As we have covered, electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials. Two commonly used methods of electrolysis is that of molten sodium chloride and aqueous sodium chloride. You might think that both methods would give you the same products, but, in fact, this not the case. Let's go through each of the methods to understand the different processes.
If sodium chloride is melted (above 801°C) and two electrodes are inserted into the melt as shown in Figure 1 and an electric current is passed through the molten salt, then chemical reactions take place at the electrodes.
Chloride ions migrate the other way, toward the anode, give up their electrons to the anode, and are oxidized to chlorine gas:
The overall reaction is the breakdown of sodium chloride into its elements:
Now what happens when we have an aqueous solution of sodium chloride as shown here Figure 2? Well, we can't forget that we now have water to factor into the equation. Since water can be oxidized and reduced it competes with the solute. Rather than producing sodium, hydrogen is produced. The reaction at the cathode is:
and at the anode:
The overall reaction is as follows:
Reduction of Na+ (E° = –2.7 v) is energetically more difficult than the reduction of water (–1.23 v), so in aqueous solution the latter will prevail. Electrolysis of salt ("brine") is carried out on a huge scale and is the basis of the chloralkali industry.