Question

$Ni$ anode is used in the electrolytic extraction of:

• A. $Al$
• B. $Mg$
• C. $Na$ by Down's process
• D. $Na$ by Castner's process

Answer 1

The correct option is D $Na$ by Castner's process

Anode is made up of Ni metal. The temperature is cooler at the bottom and hotter at the top so that the sodium hydroxide is solid in the neck (B) and liquid in the body of the vessel. Sodium metal forms at the cathode but is less dense than the fused sodium hydroxide electrolyte. Wire gauze (G) confines the sodium metal to accumulate at the top of the collection device (P).

The cathode reaction is
$2N{a}^{+}+2e^{–}\to 2Na$
The anode reaction is
$2O{H}^{–}\to \frac{1}{2}{O}_2+H_{2}O+2e^{–}$
Despite the elevated temperature some of the water produced remains dissolved in the electrolyte. This water diffuses throughout the electrolyte and results in the reverse reaction taking place on the electrolysed sodium metal:
$Na+H_{2}O\to 1/2H_2+N{a}^{+}+O{H}^{–}$
with the hydrogen gas also accumulating at (P). This, of course, reduces the efficiency of the process.