Notes On Group 16: Oxygen - Dioxygen - CBSE Class 12 Chemistry
Preparation of dioxygen:
The most convenient method for preparing dioxygen in the laboratory involves the catalytic decomposition of solid potassium chlorate. Manganese dioxide is used as the catalyst in this reaction.
                
                   Δ 
2KClO3       →        2KCl + 3O2
                 MnO2

The other laboratory method is the thermal decomposition of oxides of metals from the lower part of the electrochemical series. EX: The thermal decomposition of silver oxide or mercuric oxide gives dioxygen.

    2Ag2O           →  4Ag     +   O2
Silver oxide         Δ     Silver        Dioxygen

     2HgO                →    2Hg     +   O2
Mercuric oxide        Δ     Mercury        Dioxygen

Dioxygen may also be obtained in the laboratory by heating the higher oxides of some metals like lead, barium and manganese.

  2PbO2                    →       2PbO                 +   O2
Lead (IV) oxide           Δ     Lead (II) oxide        Dioxygen

   2BaO2                     →       2BaO                 +   O2
Barium peroxide            Δ     Barium oxide        Dioxygen

   2MnO2          + 2H2SO4      →  2MnSO4         + 2H2O    + O2
Manganese       Sulphuric        Δ    manganese        Water     Dioxygen
  (IV) oxide                 acid         (II) sulphate

The thermal decomposition of salts rich in oxygen, such as nitrates, and permanganates also yields dioxygen.

2KNO3             → 2KNO2             +  O2
Potassium        Δ     Potassium         Dioxygen
      nitrate                    nitrate

2NaNO3             → 2NaNO2             +  O2
Sodium               Δ     Sodium         Dioxygen
    nitrate                    nitrate

2KMnO4             → K2MnO4          +  MnO2                    +  O2
Potassium            Δ     Potassium              manganese             Dioxygen
    permanganate            manganate                 (IV) oxide
 
Commercially, dioxygen is prepared by either the fractional distillation of liquid air or by the electrolysis of water.

Properties of dioxygen:
  • Dioxygen is a colourless, tasteless and odourless gas.
  • It is slightly heavier than air.
  • It is slightly soluble in water .This small quantity of dissolved dioxygen is just sufficient to support marine and aquatic life.
  • It can be liquefied under pressure to a pale blue liquid by compressing the gas at 90K. It can also be solidified into a bluish white solid at 55K.
 
Isotopes:
It has three isotopes forms with mass numbers 16, 17 and 18. Dioxygen exhibits paramagnetic behaviour.

Chemical properties of dioxygen:
Dioxygen is a very reactive element and reacts directly with nearly all metals and non-metals. It does not react directly with some metals like gold and platinum, and some noble gases like helium, neon and argon.

Reaction of dioxygen with metals:
Most metals burn in dioxygen and form oxides that are mostly basic in nature.

Metal             Dioxygen        Metal-oxide
4M        +          O2         →     2M2O
                                          Δ
2M        +          O2         →     2MO
                                          Δ
4M        +          3O2        →     2M2O3
                                          Δ

Most of non-metals burn in the presence of dioxygen forms acidic oxides. EX: Sulphur burns in the presence of oxygen gives sulphur dioxide.
     
                                  Δ
S        +      O2          →    SO2

Reactions of dioxygen with some compounds:
Sulphur dioxide undergoes catalytic oxidation in the presence of vanadium pentoxide to form sulphur trioxide. This is an important step in the manufacture of sulphuric acid by the contact process.

                                     V2O5
2SO2        +      O2            →       2SO3

Dioxygen reacts with several organic compounds, such as hydrocarbons and carbohydrates, at elevated temperatures or on ignition, forming carbon dioxide and water.
 
                              High Temperature
CH4         +    2O2            →                  CO2       +     2H2O
Methane       Dioxygen                        Carbon               Water
                                                           dioxide

                                    High Temperature
C6H12O6         +    6O2            →                  6CO2       +     6H2O
Glucose          Dioxygen                                 Carbon               Water
                                                                     dioxide


Uses of dioxygen:
  • Dioxygen lies in its support to vital processes such as combustion and respiration.
  • The main importance of dioxygen lies in its support to vital processes such as combustion and respiration.
  • Dioxygen mixed with carbon dioxide or helium is used for artificial respiration.
  • It is used in the manufacture of many metals.
  • It is used in oxy-acetylene welding and metal cutting.
  • It is used to oxidise ammonia in the manufacture of nitric acid.
  • Oxygen cylinders are widely used in hospitals, high-altitude flying and in mountaineering.
  • Liquid oxygen is an important constituent of the fuel used in rockets.

