Sketch a well labeled diagram for the manufactured of NaOH by Castner Kellner's method and explain the principle and procedure for it. What in the action of caustic soda on [6+2+2] i) Chlorine ii)…

2NaCl (aq)+H₂O (l)Mercury cathode (–), Graphite anode (+), ~400°CElectricity passed through molten NaCl2NaOH (aq)+2H₂ (g)+Cl₂ (g)

Manufacture of NaOH by Castner-Kellner’s Method

Principle

The Castner-Kellner process is an electrolytic method used for the large-scale production of sodium hydroxide ($\mathrm{NaOH}$) from brine ($\mathrm{NaCl}$ solution). It operates on the principle of electrolysis, where an electric current is passed through molten or aqueous sodium chloride to decompose it into its constituent elements. The key features of this method are:

1. Use of mercury cathode: Mercury is used as the cathode because it forms an amalgam with sodium, which prevents the recombination of sodium with water.
2. Graphite anode: Chlorine gas is liberated at the anode.
3. Overall reaction: $$2\mathrm{NaCl}(\mathrm{aq})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{NaOH}(\mathrm{aq})+\mathrm{H}{}_2(\mathrm{g})+\mathrm{Cl}{}_2(\mathrm{g})$$

Procedure

The process involves the following steps:

1. Preparation of Brine Solution:

   • A concentrated solution of sodium chloride ($\mathrm{NaCl}$) is prepared.
   • The brine is purified to remove impurities like calcium and magnesium ions, which would otherwise interfere with the electrolysis process.

2. Electrolysis in Castner-Kellner Cell:

   • The purified brine is fed into an electrolytic cell where a mercury cathode and a graphite anode are used.
   • When an electric current is passed through the cell:
     • At the cathode (mercury): $$\mathrm{Na}{}^{+}+\mathrm{e}{}^{-}\to \mathrm{Na}(\mathrm{amalgam})$$ Sodium ions are reduced to sodium metal, which dissolves in mercury to form a sodium-mercury amalgam.
     • At the anode (graphite): $$2\mathrm{Cl}{}^{-}\to \mathrm{Cl}{}_2(\mathrm{g})+2\mathrm{e}{}^{-}$$ Chloride ions are oxidized to chlorine gas.
     • Water undergoes electrolysis to produce hydrogen gas and hydroxide ions: $$2\mathrm{H}{}_2\mathrm{O}\to \mathrm{H}{}_2(\mathrm{g})+2\mathrm{OH}{}^{-}$$
   • The sodium-mercury amalgam is then reacted with water in a decomposer to produce sodium hydroxide: $$2\mathrm{Na}(\mathrm{amalgam})+2\mathrm{H}{}_2\mathrm{O}\to 2\mathrm{NaOH}(\mathrm{aq})+\mathrm{H}{}_2(\mathrm{g})$$
   • The mercury is recycled back into the cell.

3. Collection of Products:

   • Chlorine gas ($\mathrm{Cl}{}_2$): Collected at the anode and used for the manufacture of bleaching powder, hydrochloric acid, or PVC.
   • Hydrogen gas ($\mathrm{H}{}_2$): Collected at the cathode and used as a fuel or in the Haber process.
   • Sodium hydroxide ($\mathrm{NaOH}$): Collected as a concentrated solution (50% w/w) and further purified by evaporation to obtain solid $\mathrm{NaOH}$.

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Action of Caustic Soda ($\mathrm{NaOH}$) on

Cl₂ (g)+2NaOH (aq)Cold and diluteNaCl (aq)+NaOCl (aq)+H₂O (l)

Zn (s)+NaOH (aq)Excess NaOHNa₂[Zn(OH)₄] (aq)

(i) Chlorine ($\mathrm{Cl}{}_2$)

When chlorine gas is passed through a cold, dilute solution of sodium hydroxide, it undergoes disproportionation to form sodium chlorate ($\mathrm{NaClO}{}_3$), sodium chloride ($\mathrm{NaCl}$), and water: $$3\mathrm{Cl}{}_2+6\mathrm{NaOH}\to \mathrm{NaClO}{}_3+5\mathrm{NaCl}+3\mathrm{H}{}_2\mathrm{O}$$ Explanation:

• Chlorine acts as both an oxidizing agent and a reducing agent in this reaction.
• In cold and dilute conditions, chlorine is reduced to $\mathrm{Cl}{}^{-}$ (chloride ion) and oxidized to $\mathrm{ClO}{}_3{}^{-}$ (chlorate ion).

(ii) Zinc ($\mathrm{Zn}$)

When zinc reacts with sodium hydroxide, it forms sodium zincate ($\mathrm{Na}{}_2\mathrm{ZnO}{}_2$), hydrogen gas, and water: $$\mathrm{Zn}+2\mathrm{NaOH}+2\mathrm{H}{}_2\mathrm{O}\to \mathrm{Na}{}_2\mathrm{ZnO}{}_2+2\mathrm{H}{}_2$$ Explanation:

• Zinc is an amphoteric metal, meaning it reacts with both acids and bases.
• In this reaction, zinc dissolves in $\mathrm{NaOH}$ to form a soluble complex, sodium zincate, with the evolution of hydrogen gas.

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Conversion of Caustic Soda ($\mathrm{NaOH}$) into Sodium Silicate ($\mathrm{Na}{}_2\mathrm{SiO}{}_3$)

Sodium silicate (water glass) is prepared by reacting sodium hydroxide with silica ($\mathrm{SiO}{}_2$): $$2\mathrm{NaOH}+\mathrm{SiO}{}_2\to \mathrm{Na}{}_2\mathrm{SiO}{}_3+\mathrm{H}{}_2\mathrm{O}$$ Procedure:

1. Mixing: Silica (sand, $\mathrm{SiO}{}_2$) is mixed with a concentrated solution of sodium hydroxide.
2. Heating: The mixture is heated to ~100°C to facilitate the reaction.
3. Formation of Sodium Silicate: The reaction produces sodium silicate ($\mathrm{Na}{}_2\mathrm{SiO}{}_3$), which dissolves in water to form a viscous solution known as water glass.
4. Applications: Sodium silicate is used in detergents, adhesives, and as a preservative.

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2NaOH (aq)+SiO₂ (s)Heating (~100°C)Na₂SiO₃ (aq)+H₂O (l)