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The s-Block Elements

Long Answer Type

131.

What happens when:
(i) sodium metal is dropped in water?
(ii) sodium metal is heated in free supply of air?
(iii) sodium peroxide dissolves in water?

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Short Answer Type

132.

Write balanced equation for reactions between:
Na₂O₂ and water

212 Views

133.

Write balanced equation for reactions between:
KO₂ and water

216 Views

134.

Write balanced equation for reactions between:
Na₂O and CO₂.

175 Views

135.

Account for the following:
(i) Sodium imparts colour to the flame.
(ii) Sodium acts as a strong reducing agent.

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136.

Name a few important uses of sodium ?

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Long Answer Type

137.
Discuss in brief the Solvay process for the manufacture of washing soda.

Ammonia-soda or Solvay process is the modern method for the manufacture of sodium carbonate (washing soda i.e. Na₂CO₃.10H₂O) from common salt.

Principle:The process is based upon the low solubility of sodium bicarbonate especially in the presence of CO₂, ammonia and carbon dioxide are passed through a saturated solution of sodium chloride, sodium bicarbonate is produced.

Sodium bicarbonate formed is least soluble in the presence of an excess of carbon dioxide and NaCl (common ion effect) and hence filtered.
Sodium bicarbonate is then heated strongly to produce sodium carbonate, carbon dioxide (which is used again) and steam.

Ammonium is recovered by heating ammonium chloride solution with slaked lime.

Ammonia gas is used again.

Raw materials to prepare sodium carbonate are:
(i) Common salt (NaCl)
(ii) Lime - stone (CaCO₃)
(iii) Ammonia gas (NH₃)
(iv) Coke for heating.

Process. The actual manufacture of sodium carbonate is carried out as follows:
1. Saturating or ammoniation tank. It is an upright iron cylinder having a conical base. The brine solution is taken in this tower and a mixture of ammonia gas and carbon dioxide is bubbled through it. Any impurities of calcium and magnesium salts in the brine solution are precipitated as carbonates and removed with the help of filter press.

2. Carbonating tower. It is provided with perforated horizontal plates. The clear ammoniacal brine after cooling flows downward slowly and carbon dioxide (from lime kiln) introduced at the base of the tower rises in small bubbles. Ammonical brine and carbon dioxide move in opposite directions and therefore, the two react on the principle of counter currents. Sodium bicarbonate is formed which is least soluble in excess of carbon dioxide and sodium chloride and hence precipitated.

3. Rotary vacuum filter. The milky liquid containing small crystals of sodium bicarbonate is drawn off at the base of the carbonating tower. It is filtered by means of a rotary vacuum filter and then scrapped off. The remaining liquor containing NH₄Cl is pumped to the top of the ammonia recovery tower.

4. Calcination of sodium bicarbonate. The sodium bicarbonate is calcined in a covered pan or a rotary furnace. It undergoes decomposition to form sodium carbonate, carbon dioxide and stream.

Carbon dioxide set free is again used in the carbonating tower together with the gas(from lime kiln).

5. Limekiln. Here lime -stone is heated to get carbon dioxide and calcium oxide.

Carbon dioxide formed is passed through the base of carbonating tower to form NaHCO₃. The CaO is treated with a large quantity of water to get Ca(OH)₂ which is pumped on to the ammonia recovery tower.

6. Ammonia recovery tower. The filtrate from the vacuum filter (Step 3) containing ammonium chloride is heated (using steam) along with Ca(OH)₂ (obtained from lime kiln) to get a mixture of ammonia with a small amount of carbon dioxide gas.

The ammonia with a small amount of carbon dioxide evolved is returned to the saturating tank for re-use. CaCl₂ is the only waste product of this process.
Thus, the ammonia-soda process is very cheap, self-contained and self-sufficient and sodium carbonate formed is quite pure.