Notes On Group 16: Sulphur - Sulphur Dioxide - CBSE Class 12 Chemistry

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
Fe2S3  +  4O2         →        FeO +  3SO2

              Liquefaction 25 atm
So2 (g)              →                       SO2(l)

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

                           Δ
S        +    O2      →        SO2

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.

       SO2             +       H2O     →     H2SO3
Sulphur dioxide           Water         Sulphurous acid

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

       SO2             +       2NaOH                   →     Na2SO3          + H2O
Sulphur dioxide            Sodium hydroxide         Sodium sulpahte          Water

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

       SO2             +         Na2SO3                   →         H2O     +         2NaHSO3  
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:
 
 2Fe3+  +  SO2     + 2H2O     →   2Fe2+     +  SO42-   +   4H+
Ferric     Sulphur       water           Ferrous
   salt        dioxide                               salt

X2           +   SO2     + 2H2O     →   SO42-     +  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.

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
Fe2S3  +  4O2         →        FeO +  3SO2

              Liquefaction 25 atm
So2 (g)              →                       SO2(l)

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

                           Δ
S        +    O2      →        SO2

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.

       SO2             +       H2O     →     H2SO3
Sulphur dioxide           Water         Sulphurous acid

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

       SO2             +       2NaOH                   →     Na2SO3          + H2O
Sulphur dioxide            Sodium hydroxide         Sodium sulpahte          Water

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

       SO2             +         Na2SO3                   →         H2O     +         2NaHSO3  
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:
 
 2Fe3+  +  SO2     + 2H2O     →   2Fe2+     +  SO42-   +   4H+
Ferric     Sulphur       water           Ferrous
   salt        dioxide                               salt

X2           +   SO2     + 2H2O     →   SO42-     +  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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