Electrolysis of Molten sodium chloride
Figure 1.53 on the next page shows how molten sodium chloride can be electrolysed in the laboratory. Sodium chloride is heated very strongly until it melts. Then, the current is passed through the molten sodium chloride. A silvery liquid forms at the cathode (negative electrode); this is molten sodium. Bubbles of a yellow green gas appear around the anode (positive electrode), the gas is chlorine.
Let us explain these observations.
What are the ions present in sodium chloride?
Sodium chloride is an ionic compound consisting of sodium ions, Nat, and chloride ions, Cl In the solid state, the oppositely charged ions attract each other strongly and are thus not free to move. In the molten state, however, the ions can move around freely.
What happens at the cathode (-)?
The battery pushes electrons into the cathode (Figure 1.54). The cathode therefore becomes negatively charged. The positive ions are attracted to the negatively charged cathode. At the cathode, the sodium ions take electrons from the electrode to form sodium atoms.
Na+ (l) + e- → Na (l) reduction
We say the sodium ions have been discharged. As the sodium ions gain electrons, reduction occurs at the cathode. Tiny globules of molten sodium float to the surface of the molten sodium chloride and burn in air with flashes of yellow light.
What happens at the anode (+)?
Electrons flow out of the anode back to the battery. The anode is therefore positively charged. The negative chloride ions (anions) are attracted to the positively charged anode. At the anode, the chloride ions give up electrons to the electrode to form chlorine atoms.
Cl- (l) → Cl (l) + e- oxidation
As chloride ions lose electrons, oxidation occurs at the anode. The chlorine atoms then combine to form chlorine molecules.
2Cl (l) → Cl2 (g)
Chlorine is seen as a pale yellow-green gas around the anode during the electrolysis process.
The overall chemical change during the electrolysis is:
2NaCl (l) → 2Na (l) + Cl2 (g)
Therefore, the reaction that takes place during the electrolysis is a redox reaction.
The carbon electrodes are called inert electrodes because they do not react in the electrolysis.
An electric current is produced during electrolysis. Electric current is the movement of charged particles. In the molten electrolyte, the moving charged particles are ions. In the electrodes and the external circuit (connecting wires and battery), it is the electrons that move.