electrolysis of dilute NaCl

In the solution of sodium chloride, the sodium chloride is dissociated to give Na+(aq) and Cl-(aq) ions, while water is slightly ionized to give H+(aq) and OH-(aq) ions.

Cathode in electrolysis:
Na+(aq) and H+(aq) ions move to the cathode. No matter how concentrated or dilute the Na+(aq) ions are, H+(aq) ions are preferentially discharged to give H2(g) because H+(aq) ions are reduced more readily than Na+(aq) ions.
2H+(aq) + 2e- → H2(g)

Anode in electrolysis:
OH‑(aq) and Cl-(aq) ions move to the anode. When brine (concentrated sodium chloride solution) is used, Cl-(aq) ions are preferentially discharged to give Cl2(g) because Cl-(aq) ions are much more concentrated than OH-(aq) ions.
2Cl-(aq) → Cl2(g) + 2e-
When dilute sodium chloride solution is used, OH-(aq) ions are preferentially discharged to give O2(g) because OH-(aq) ions are oxidized more readily than Cl-(aq) ions, as the concentrations of the two ions are comparable.
4OH-(aq) → 2H2O(l) + O2(g) + 4e-
=

2009-03-29 18:43:07 補充：
Carbon electrodes are used in the electrolysis.