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Question

Describe the preparation of sodium hydroxide by Castner-Kellner process. ### Explain the electrolysis method for preparation of sodium hydroxide.

Accepted Answer

i. Sodium hydroxide (caustic soda) is commercially obtained by the electrolysis of aqueous sodium chloride solution (brine) in Castner-Kellner cell (mercury cathode cell).

ii. In Castner-Kellner cell, mercury is used as cathode, carbon rod as anode and brine solution is used as electrolyte which is subjected to electrolysis.

iii. During electrolysis, the following reactions take place:
a. At cathode: Sodium ions get reduced to metallic sodium, which combines with mercury to form sodium amalgam (Na-Hg).
$Na ^{+}+ e ^{-} \xrightarrow{ Hg } Na -$-amalgam
b. At anode: Chloride ions are oxidized and thus, chlorine gas is evolved.
$Cl ^{-} \longrightarrow \frac{1}{2} Cl _2+ e ^{-}$

iv. Sodium amalgam is then treated with water to obtain sodium hydroxide and hydrogen gas.
$\underset{\begin{array}{c}\text { Sodium } \\ \text { amalgam }\end{array}}{2 Na - Hg }+2 H _2 O \longrightarrow \underset{\begin{array}{c}\text { Sodium } \\ \text { hydroxide }\end{array}}{2 NaOH +2 Hg + H _2 \uparrow}$

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