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Sodium and Its Compounds, NaOH, Na₂CO₃, NaHCO₃

Sodium is an alkali metal. It is a very reactive metal, hence it does not occur freely. It is found as NaCl, NaBr and NaI in the sea.

It is found also in deposit as NaCl (also known as rock salt); NaNO₃ (also known as chile salt petre); and Na₂CO₃. It is found in borax and complex silicates found in clay soil.

Sodium dissolves readily in cold water to give an alkaline solution.

 Sodium hydroxide is an important compound of sodium with a wide range of usage.

Production of NaOH by Electrolysis of Brine:

This process is used for the industrial production of NaOH

Action of NaOH On Aluminium:

Aluminium readily dissolves in sodium hydroxide solution to form sodium aluminate and hydrogen.

2Al_(s) + 2NaOH_(aq) + 6H₂O_(l) → 2NaAl(OH)_4(aq) + 3H_2(g)

On Zinc: zinc also dissolves readily in sodium hydroxide solution to form sodium zincate and hydrogen.

Zn_(s) + 2NaOH_(aq) + 2H₂O_(l) → Na₂Zn(OH)_4(aq) + H_2(g)

On Lead: lead as well dissolves in sodium hydroxide solution, forming sodium leadate and hydrogen.

Pb_(s) + 2NaOH_(aq) + 2H₂O_(l) → Na₂Pb(OH)_4(aq) + H_2(g)

On Tin: tin dissolves in conc. sodium hydroxide solution to form trioxostannate(IV) salts and hydrogen.

Sn_(s) + 2NaOH_(aq) + H₂O_(l) → Na₂SnO_3(aq) + 2H_2(g)

Uses of NaOH

1. To precipitate insoluble metallic hydroxides - certain metallic hydroxides which are insoluble in water are precipitated from their salt solutions by NaOH .

Example, ZnSO_4(aq) + 2NaOH_(aq) → Zn(OH)_2(s) + Na₂SO_4(aq)

Pb(NO₃)_2(aq) + 2NaOH_(aq) → Pb(OH)_2(s) + 2NaNO_3(aq)

AlCl_3(aq) + 3NaOH_(aq) → Al(OH)_3(s) + 3NaCl_(aq)

FeSO_4(aq) + 2NaOH_(aq) → Fe(OH)_2(s) + Na₂SO_4(aq)

FeCl_3(aq) + 3NaOH_(aq) → Fe(OH)_3(s) + 3NaCl_(aq)

Note: the hydroxides of zinc, aluminium and lead are amphoteric. They will dissolve in excess of sodium hydroxide solution to form complex salts. Example,

Zn(OH)_2(s) + 2NaOH_(aq) → Na₂Zn(OH)_4(aq)

2. For absorbing carbon(IV) oxide in the laboratory.

3. For the manufacture of soap, paper, rayon (artificial silk), and for the manufacture of different other chemical compounds, such as sodium chlorate(V), sodium mathanoate and phosphine.

4. In the purification of bauxite.

5. In the refining of petroleum.

6. In the bleaching of cotton and textiles.

Sodium trioxocarbonate(IV), Na₂CO₃ can exist in different forms: in anhydrous state, i.e., dry or powdery (this is called soda ash) and in crystalline state.

There are two forms of crystal: a. Sodium trioxocarbonate(IV) decahydrate (washing soda), Na₂CO₃ . 10H₂O. This may lose 9 molecules of water when exposed to the atmosphere to give the second form.

b. Sodium trioxocarbonate(IV) monohydrate, Na₂CO₃.H₂O.

Industrial Preparation of Na₂CO₃

Sodium trioxocarbonate(IV) is manufactured industrially by a process called “the Solvay process”.

Ammonical brine, which is made by dissolving ammonia gas in brine (25% NaCl solution) is made to run down a solvay tower, up which carbon(IV) oxide is forced.

The towers are fitted at intervals with perforated mushroom-shaped baffle-plates. These plates slow down the flow of liquid and gas, as well as present a larger surface area for the reaction.

The chemical processes for the production of Na₂CO₃ is thus:

(1). The CO₂ reacts with the NH₃ in the ammonical brine to form ammonium hydrogen trioxocarbonate(IV).

NH3(g) + CO2(g) + H2O(l)

NH4HCO3(aq)

(2). The NH₄HCO₃ reacts with NaCl to form sodium hydrogen trioxocarbonate(IV).

NaCl_(aq) + NH₄HCO_3(aq) → NaHCO_3(s) + NH₄Cl(aq)

The formed NaHCO₃ is insoluble and is filtered off. It is washed and heated to obtain the anhydrous Na₂CO₃, together with water and CO₂ (this is passed back into the tower for further use).

