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Group 16: Oxygen - Dioxygen

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Group 16: Oxygen - Dioxygen - Lesson 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 
2KClO 3       →        2KCl + 3O 2
                 MnO 2

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.

    2Ag 2O           →  4Ag     +   O 2
Silver oxide         Δ     Silver        Dioxygen

     2HgO                →    2Hg     +   O 2
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.

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

   2BaO 2                     →       2BaO                 +   O 2
Barium peroxide            Δ     Barium oxide        Dioxygen

   2MnO 2          + 2H 2SO 4      →  2MnSO 4         + 2H 2O    + O 2
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.

2KNO 3             → 2KNO 2             +  O 2
Potassium        Δ     Potassium         Dioxygen
      nitrate                    nitrate

2NaNO 3             → 2NaNO 2             +  O 2
Sodium               Δ     Sodium         Dioxygen
    nitrate                    nitrate

2KMnO 4             → K2MnO 4          +  MnO 2                    +  O 2
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        +          O 2         →     2M 2O
                                          Δ
2M        +          O 2         →     2MO
                                          Δ
4M        +          3O 2        →     2M 2O 3
                                          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        +      O 2          →    SO 2

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.

                                     V 2O 5
2SO 2        +      O 2            →       2SO 3

Dioxygen reacts with several organic compounds, such as hydrocarbons and carbohydrates, at elevated temperatures or on ignition, forming carbon dioxide and water.
 
                              High Temperature
CH 4         +    2O 2            →                  CO 2       +     2H 2O
Methane       Dioxygen                        Carbon               Water
                                                           dioxide

                                    High Temperature
C6H 12O6         +    6O 2            →                  6CO 2       +     6H 2O
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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