CHEMISTRY OF s-BLOCK ELEMENTS

CHEMISTRY OF S-BLOCK ELEMENTS
EXTRACTION OF SODIUM
EXTRACTION OF SODIUM
On industrial scale sodium metal is extracted by "Down's Process".
PRINCIPLE
Down's Process is based on the electrolysis of fused NaCl.
CONSTRUCTION OF DOWN'S CELL
Down's cell consists of a rectangular container of steel. Inside of the tank is lined with firebricks. Anode is a graphite rod which projects centrally up through the base of the cell. Cathode is a ring of iron, which surrounds the anode. The anode and cathode are separated from each other by a cylindrical steel gauze diaphragm so that Na and Cl2 are kept apart. A bell like hood is submerged over the anode.
DRAW BACKS OF DOWN'S METHOD
Melting point of NaCl is 801C. At this temperature molten NaCl and Na form a metallic fog in the container which is impossible to separate.
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STEPS TO OVER COME THIS DIFFICULTY
In order to over come this difficulty instead of only NaCl, a mixture of NaCl and CaCl2 is electrolyzed in down's cell. The melting point of this mixture is 600C. At 600C no metallic fog is formed.
COMPOSITION OF CHARGE: NaCl = 42% CaCl2 = 58%
THE PROCESS
When an electric current is passed through the molten mixture of NaCl and CaCl2, NaCl decomposes in to Na+ and Cl- ion. Na+ ions migrate towards cathode while Cl- ions towards the anode. The molten sodium collects in the cathode compartment where it rises to the top and is tapped off by a pipe. Chlorine is collected at the anode.
THE CHEMISTRY OF REACTION
Fused NaCl contains sodium and chloride ions.
2NaCl è 2Na+ + 2Cl-
ELECTROCHEMICAL CHANGES
At cathode
Na+-ions migrate to cathode where they are reduced to Na.
2Na+ + 2e- è 2Na (Reduction)
At anode
Cl--ions migrate to anode and oxidised to form chlorine gas.
2Cl- è Cl2 + 2e- (Oxidation)
Overall Reaction
2Na+ + 2e- è 2Na 2Cl- è Cl2 + 2e- __________________ 2Na+ + 2Cl- è 2Na + Cl2
Reason
During electrolysis Calcium is also obtained but it does not mix with Sodium. Explain.
Answer:
During electrolysis calcium is also obtained at cathode but sodium and calcium are separated from each other due difference in density. Density of Na is 0.67gm/cc and the density of Ca is much higher than that of Na i.e. 2.54gm/cc. That's why they do not mix with each other.