Question

In Down's process for the manufacture of sodium, $CaC{l}_2$ is added to:

• A. decrease the melting point
• B. increase the melting point
• C. decrease the interionic interactions
• D. increase the interionic interactions

Answer 1

The correct option is A decrease the melting point

$CaC{l}_2$ is added to lower the melting point from 1080 K (melting point of NaCl) to 850 K. This is due to following reasons.
a. At such high temperature, it is very difficult to maintain Na in the molten state during electrolysis.
b. Moreover, Na is voltile at this temperature and a part of it would vapourize.
c. Molten Na, gets dispersed in molten NaCl to form metallic fog at high temperature.
d. Both Na and $C{l}_2$ produced in the electrolysis have a corrosive action on the vessel used.
The over all reactions nare as follows :
At cathode : $N{a}^{\oplus }+e^{-}\to Na$
At anode : $C{l}^{\ominus }\to \frac{1}{2}C{l}_2+e^{-}$