energy per charge.

You are asking about electrolysis right?
Under the electrolysis, oxidation occurs in anode. Reduction occurs in cathode.
Take diluted NaCl as example.According to the electrochemical series, the OH ion will discharge in anode(4OH-==>O2+2H2O+4e-) Cl- ions wont be discharged because the solution consist of very little amount Cl-ions. The Cl- ions have less chance to form Cl2.
After a while, the Cl-ions will be more concentrated as the H+ions and OH-ions are discharge. Therefore the NaCl has became more concentrated. When NaCl is
concentrated (about 3M) the Cl-ions are concentrated enough and will be discharged to form Cl2 (g) in anode until the solution become diluted again.

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