Compounds of Sodium

Sodium Hydroxide (NaOH):

Also known as Caustic Soda

Preparation:

Sodium hydroxide is a strong base and is prepared by the electrolysis of aqueous solution of sodium chloride known as brine.

NaCl is mixed in water to produce brine. Composition of brine is 25% m/m.

Electrolysis of brine is carried out in an electrolytic cell known as "Diaphragm cell". When an electric current is passed through the brine, following changes take place.

Ionization of NaCl:           2NaCl → 2Na+ + 2Cl-

Ionization H2O:               2H2O → 2H+ + 2OH-

Electrochemical changes:

At cathode:              2H+ + 2e- → H2 

At anode:                  2Cl- → Cl2 + 2e-

During electrolysis, Na+ ions are not discharged but H+ ions are readily discharged. As more H+ ions are discharged, more H2O dissociates to replace it. Na+ and OH- ions accumulate in the solution. On evaporation sodium hydroxide is obtained.

Na+ + OH-→ NaOH

Physical Properties:

1. It is a white crystalline solid. It has soapy touch.
2. It is highly deliquescent.
3. It is highly soluble in water. It is bitter in taste.
4. Its density is 2.13 g/ml and melting point 3180C
5. It is corrosive in nature.
6. Its solubility is comparatively less in alcohol.

Chemical Properties:

a. Action of atmosphere: It cannot be kept in atmosphere 2NaOH + CO₂ → Na₂CO₃ + H₂O

b. Strong alkali: Sodium hydroxide is a strong alkali as it dissociates completely in water furnishing OH⁻ ions.

NaOH ⇋ Na⁺ + OH⁻

1. It reacts with acids forming corresponding salts
   
   NaOH + HCl → NaCl + H₂O 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
2. It combines with acidic oxides to form salts
   
   2NaOH + CO₂ → Na₂CO₃ + H₂O 2NaOH + SO₂ → Na₂SO₃ + H₂O 2NaOH + 2NO₂ → NaNO₃ + NaNO₂ + H₂O

c. With halogens: Cl₂ + 2NaOH → NaCl + NaClO + H₂O

Br₂ + 2NaOH → 2NaBr + H₂O

d. Action on metals: Metals like Zn, Al, Sn, Pb react with sodium hydroxide solution and evolve hydrogen. Zn + 2NaOH → Na₂ZnO₂ + H₂O
2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂
Sn + 2NaOH → Na₂SnO₂ + H₂
e. Insoluble hydroxides which dissolve in excess of NaOH:
ZnSO₄ + 2NaOH → Zn (OH)₂ + Na₂SO₄
Zn (OH)₂ + 2 NaOH → Na₂ZnO₂ + 2H₂O
Al (OH)₃ + NaOH → NaAlO₂ + 2H₂O
Sn (OH)₂ + 2 NaOH → Na₂SnO₂ + 2H₂O

Sodium carbonate (Na₂CO₃.10H₂O):

Manufacture of Sodium Carbonate:

Sodium carbonate is usually made by the Ammonia-soda process or Solvay process.

The raw materials for this process are common salt, ammonia and limestone (for supplying CO2 and quicklime).

When carbon dioxide is passed into a concentrated solution of brine saturated with ammonia, ammonium bicarbonate is produced. The ammonium bicarbonate then reacts with common salt forming sodium bicarbonate.

NH₄OH   +      H₂CO₃   →   NH₄HCO₃   +      H₂O

                                 Ammonium Bicarbonate

NH₄HCO₃   +      NaCl   →   NaHCO₃   +      NH₄Cl

                              Sodium Bicarbonate

Sodium bicarbonate being slightly soluble (in presence of sodium ions) gets precipitated. Precipitated sodium bicarbonate is removed by filtration and changed into sodium carbonate by heating.

2 NaHCO₃      →      Na₂CO₃      +      H₂O      +      CO₂

                       Sodium Bicarbonate

Physical Properties:

1. Sodium carbonate is a white crystalline solid. It is known in several hydrated forms.
2. The common form is decahydrate, Na₂CO₃.10H₂O. This form is called washing soda.
3. It also forms a heptahydrate Na₂CO₃.7H₂O  
4. On heating, the monohydrate changes into anhydrous.
5. It is soluble in water with evolution of considerable amount of heat. The solution is alkaline in nature due to hydrolysis
   
   Na₂CO₃ + 10H₂O ⇋ 2NaOH + H₂ CO₃

Chemical properties:

1. Action of acids:
   It is readily decomposed by acids with evolution of carbon dioxide. The reaction occurs in two steps.
   

   Na₂CO₃ + HCl → NaHCO₃ + NaCl

   
   

   NaHCO₃ + HCl → NaCl + H₂O + CO₂

2. Action of CO₂:
   On passing CO₂ through the concentrated solution of sodium carbonate, sodium carbonate gets precipitated.

Na₂CO₃ + H₂O + CO₂ → 2 NaHCO₃

3. Action of salts of non–alkali metals:
   Sodium carbonate reacts with metal salts (except alkali metal salts) to form insoluble normal or basic carbonates

CaCl₂ + Na₂CO₃ → CaCO₃ + 2 NaCl

BaCl₂ + NaCO₃ → BaCO₃ + 2 NaCl

2MgCl₂ + 2Na₂CO₃ + H₂O → MgCO₃.Mg (OH)₂ + 4 NaCl + CO₂

Uses:

1. Used in laundries and in softening of water as washing soda.
2. In textile and petroleum refining.
3. For the preparation of various carbonates of metals

Sodium Bicarbonate (Baking soda):

• Formula of Baking soda is NaHCO₃
• It is a white crystalline solid, sparingly soluble in water
• The solution is alkaline in nature due to hydrolysis

Preparation:

Na₂CO₃ + CO₂ + H₂O → 2 NaHCO₃ (Sparingly Soluble)

Properties:

• The solution gives yellow color with methyl orange but no color with phenolphthalein.
• On heating it loses carbon dioxide and water forming sodium carbonate.
  2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

  ZnSO₄ + 2 NaHCO₃ → ZnCO₃ + Na₂SO₄ + H₂O + CO₂

Uses:

1. It is used as medicine to neutralize the acidity in the stomach.
2. It is largely used for making baking powder. Baking powder is a mixture of potassium hydrogen tartrate and sodium bicarbonate.
3. It is used in making effervescent drinks.
4. It is used in fire exhausters.
5. It is used for production of carbon dioxide.