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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, NaOH

 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) + 3H2(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) + H2(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) + H2(g)

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

Sn(s) + 2NaOH(aq) + H2O(l) → Na2SnO3(aq) + 2H2(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, ZnSO4(aq) + 2NaOH(aq) → Zn(OH)2(s) + Na2SO4(aq)

Pb(NO3)2(aq) + 2NaOH(aq) → Pb(OH)2(s) + 2NaNO3(aq)

AlCl3(aq) + 3NaOH(aq) → Al(OH)3(s) + 3NaCl(aq)

FeSO4(aq) + 2NaOH(aq) → Fe(OH)2(s) + Na2SO4(aq)

FeCl3(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  

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)   reversible reaction arrow  NH4HCO3(aq)    

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

NaCl(aq) + NH4HCO3(aq) → NaHCO3(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).

2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g)

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

Na2CO3(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) → CaCl2(aq) + 2H2O(l) + 2NH3(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. Na2CO3(s) + SiO2(s) → Na2SiO3(s) + CO2(g)

Na2SiO3(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)

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.

Na2CO3(aq) + H2O(l) + CO2(g) → 2NaHCO3(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

2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)

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

NaHCO3(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’.

To Distinguish Between Na2CO3 and NaHCO3

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.

MgSO4(aq) + Na2CO3(aq) → MgCO3(s) + Na2SO4(aq) 

MgCO3(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 → ZnCO3(s) + Na2SO4(aq) + H2O(l) + CO2(g)

Sodium Chloride, NaCl

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.

  

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