2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(g) + CO_2(g)

If the crystals are required, the soda ash is dissolved in hot water.

Na₂CO_3(s) + 10H₂O_(l) → Na₂CO₃.10H₂O_(aq)

Efficiency of the Process (1). The major raw materials, NaCl and CaCO₃ are cheap and easily obtainable. The required CO₂ is obtained by heating limestone (i.e. CaCO₃), also about half of it is recovered from heating NaHCO₃.

Most of the ammonia used are recovered from NH₄Cl (by heating and washing with Ca(OH)₂).

It is then returned to the ammoniating tower.

2NH₄Cl_(aq) + Ca(OH)_2(aq) → CaCl_2(aq) + 2H₂O_(l) + 2NH_3(g)

(2). The process is a continuous flow - it uses minimum labour.

Properties of Na₂CO₃

(1). Both the anhydrous (soda ash) and crystalline (washing soda) form alkaline solution in water.

(2). Na₂CO₃ does not decompose by heat. (3). Both the anhydrous and crystalline forms react with acids to liberate carbon(IV) oxide.

Uses of Sodium Trioxocarbonate(IV), Na₂CO₃

(1). In the manufacture of glass - ordinary glass is made by fusing together Na₂CO₃, CaCO₃, SiO₂ and a little carbon (as a reducing agent).

To facilitate fusion, broken glass (cullet) is added. Na₂CO_3(s) + SiO_2(s) → Na₂SiO_3(s) + CO_2(g)

Na₂SiO_3(s) - Sodium trioxosilicate(IV) is the glass.

(2). In domestic water softening process, Na₂CO₃ precipitates the Ca²⁺ ions which cause hardness in water.

(3). In the manufacture of water glass.

(4). In the manufacture of NaOH, sodium silicate(VI) and borax.

(5). In the manufacture of soap and paper.

(6). Used in standardizing acids for titrimetry analysis in the laboratory.

Sodium hydrogen trioxocarbonate(IV) is also known as baking soda, and has the molecular formula NaHCO₃.

Industrial Preparation of NaHCO₃

The salt is manufactured industrially by the solvay process by saturating a wet mush of Na₂CO₃ obtained from the solvay process with CO₂.

Na₂CO_3(aq) + H₂O_(l) + CO_2(g) → 2NaHCO_3(s)

Properties of NaHCO₃

Sodium hydrogen trioxocarbonate(IV) shows the following properties:

(1). It is a white crystalline solid.

(2). It is slightly soluble in water.

(3). It hydrolysis in water to give alkaline solution (even though it is an acid salt), hence its solution turns red litmus paper blu

(4). On heating, it decomposes to release CO₂

2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(l) + CO_2(g)

(5). With acids, NaHCO₃ produces water, CO₂ and a salt.

NaHCO_3(aq) + HCl_(aq) → NaCl_(aq) + H₂O(l) + CO₂

Uses of NaHCO₃

It is used in the manufacture of baking powder. As we have stated, it produces CO₂ when heated. The CO₂ causes the cake to rise. This is the reason it is called ‘baking soda’.

The following test can be used to identify either of Na₂CO₃ or NaHCO₃ from the other:

(1). Application of heat

When both substances are heat only NaHCO₃ decomposes. It produces Na₂CO₃, H₂O and CO₂.

(2). Addition of magnesium sulphate solution

 In the presence of magnesium sulphate solution only Na₂CO₃ reacts to form a white precipitate.

MgSO_4(aq) + Na₂CO_3(aq) → MgCO_3(s) + Na₂SO_4(aq) 

MgCO_3(s) is the white precipitate

(3). Addition of a solution of a metallic salt (other than that of magnesium or aluminium)

Only NaHCO₃ solution reacts to liberate CO₂ Example,

ZnSO4(aq) + 2NaHCO₃ → ZnCO_3(s) + Na₂SO_4(aq) + H₂O_(l) + CO_2(g)

Sodium chloride is found to the extent of about 3% by mass in sea water. It can be obtained by the vaporization of sea water by the sun - this is suitable in the part of the world where the sun’s heat energy is intense.

Uses of NaCl

(1). It is converted into a number of useful compounds, such as NaOH, Na₂CO₃, NaHCO₃, Na₂SO₄, Cl₂, HCl and NaOCl.

(2). It is used to ‘salt-out’ soap in the manufacture of soap.

(3). As a food preservative.

(4). In regenerating water softeners.

(5). In glazing earth ware.