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Group 16: Sulphur - Sulphur Dioxide

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Group 16: Sulphur - Sulphur Dioxide - Lesson Summary

Methods to prepare sulphur dioxide gas:
Sulphur dioxide prepared in the laboratory by the action of dilute sulphuric acid on sulphites.

Ex: sodium sulphite on treatment with dilute sulphuric acid, gives sulphur dioxide

     Na2SO3        +       H2SO4            → Na2SO4                    + H2O           + SO2
Sodium sulphite         Sulphuric acid      Sodium sulphate      water          Sulphur dioxide

Industrially, large volumes of sulphur dioxide are prepared by roasting a sulphide ore, such as iron sulphide.

After drying, the gas is liquefied under 25 atmospheres pressure and stored in steel cylinders.

                         Roasting
Fe 2S 3  +  4O 2         →        FeO +  3SO 2

              Liquefaction 25 atm
So 2 (g)              →                       SO 2(l)

Sulphur dioxide is also obtained on a large scale by burning sulphur in air.

                           Δ
S        +    O 2      →        SO 2

Physical properties of sulphur dioxide:

  • Sulphur dioxide is a colourless gas with a pungent smell of burning sulphur.
  • It is one of the easiest gases to liquefy, since it condenses at room temperature under a pressure of 2 atm.


Chemical properties of sulphur dioxide:

  • It is an acidic oxide that is highly soluble in water.
  • It dissolves in water to give sulphurous acid.

       SO 2             +       H 2O     →     H 2SO 3
Sulphur dioxide           Water         Sulphurous acid

It reacts readily with sodium hydroxide solution and forms sodium sulphite.

       SO 2             +       2NaOH                   →     Na 2SO 3          + H 2O
Sulphur dioxide            Sodium hydroxide         Sodium sulpahte          Water

If more sulphur dioxide is passed into this solution, sodium hydrogen sulphite is formed.

       SO 2             +         Na 2SO 3                   →         H 2O     +         2NaHSO 3  
Sulphur dioxide            Sodium sulphate                      Water           Sodium hydrogen sulpahte  
  (excess)

The sulphur atom in a sulphur dioxide molecule is tetravalent. Hence, it can extend

It's covalency to six by directly combining with elements like O2 &Cl2 to form the corresponding addition compounds.

Ex: It reacts with chlorine in the presence of charcoal as a catalyst to give sulphuryl chloride (SO2Cl2).

                                                          Charcoal
         SO2             +           Cl2                →              SO2Cl2
Sulphur dioxide                 Chlorine       (Catalyst)      Sulphuryl Chloride

In the presence of vanadium pent oxide as a catalyst, it gives sulphur trioxide.

                                                              V2O5
         2SO2             +           O2                →              SO3
Sulphur dioxide                 Oxygen                           Sulphur trioxide

In the presence of moisture, it can liberate nascent hydrogen, and, thus, act as a reducing agent.

Ex: It reduces ferric salts to ferrous salts, and halogens to halogen acids.

Example:
 
 2Fe 3+  +  SO 2     + 2H 2O     →   2Fe 2+     +  SO 4 2-   +   4H +
Ferric     Sulphur       water           Ferrous
   salt        dioxide                               salt

X 2           +   SO 2     + 2H 2O     →   SO 4 2-     +  2X -   +   4H +
Halogen     Sulphur       water                                   halogen
                     dioxide                                                   acid

Identifying tests for sulphur dioxide gas:
It de colourises acidified KMnO4 solution.

    5SO2     + 2KMnO4       + 2H2O  → K2SO4     +      2 MNSO4 + 2H2SO4
Sulphur       potassium                       potassium       Manganese       Sulphuric
dioxide       permanganate                     sulphate            sulphate           acid

It turns a filter paper moistened with acidified K2Cr2O7 solution green.

    3SO2     +   K2Cr2O7       +    H2SO4          →  K2SO4     +      2Cr2(SO4)3 +     H2O
Sulphur       Potassium            Sulphuric             potassium        chromium      
dioxide           dichromate               acid               sulphate            sulphate      

It turns starch iodate paper blue.

   5SO2   +    2KIO3   +  4H2O →  2KHSO4     +   3H2SO4     +   I2
Sulphur     potassium     Water      potassium       Sulphuric        Iodine
 dioxide        iodate                       hydrogen            acid
                                                     sulphate

Structure of sulphur dioxide:
It is angular in shape with an O-S-O bond angle of 119.50. Note that although sulphur dioxide has two different types of pi bonds, i.e. p pi - p pi and d pi - p pi.

The two sulphur-oxygen bond lengths are equal. This indicates that sulphur dioxide is a resonance hybrid of two canonical forms.

Uses:

• Sulphur dioxide is used in the manufacture of industrial chemicals like sulphuric acid, sodium hydrogen sulphite and calcium hydrogen sulphite.

• It is used to refine sugar and petroleum.

• It is used to bleach delicate material like wool and silk.

• It is used as a disinfectant for killing germs, fungi and certain moulds.

• It is used as an antichlor for removing excess chlorine from a fabric after bleaching.

• Liquid sulphur dioxide is used as a solvent to dissolve a number of organic and inorganic compounds.

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