Extraction of sodium metal by down’s process

Introduction:
The sodium metal is extcrated from common salt (rock salt) by down’s electrolytic process. The direct electrolysis of molten NaCl is not possible because it creates following hindrances.

• The melting point of ‘NaCl’ is very high i.e 800 centigrade, which is nearer to the boiling point of sodium (883 centigrade).
• The sodium becomes volatile at high temperature and it vaporizes easily.
• Sodium metal forms a colliodal solution with the molten NaCl and makes impossible to separate sodium from it.
• Molten sodium is not a good electrolyte.

Extraction of sodium metal by down’s process

All these hindrances can be removed by lowering down the melting point of NaCl. For this purpose 58% CaCl₂ is added which lowers the melting point of NaCl to 590 centigrade and also increases the electrical conductivity.

Construction Of the Down’s Electrolytic Cell

The cell consists of a steel box, lined inside with fire bricks. The cell is fitted with a central graphite anode and a surrounding iron cathode. The iron gauze diaphragm separates the two electrodes. This keeps away the molten sodium metal from gaseous chlorine. The cathode is provided with hood through which molten sodium rises to the receiver, connected with one of the sides. Similarly the upper end of the anode is covered with iron hood, to collect chlorine gas.
Electrochemical Process:
When electric current is passed through the fused mixture, NaCl gets decomposed and sodium is liberated at cathode. Being lighter, it rises up in the pipe and goes into the receiver.  Chlorine is liberated at anode and is allowed to go out of the hood.
NaCl ——> Na⁺  + Cl^-
At cathode:        Na⁺  + 2e^- ——> 2Na    (Reduction)
At Anode:            2Cl^- ——> Cl₂ + 2e^{-          (Oxidation)}
Over all reaction:    2Na⁺  + 2Cl^- ——> 2Na + Cl₂

Reaction of sodium

Reaction with Water:
It reacts vigorously with water forming caustic soda and hydrogen gas. The process is exothermic, So hydrogen gas catches fire.
2Na + 2H2O ——> 2NaOH + H₂ + Heat

Reaction with Oxygen:
On heating in air or oxygen it gives a mixture of normal and per oxide.
4Na + O₂ ——> 2Na₂O (Sodium Oxide)
2Na + O₂ ——> 2Na₂O (sodium per oxide)

Reaction with Chlorine:
It directly reacts with chlorine to form sodium chloride.
2Na + Cl₂ ——> 2NaCl

Reaction with Hydrogen:
It reacts with hydrogen at high temperature to form sodium hydride.
2Na + H₂ ——> 2NaH + H₂