The ions present are: Ions from sodium chloride - Na+(aq) and Cl-(aq) Ions from water - H+(aq) and OH-(aq)
At the anode, OH- and Cl- ions are attracted to the carbon anode. OH- ions give up electrons more readily than Cl- ions. Thus, OH- ions are preferentially discharged to form water and oxygen gas. Ionic half equation: 4 OH-(aq) --> 2 H2O(l) + O2(g) + 4 e- The Cl- ions remain in solution.
At the cathode, H+ and Na+ ions are attracted to the carbon cathode. Sodium is higher in the reactivity series. Thus, Na+ ions are not discharged. H+ ions are preferentially discharged as hydrogen gas. Ionic half equation: 2 H+(aq) + 2 e- --> H2(g) The Na+ ions remain in solution.
The overall chemical equation: 2 H2O(l) --> 2 H2(g) + O2(g) In the electrolysis of dilute sodium chloride solution, The ratio of H2 to O2 collected in the electrolysis is 2:1. In this electrolysis, it is the water molecules being electrolysed.Hence, the electrolysis of dilute sodium chloride solution is also known as the electrolysis of water. As water is removed (by decomposition into hydrogen and oxygen), the solution eventually becomes concentrated sodium chloride solution. Carbon (or graphite) is needed to be replaced regularly, as oxygen reacts with carbon to form carbon dioxide.