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.
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