Write the principle and process along with self explanatory diagram for the manufacture of sodium by Down's method. Why is sodium fire at laboratory not extinguished by adding water? Mention any two…

NaCl (molten, 60%)+CaO (40%)1123 K (850°C)Na (liquid)+CaCl₂ (molten)+Cl₂ (gas)

Principle and Process of Sodium Manufacture by Down’s Method

Principle

Sodium is manufactured by the electrolysis of molten sodium chloride (NaCl) in the Downs cell. The process relies on Faraday’s laws of electrolysis:

• At the cathode, sodium ions ($\mathrm{Na}{}^{+}$) gain electrons to form liquid sodium metal: $$\mathrm{Na}{}^{+}+\mathrm{e}{}^{-}\to \mathrm{Na}(\mathrm{l})$$
• At the anode, chloride ions ($\mathrm{Cl}{}^{-}$) lose electrons to form chlorine gas: $$2\mathrm{Cl}{}^{-}\to \mathrm{Cl}{}_2(\mathrm{g})+2\mathrm{e}{}^{-}$$

To make the process economically viable, calcium oxide (CaO, lime) is added to molten NaCl (60% NaCl + 40% CaO). This serves three key purposes:

1. Lowers the melting point of NaCl from 1073 K (800°C) to 1043 K (770°C), reducing energy costs.
2. Prevents sodium from burning in chlorine gas by forming a protective layer of calcium chloride ($\mathrm{CaCl}{}_2$).
3. Acts as a buffer to absorb moisture and impurities.

Process

1. Preparation of Molten Mixture:

   • Pure NaCl is mixed with 40% CaO and heated to 1123 K (850°C) in a steel-lined iron cell.
   • The mixture is kept molten in a graphite-lined steel container to prevent corrosion.

2. Electrolysis:

   • A graphite anode and an iron cathode are immersed in the molten mixture.
   • When a voltage of ~7 V is applied, sodium ions migrate to the cathode, where they are reduced to liquid sodium (density 0.97 g/cm³), which floats to the surface and is tapped off.
   • Chloride ions migrate to the anode, where they oxidize to chlorine gas (used to manufacture bleach, PVC, etc.).
   • Calcium ions ($\mathrm{Ca}{}^{2+}$) remain in the melt, forming $\mathrm{CaCl}{}_2$, which does not interfere with the process.

3. Product Collection:

   • Liquid sodium is collected in a separate chamber to prevent contact with air or chlorine.
   • Chlorine gas is collected, dried, and compressed for industrial use.

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Why Sodium Fire Cannot Be Extinguished by Water

When sodium burns in air, it reacts violently with oxygen and moisture to form sodium hydroxide (NaOH) and hydrogen gas ($\mathrm{H}{}_2$): $$2\mathrm{Na}+2\mathrm{H}{}_2\mathrm{O}\to 2\mathrm{NaOH}+\mathrm{H}{}_2(\text{highly exothermic})$$

• The reaction is extremely exothermic, releasing enough heat to sustain the fire.
• Hydrogen gas produced is highly flammable, which can explode if ignited.
• Sodium hydroxide formed is corrosive, making the situation worse.

2Na (s)+2H₂O (l)Exothermic reaction2NaOH (aq)+H₂ (g)

Correct extinguishing method:

• Use sand, dry fire extinguishers (Class D), or mineral oil to smother the fire and prevent contact with air/moisture.

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Two Advantages of Down’s Process

1. High Purity Sodium:

   • The process yields 99.9% pure sodium, suitable for industrial applications (e.g., soap, paper, titanium production).

2. Byproduct Utilization:

   • Chlorine gas produced is a valuable byproduct, used in bleach, PVC, solvents, and disinfectants, making the process economically viable.

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Conversion of Sodium to Washing Soda

Sodium hydroxide (caustic soda) obtained from sodium is converted to washing soda ($\mathrm{Na}{}_2\mathrm{CO}{}_3\cdot 10\mathrm{H}{}_2\mathrm{O}$) via the following steps:

NaCl (aq)+NH₃ (aq)+CO₂ (g)+H₂O (l)Solvay process (low temperature, NH₃ recycling)NaHCO₃ (s)+NH₄Cl (aq)

1. Formation of Sodium Carbonate (Solvay Process):

   • Sodium hydroxide reacts with carbon dioxide to form sodium carbonate: $$2\mathrm{NaOH}+\mathrm{CO}{}_2\to \mathrm{Na}{}_2\mathrm{CO}{}_3+\mathrm{H}{}_2\mathrm{O}$$
   • Alternatively, sodium chloride reacts with ammonia and carbon dioxide in the Solvay process: $$2\mathrm{NaCl}+\mathrm{CO}{}_2+\mathrm{H}{}_2\mathrm{O}+\mathrm{NH}{}_3\to \mathrm{Na}{}_2\mathrm{CO}{}_3+2\mathrm{NH}{}_4\mathrm{Cl}$$

2. Crystallization:

   • The sodium carbonate solution is evaporated and crystallized to form washing soda decahydrate ($\mathrm{Na}{}_2\mathrm{CO}{}_3\cdot 10\mathrm{H}{}_2\mathrm{O}$), a white crystalline solid used in detergents, glass manufacturing, and water softening.

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Key Points Summary (for NEB Exam Focus)