SULPHURIC ACID (H2SO4)

    On industrial scale, sulphuric acid can be prepared by the following two methods.
 Contact process
 Lead Chamber process
    

CONTACT PROCESS

    Now a days, sulphuric acid is prepared by contact process all over the world. 
    Preparation of sulphuric acid by contact process is based upon the catalytic oxidation of SO2 to SO3.

DETAILS OF CONTACT PROCESS :

 Following steps are involved in the preparation of H2SO4.
 PREPARATION OF SO2.
 PURIFICATION OF SO2.
 OXIDATION OF SO2.
 ABSORPTION OF SO3.
 DILUTION OF OLEUM.

PREPARATION OF SO2

  SO2 is obtained by burning sulphur or by heating iron pyrite (FeS2) in pyrite burner.

 S + O2 → SO2

 4FeS2 + 11O2 →  2Fe2O3 + 8SO2

PURIFICATION OF SO2

  SO2 contains a number of impurities such as dust particles, Arsenous oxide, vapours, sulphur etc. These     impurities must be removed otherwise catalyst loses its efficiency (catalyst poisoning).

    DUST CHAMBER:

    SO2 is first passed through the dust chamber where steam is spread over the gas to remove dust     particles, which settle down. Fe(OH)3 also sprayed over to remove oxides of Arsenic.

    WASHING TOWER:

    SO2 is then passed through a washing tower after cooling. Here it is sprayed by water to remove any     other soluble impurities.

    DRYING TOWER:
    
The gas is now dried by passing through drying tower where conc. H2SO4 (dehydrating agent) is     sprayed. H2SO4 removes moisture from SO2.

    ARSENIC PURIFIER:

    Arsenic oxide is a poison for the catalyst. It is removed when the gas is passed over ferric hydroxide.

  As2O3 + 2Fe(OH)3 →  2FeAsO3 + 3H2O.

    In order to remove traces of As2O3, it is passed through a test box, where a strong beam of light is thrown against the gas. If there is no scattering of light in the box, it indicates that gas is free from     As2O3.

OXIDATION OF SO2 TO SO3

    CONTACT TOWER:
    Oxidation of SO2 is carried out in contact tower where V2O5 is filled in different pipes. SO2 here reacts     with air (O2) to produce SO3. Under above conditions 98% SO2 is converted into SO3.

 2SO2 + O2  → 2SO3 + 45Kcal

CONDITIONS NECESSARY FOR MAXIMUM YIELD OF SO3:

    Oxidation of SO2 is a reversible and exothermic process in which volume of product is less than the     volumes of reactants. In order to obtain maximum amount of SO3, according to Le-Chatelier’s Principle  following conditions are necessary.

    CONCENTRATION:
    Excess of O2.

    TEMPEATURE:

    A decrease in temperature favours reaction in forward direction. Optimum temperature for this process is     450oC to 500oC.

    PRESSURE:

    Since volumes of reactants are greater than the product (3:2), therefore, according to Le-Chatelier’s     Principle a high pressure is favourable. Optimum pressure is about 1.5 to 1.7 atmosphere.

    USE OF CATALYST:
    
At low temperature, rate of reaction decreases. To increase rate of reaction a catalyst vanadium     pentaoxide (V2O5) is used.

ABSORPTION OF SO3 IN H2SO4

    SO3 is not directly passed in water, because a dense fog of minute particles of H2SO4 is produced. It is     therefore, dissolved in conc.H2SO4 to form pyrosulphuric acid (oleum).

                                    SO3 + H2SO4 →  H2S2O7 (OLEUM)

DILUTION OF OLEUM

    Oleum is now diluted with water to form H2SO4 of required concentration.

H2S2O7 + H2O →  2H2SO4

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DOWN'S PROCESS

INTRODUCTION:

J.C Down developed a process for the manufacture of sodium metal known as DOWN's Process.

PRINCIPLE:

The process based upon the principle of electrolysis of Aqeous sodium chloride solution

 NaCl     →   2Na + Cl2

RAW MATERIALS:

• Fused sodium chloride is used as the electrolyte in the process.
• As the sodium chloride is has the melting point of 801 degree celcius some amount of calcium chloride is added to lower the melting point to about 600 degree celcius which makes the process feasible.

