10.4 Some Important Compounds Of Sodium

Industrially important compounds of sodium include sodium carbonate, sodium hydroxide, sodium chloride and sodium bicarbonate. The large scale production of these compounds and their uses are described below:

Sodium Carbonate (Washing Soda), Na2CO3·10H2O

Sodium carbonate is generally prepared by Solvay Process. In this process, advantage is taken of the low solubility of sodium hydrogencarbonate whereby it gets precipitated in the reaction of sodium chloride with ammonium hydrogencarbonate.

The latter is prepared by passing CO2 to a concentrated solution of sodium chloride saturated with ammonia, where ammonium carbonate followed by ammonium hydrogencarbonate are formed.

The equations for the complete process may be written as:

2NH3 + H2O + CO2 → (NH4)2CO3

(NH4)2CO3 + H2O + CO2 → 2NH4HCO3

NH4HCO3 + NaCl → NH4Cl + NaHCO3

Sodium hydrogencarbonate crystal separates. These are heated to give sodium carbonate.

2NaHCO3 → Na2CO3 + CO2 + H2O

In this process NH3 is recovered when the solution containing NH4Cl is treated with Ca(OH)2. Calcium chloride is obtained as a by-product.

2NH4Cl + Ca(OH)2 → 2NH3 + CaCl2 + H2O

It may be mentioned here that Solvay process cannot be extended to the manufacture of potassium carbonate because potassium hydrogencarbonate is too soluble to be precipitated by the addition of ammonium hydrogencarbonate to a saturated solution of potassium chloride.

Properties : Sodium carbonate is a white crystalline solid which exists as a decahydrate, Na2CO3·10H2O. This is also called washing soda. It is readily soluble in water.

On heating, the decahydrate loses its water of crystallisation to form monohydrate. Above 373K, the monohydrate becomes completely anhydrous and changes to a white powder called soda ash.

Na2CO3.10H2O → Na2CO3.H2O + 9H2O

Na2CO3.H2O → Na2CO3 + H2O

Carbonate part of sodium carbonate gets hydrolysed by water to form an alkaline solution.

CO32- + H2O → HCO3- + OH-

Uses:

  • It is used in water softening, laundering and cleaning.
  • It is used in the manufacture of glass, soap, borax and caustic soda.
  • It is used in paper, paints and textile industries.
  • It is an important laboratory reagent both in qualitative and quantitative analysis.

 

Sodium Chloride, NaCl

The most abundant source of sodium chloride is sea water which contains 2.7 to 2.9% by mass of the salt. In tropical countries like India, common salt is generally obtained by evaporation of sea water. Approximately 50 lakh tons of salt are produced annually in India by solar evaporation.

Crude sodium chloride, generally obtained by crystallization of brine solution, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities.

Calcium chloride, CaCl2, and magnesium chloride, MgCl2 are impurities because they are deliquescent (absorb moisture easily from the atmosphere).

To obtain pure sodium chloride, the crude salt is dissolved in minimum amount of water and filtered to remove insoluble impurities. The solution is then saturated with hydrogen chloride gas. Crystals of pure sodium chloride separate out.

Calcium and magnesium chloride, being more soluble than sodium chloride, remain in solution. Sodium chloride melts at 1081K. It has a solubility of 36.0 g in 100 g of water at 273 K. The solubility does not increase appreciably with increase in temperature.

Uses :

  • It is used as a common salt or table salt for domestic purpose.
  • It is used for the preparation of Na2O2, NaOH and Na2CO3.

 

Sodium Hydroxide (Caustic Soda), NaOH

Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell.

A brine solution is electrolysed using a mercury cathode and a carbon anode. Sodium metal discharged at the cathode combines with mercury to form sodium amalgam. Chlorine gas is evolved at the anode.

Cathode: Na+ + e- → Na – amalgam

Anode: Cl- → 1/2 Cl2 + e-

The amalgam is treated with water to give sodium hydroxide and hydrogen gas.

Sodium hydroxide is a white, translucent solid. It melts at 591 K. It is readily soluble in water to give a strong alkaline solution. Crystals of sodium hydroxide are deliquescent.

The sodium hydroxide solution at the surface reacts with the CO2 in the atmosphere to form Na2CO3.

Uses: It is used in

(i) the manufacture of soap, paper, artificial silk and a number of chemicals,

(ii) in petroleum refining,

(iii) in the purification of bauxite,

(iv) in the textile industries for mercerising cotton fabrics,

(v) for the preparation of pure fats and oils, and

(vi) as a laboratory reagent.

 

Sodium Hydrogencarbonate (Baking Soda), NaHCO3

Sodium hydrogencarbonate is known as baking soda because it decomposes on heating to generate bubbles of carbon dioxide (leaving holes in cakes or pastries and making them light and fluffy).

Sodium hydrogencarbonate is made by saturating a solution of sodium carbonate with carbon dioxide. The white crystalline powder of sodium hydrogencarbonate, being less soluble, gets separated out.

Na2CO3 + H2O + CO2 → 2NaHCO3

Sodium hydrogen carbonate is a mild antiseptic for skin infections. It is used in fire extinguishers.

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