11. Dioxygen

DIOXYGEN (O₂)

It differs from the remaining elements of the VI^th group because  of the following properties.

(A) small size

(B) high electronegativity and

(C) non-availability of d-orbitals.

Preparation :

(i) By thermal decomposition of oxides of metals.

2 HgO   2 Hg + O₂

2 Ag₂O   4 Ag + O₂ 

(ii) By thermal decomposition of oxygen rich compounds.                                                 

KClO₃     2 KCl + 3O₂ (laboratory method)

(iii) 2H₂O₂(aq.)  2H₂O(l) + O₂(g)

(iv) Industrial method :

(a) Electrolysis of water leads to the release of hydrogen at the cathode and oxygen at the anode.

 (b) Oxygen is obtained by liquification of air and then its fractional distillation.

Physical properties :

Colourless , odourless and tasteless gas. It is paramagnetic and exhibits allotropy. Three isotopes of oxygen are ,  and . Oxygen does not burn but is a strong supporter of combustion.

Chemical properties :

(i) Reaction with metals :

2Ca + O₂  —® 2CaO  

4Al + 3O₂ —® 2Al₂O₃

(ii) Reaction with non-metals :

P₄ + 5O₂ —®  P₄O₁₀

C + O₂ —® CO₂

 (iii) Reaction with compounds :

2ZnS + 3O₂ —®  2ZnO + 2SO₂ 

CH₄ + 2O₂ —®CO₂ + 2H₂O

2SO₂ + O₂  2SO₃

4HCl + O₂   2Cl₂ + 2H₂O

Note : It has been observed that its combination with other elements is often strongly exothermic which helps in sustaining the reaction. However, to initiate the reaction, some external heating is required as bond dissociation enthalpy of oxygen-oxygen double bond is high (493.4 kJ mol^–1).

Use : 

1. Oxygen mixed with helium or CO₂ is used for artificial respiration.

2. Liquid oxygen (with combustion fuel hydrazine) is used as oxidising agent in rocket fuels.

3. Oxygen is used for production of oxy-hydrogen or oxy-acetylene flames employed for cutting and welding.

4. Pure dioxygen is used to convert pig iron into steel in the basic oxygen process which are kaldo and LD process.