Working:

When an electric current is passed through the molten mixture of NaCl and CaCl2, NaCl decomposes in to   Na+ and Cl- ion. Na+ ions migrate towards cathode while Cl- ions towards the anode. The molten sodium   collects in the cathode compartment where it rises to the top and is tapped off by a pipe. Chlorine is   collected at the anode.

ELECTROCHEMICAL CHANGES

 At cathode

 

Na+-ions migrate to cathode where they are reduced to Na.

2Na+ + 2e    →  2Na (Reduction)

At anode 

Cl--ions migrate to anode and oxidised to form chlorine gas.

2Cl-   →  Cl2 + 2e- (Oxidation)

Overall Reaction

 2Na+ + 2e-    →   2Na

2Cl-   →  Cl2 + 2e-

__________________ 

2Na+ + 2Cl-   →  2Na + Cl2

 During electrolysis calcium is also obtained at cathode but sodium and calcium are separated from each other due difference in density. Density of Na is 0.67gm/cc and the density of Ca is much higher than that of Na i.e. 2.54gm/cc. That's why they do not mix with each other.

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On industrial scale SODA ASH is prepared by Ammonia Solvay Process.

Raw Materials

1. Sodium Chloride
2. Lime Stone (CaCO3)
3. Ammonia

Details of the process

At first stage a saturated solution of sodium chloride is prepared which is also known "BRINE". 
Composition of solution of NaCl (brine) is 28% m/m.
Steps of preparation

Ammoniation of Brine

In this stage saturated brine is allowed to flow down an ammoniating tower. This tower is fitted with mushroom shaped baffles. These baffles control the flow of brine and ensure the proper mixing and saturation of ammonia.

Carbonation of ammoniated brine

In the second step, ammoniated brine is allowed to trickle down a carbonating tower known as solvay tower. This tower is also fitted with baffle plates. Here brine is mixed with carbon dioxide gas, produced by heating lime stone in a separate chamber called "kiln".
                                       CaCO3  → CaO + CO2

the baffle plates ensure the flow of solution and breaks up carbon dioxide into small bubbles to produce good conditions for reaction.

Chemistry of Solvay tower

CO2 reacts with ammonia to form ammonium carbonate.

2NH3 + CO2 + H2O →  (NH4)2CO3

Ammonium carbonate further reacts with CO2 to form ammonium bicarbonate.(NH4)2CO3 + CO2 + H2O →  2NH4HCO3 (aq.)

Ammonium bicarbonate then react with NaCl to form sodium bicarbonate.
                           NH4HCO3 + NaCl  → NaHCO3 + NH4Cl
Due to exothermic nature of above reactions, solubility of NaHCO3 increases. To counter this effect , lower part of Solvay tower is cooled , ppt of NaHCO3 are separated by vacuum filtration and washed to remove ammonium salts.
Conversion of NaHCO3 to Na2 CO3

Dry sodium bicarbonate is heated in rotary furnace called "CALCINER" to give anhydrous sodium carbonate or soda ash. Carbon dioxide is recirculated to carbonation tower.
                            2NaHCO3 →  Na2 CO3 + CO2 + H2O

Ammonia recovery process

When CaCO3 is heated, CaO is obtained along with CO2 . CaO is treated with water to form Ca(OH)2 .
                                    CaO + H2O  → Ca(OH)2
Quick lime is heated with NH4 Cl to form NH3 and calcium chloride (by product) . Ammonia is used again in this process.

                 2NH4Cl + Ca(OH)2 →  CaCl2 + 2NH3 + 2H2 O

this is the method for the production of SODA ASH 
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Since the atoms of alkaline earth metals are smaller in size and have one  valance electron more than the alkali metals of group IA,therefore they are more closely packed and less tightly held. Hence the inner atomic attraction in the alkaline earth metals is greater and they are harder than alkali metals.

As we go down the group the atomic size increases and the atoms are packed less and closely and held less and less tightly.Thus the atomic attraction decreases and hardness decreases.

 Oxides of group 2 were originally thought to be elements, not compounds.  These "elements" were called alkaline earths: "alkaline" because they're basic when added to water, and 'earth' because that's what they called oxides that they thought were elements.