Summary

Preparation of dioxygen:
The most convenient method for preparing dioxygen in the laboratory involves the catalytic decomposition of solid potassium chlorate. Manganese dioxide is used as the catalyst in this reaction.
                
                   Δ 
2KClO3       →        2KCl + 3O2
                 MnO2

The other laboratory method is the thermal decomposition of oxides of metals from the lower part of the electrochemical series. EX: The thermal decomposition of silver oxide or mercuric oxide gives dioxygen.

    2Ag2O           →  4Ag     +   O2
Silver oxide         Δ     Silver        Dioxygen

     2HgO                →    2Hg     +   O2
Mercuric oxide        Δ     Mercury        Dioxygen

Dioxygen may also be obtained in the laboratory by heating the higher oxides of some metals like lead, barium and manganese.

  2PbO2                    →       2PbO                 +   O2
Lead (IV) oxide           Δ     Lead (II) oxide        Dioxygen

   2BaO2                     →       2BaO                 +   O2
Barium peroxide            Δ     Barium oxide        Dioxygen

   2MnO2          + 2H2SO4      →  2MnSO4         + 2H2O    + O2
Manganese       Sulphuric        Δ    manganese        Water     Dioxygen
  (IV) oxide                 acid         (II) sulphate

The thermal decomposition of salts rich in oxygen, such as nitrates, and permanganates also yields dioxygen.

2KNO3             → 2KNO2             +  O2
Potassium        Δ     Potassium         Dioxygen
      nitrate                    nitrate

2NaNO3             → 2NaNO2             +  O2
Sodium               Δ     Sodium         Dioxygen
    nitrate                    nitrate

2KMnO4             → K2MnO4          +  MnO2                    +  O2
Potassium            Δ     Potassium              manganese             Dioxygen
    permanganate            manganate                 (IV) oxide
 
Commercially, dioxygen is prepared by either the fractional distillation of liquid air or by the electrolysis of water.

Properties of dioxygen:
  • Dioxygen is a colourless, tasteless and odourless gas.
  • It is slightly heavier than air.
  • It is slightly soluble in water .This small quantity of dissolved dioxygen is just sufficient to support marine and aquatic life.
  • It can be liquefied under pressure to a pale blue liquid by compressing the gas at 90K. It can also be solidified into a bluish white solid at 55K.
 
Isotopes:
It has three isotopes forms with mass numbers 16, 17 and 18. Dioxygen exhibits paramagnetic behaviour.

Chemical properties of dioxygen:
Dioxygen is a very reactive element and reacts directly with nearly all metals and non-metals. It does not react directly with some metals like gold and platinum, and some noble gases like helium, neon and argon.

Reaction of dioxygen with metals:
Most metals burn in dioxygen and form oxides that are mostly basic in nature.

Metal             Dioxygen        Metal-oxide
4M        +          O2         →     2M2O
                                          Δ
2M        +          O2         →     2MO
                                          Δ
4M        +          3O2        →     2M2O3
                                          Δ

Most of non-metals burn in the presence of dioxygen forms acidic oxides. EX: Sulphur burns in the presence of oxygen gives sulphur dioxide.
     
                                  Δ
S        +      O2          →    SO2

Reactions of dioxygen with some compounds:
Sulphur dioxide undergoes catalytic oxidation in the presence of vanadium pentoxide to form sulphur trioxide. This is an important step in the manufacture of sulphuric acid by the contact process.

                                     V2O5
2SO2        +      O2            →       2SO3

Dioxygen reacts with several organic compounds, such as hydrocarbons and carbohydrates, at elevated temperatures or on ignition, forming carbon dioxide and water.
 
                              High Temperature
CH4         +    2O2            →                  CO2       +     2H2O
Methane       Dioxygen                        Carbon               Water
                                                           dioxide

                                    High Temperature
C6H12O6         +    6O2            →                  6CO2       +     6H2O
Glucose          Dioxygen                                 Carbon               Water
                                                                     dioxide


Uses of dioxygen:
  • Dioxygen lies in its support to vital processes such as combustion and respiration.
  • The main importance of dioxygen lies in its support to vital processes such as combustion and respiration.
  • Dioxygen mixed with carbon dioxide or helium is used for artificial respiration.
  • It is used in the manufacture of many metals.
  • It is used in oxy-acetylene welding and metal cutting.
  • It is used to oxidise ammonia in the manufacture of nitric acid.
  • Oxygen cylinders are widely used in hospitals, high-altitude flying and in mountaineering.
  • Liquid oxygen is an important constituent of the fuel used in rockets.

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