When aqueous solutions of ionic compounds are electrolyzed, the anode and cathode half-reactions may involve the electrolysis of either water species (H2O, H+, OH–) or solute species (the cations and anions of the compound). As an example, the electrolysis of aqueous sodium chloride could involve either of these two anode reactions:
(i) 2Cl−(aq) ⟶ Cl2(g) + 2e− Eanode°= +1.35827 V
(ii) 2H2O(l) ⟶ O2(g) + 4H+(aq) + 4e− Eanode°= +1.229 V
The standard electrode (reduction) potentials of these two half-reactions indicate water may be oxidized at a less negative/more positive potential (–1.229 V) than chloride ion (–1.358 V). Thermodynamics thus predicts that water would be more readily oxidized, though in practice it is observed that both water and chloride ion are oxidized under typical conditions, producing a mixture of oxygen and chlorine gas.
Turning attention to the cathode, the possibilities for reduction are:
(iii) 2H+(aq) + 2e− ⟶ H2(g) Ecathode°= 0 V
(iv) 2H2O(l) + 2e− ⟶ H2(g) + 2OH−(aq) Ecathode°= −0.8277 V
(v) Na+(aq) + e− ⟶ Na(s) Ecathode°= −2.71 V
Comparison of these standard half-reaction potentials suggests the reduction of hydrogen ion is thermodynamically favored. However, in a neutral aqueous sodium chloride solution, the concentration of hydrogen ion is far below the standard state value of 1 M (approximately 10-7 M), and so the observed cathode reaction is actually reduction of water. The net cell reaction in this case is then cell: 2H2O(l) + 2Cl−(aq) ⟶ H2(g) + Cl2(g) + 2OH−(aq) Ecell°= −2.186 V
This electrolysis reaction is part of the chlor-alkali process used by industry to produce chlorine and sodium hydroxide (lye).