Dioxygen – Preparation, Properties, and Uses 

1.0What is Dioxygen?

Dioxygen (O₂) is the most stable and abundant molecular form of oxygen found in nature. It exists as a diatomic molecule, meaning two oxygen atoms are chemically bonded together. About 21% of Earth’s atmosphere is made up of dioxygen, making it one of the most important gases for life, combustion, and industrial processes.

2.0Occurrence of Dioxygen in Nature

Dioxygen is the most abundant element by mass in the Earth’s crust and the second most abundant in the atmosphere. It is found in:

• Atmosphere: As a diatomic molecule (O₂), making up about 21% by volume.
• Water: As H₂O, oxygen constitutes about 89% by mass.
• Minerals: Found in silicates, oxides, carbonates, and sulfates.
• Living Organisms: Essential components of carbohydrates, proteins, fats, and nucleic acids.

3.0Preparation of Dioxygen

Laboratory Preparation of Dioxygen

In laboratory conditions, dioxygen is prepared by decomposition of oxygen-rich compounds.

1. Thermal decomposition of potassium chlorate (KClO₃):

$2{\mathrm{KClO}}_3\stackrel{{\mathrm{MnO}}_2,\Delta }{\to }2\mathrm{KCl}+3{\mathrm{O}}_2$

MnO₂ acts as a catalyst.

This is the most common laboratory method.

2. Decomposition of hydrogen peroxide (H₂O₂):

$2{\mathrm{H}}_2{\mathrm{O}}_2\stackrel{{\mathrm{MnO}}_2}{\to }2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$

Provides a convenient source of oxygen in labs.

3. Decomposition of nitrates and nitrites:
   Example:

$2\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\stackrel{\Delta }{\to }2\mathrm{PbO}+4{\mathrm{NO}}_2+{\mathrm{O}}_2$

Industrial Preparation of Dioxygen

Industrially, dioxygen is obtained in large quantities by:

• Fractional distillation of liquid air:
• Air is liquefied and cooled.
• Nitrogen (bp −196 °C) distills off first.
• Oxygen (bp −183 °C) is then collected in pure form.
• Electrolysis of water:

$2{\mathrm{H}}_2\mathrm{O}\stackrel{\text{ electricity }}{\to }2{\mathrm{H}}_2+{\mathrm{O}}_2$

Hydrogen and oxygen gases are liberated at electrodes.

Preparation of Dioxygen from Oxidizing Agents

Strong oxidizing agents release dioxygen on decomposition:

• Potassium permanganate (KMnO₄):

$2{\mathrm{KMnO}}_4\stackrel{\Delta }{\to }{\mathrm{K}}_2{\mathrm{MnO}}_4+{\mathrm{MnO}}_2+{\mathrm{O}}_2$

• Potassium dichromate (K₂Cr₂O₇):

$2{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7+3{\mathrm{H}}_2{\mathrm{SO}}_4\stackrel{\Delta }{\to }{\mathrm{Cr}}_2{\left({\mathrm{SO}}_4\right)}_3+{\mathrm{K}}_2{\mathrm{SO}}_4+3{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$

4.0Physical Properties of Dioxygen

• Color and odor: Colorless, odorless gas.
• Taste: Tasteless.
• Density: Slightly heavier than air (density = 1.429 g/L at STP).
• Solubility: Slightly soluble in water (supports aquatic life).
• Boiling point: −183 °C.
• Melting point: −218 °C.
• Paramagnetism: Dioxygen is paramagnetic due to two unpaired electrons in antibonding molecular orbitals (confirmed by molecular orbital theory).

5.0Chemical Properties of Dioxygen

Dioxygen is a highly reactive oxidizing agent. It forms oxides with almost all elements.

Reaction with Non-metals

• With hydrogen:

$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$

A highly exothermic reaction.

• With carbon:

$\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$

In limited supply of oxygen:

$2\mathrm{C}+{\mathrm{O}}_2\to 2\mathrm{CO}$

• With sulfur:

$\mathrm{S}+{\mathrm{O}}_2\to {\mathrm{SO}}_2$

Reaction with Metals

• With sodium:

$4\mathrm{Na}+{\mathrm{O}}_2\to 2{\mathrm{Na}}_2\mathrm{O}$

• With calcium:

$2\mathrm{Ca}+{\mathrm{O}}_2\to 2\mathrm{CaO}$

• With magnesium:

$2\mathrm{Mg}+{\mathrm{O}}_2\to 2\mathrm{MgO}$

Reaction with Compounds

• With ammonia (at high temperature, Pt catalyst):

$4{\mathrm{NH}}_3+3{\mathrm{O}}_2\to 2{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$

• With methane:

${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$

Combustion Reactions

• Dioxygen supports combustion of fuels, hydrocarbons, and other organic compounds.
• Complete combustion produces CO₂ and H₂O.
• Incomplete combustion produces CO and soot.

Redox Behavior

Dioxygen acts as an oxidizing agent:

${\mathrm{O}}_2+4e^{-}+4{\mathrm{H}}^{+}\to 2{\mathrm{H}}_2\mathrm{O}$

It also shows variable oxidation states in oxides (−2 in H₂O, −1 in peroxides, +2 in OF₂).

6.0Uses of Dioxygen

Dioxygen has a wide range of applications in industry, medicine, and daily life:

• Respiration: Essential for survival of living organisms.
• Combustion: Supports burning of fuels for energy.
• Industrial uses:
• Steel industry: Used in Bessemer process for removing impurities.
• Welding and cutting: Oxy-acetylene flame for welding metals.
• Chemical industry: For production of HNO₃, SO₂, H₂O₂.
• Medical applications:
• Oxygen cylinders for patients with respiratory problems.
• Used in anesthesia (with nitrous oxide).
• Aerospace: Liquid oxygen used as rocket fuel oxidizer.
• Water treatment: Ozone (O₃), derived from oxygen, is used for disinfection.

7.0Importance of Dioxygen in Chemistry and Daily Life

• Acts as a universal oxidizing agent in chemical reactions.
• Vital for metabolism and energy production in living organisms.
• Plays a role in environmental cycles – respiration, combustion, and photosynthesis.
• Integral to industrial chemistry, supporting large-scale processes.
• Strongly emphasized in JEE Chemistry due to its fundamental reactivity.