Alkali Metals: Sodium and Its Compounds

Sodium carbonate

It exists in anhydrous salt (Na₂CO₃) and also in hydrated salt (Na₂CO₃.10H₂O). The decahydrated sodium carbonate is called washing soda.

Manufacture of washing soda by Solvay or ammonia soda process

Principle:

Brine solution saturated with ammonia gas reacts with carbon dioxide to form sodium bicarbonate.
NH₃ + H₂O + CO₂ → NH₄HCO₃
NaCl + NH₄HCO₃ → NaHCO₃ + NH₄Cl

Sodium bicarbonate is sparingly soluble in water due to unreacted NaCl.The filtered product on heating gives Na₂CO₃.

Sodium carbonate solution is crystallized to get washing soda.

Process:

1. Saturation of brine with ammonia: In the ammonia absorber, a brine solution is saturated with ammonia gas. Impurities like CaCl₂ and MgCl₂ present in brine are precipitated as hydroxides and carbonates, are removed.

2NH₃ + H₂O + CO₂ → (NH₄)₂CO₃
CaCl₂ + (NH₄)₂CO₃ → CaCO₃ + 2NH₄Cl
MgCl₂ + (NH₄)2CO₃ → MgCO₃ + 2NH₄Cl

2. Carbonation: Ammoniated brine is made to trickle down from the top of a tower known as a carbonation tower packed with perforated plates. Brine solution saturated with ammonia flows down and meets CO₂ results in the formation of insoluble sodium bicarbonate.

NH₃ + H₂O + CO₂ → NH₄HCO₃
NH₄HCO₃ + NaCl → NaHCO₃ + NH₄Cl

Sodium bicarbonate is filtered and the filtrate is pumped to ammonia generator.

3. Recovery of ammonia: In an ammonia generator, ammonium bicarbonate is decomposed by heat and ammonium chloride by slacked lime obtained from the lime kiln.

NH₄HCO₃ → NH₃ + CO₂ + H₂O
2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + H₂O

CaCl₂ is obtained as biproduct.

4. Generation of CO₂: In lime kiln, limestone is heated to produce CO₂ required for carbonation.

CaCO₃ → CaO + CO₂

CaO is treated with water to get slacked lime and pumped to an ammonia generator.
CaO + H₂O → Ca(OH)₂

5. Calcination: Sodium bicarbonate obtained by filtration is heated to get aqueous sodium carbonate.

6. Crystallization: Sodium carbonate solution is subjected to crystallization to obtain crystalline sodium carbonate (washing soda).

Merits of Solvay process:

i. Pure product is obtained.
ii. There is no pollution problem.
iii. Raw materials are cheap and easily available.

Physical properties:

1. It is white crystalline efflorescent solid. It loses water of crystallization when exposed to air.

2. It is soluble in water.

Chemical properties:

1. Action with water:

Na₂CO₃ + H₂O ⇌ 2NaOH + H₂CO₃

2. Action with acids:

Na₂CO₃ + 2HCl → 2NaCl + H2O + CO2

3. Action with lime:

Na₂CO₃ + Ca(OH)₂ ⇌ CaCO₃ + 2NaOH

4. Action with carbon dioxide:

Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃

5. Action with salts (precipitation reaction):

Na₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl
Na₂CO₃ + MgCl₂ → MgCO₃ + 2NaCl
Na₂CO₃ + ZnCl₂ → ZnCO₃ + 2NaCl

Uses

• Manufacture of glass.
• Making wood, paper, detergents, etc.
• Softening of hard water.

Some Important Questions

1. Why is sodium not extracted by chemical reduction using common reducing agents? Can aqueous NaCl be used for the extraction of sodium metal? What are the difficulties for the extraction of sodium by electrolysis of molten NaCl? How are these difficulties overcome?
2. How is sodium manufactured by Down’s process?
3. Write the chemical reactions to convert washing soda into caustic soda and baking soda.
4. Why sodium is stored in kerosene oil?