Dioxygen

Dioxygen (O₂), known as molecular oxygen or oxygen gas, is the most common allotrope of oxygen. As one of the most electronegative elements, dioxygen is highly reactive. It readily reacts with nearly all metals and non-metals, except noble metals such as gold (Au), platinum (Pt), and noble gases. The compounds formed from these reactions are known as oxides. Oxygen constitutes nearly half of the Earth's crust, predominantly in the form of oxides.

1.0Introduction 

Dioxygen (O₂) is a colourless and odourless gas. Its solubility in water is 3.08 cm³ in 100 cm³ at 293 K, sufficient to support marine and aquatic life. Dioxygen liquefies at 90 K and freezes at 55 K. The oxygen atom has three stable isotopes: ¹⁶O, ¹⁷O, and ¹⁸O. Molecular oxygen (O₂) is unique in being paramagnetic despite having an even number of electrons.

Dioxygen readily reacts with nearly all metals and non-metals, except for gold (Au), platinum (Pt), and some noble gases. These reactions are often strongly exothermic, which helps sustain the reaction. However, initiating the reaction requires some external heating due to the high bond dissociation enthalpy of the oxygen-oxygen double bond (493.4 kJ/mol).

2.0Structure of Dioxygen

Two oxygen atoms are covalently bonded in the diatomic oxygen molecule (O₂). The bond length of the O₂ molecule is 121 pm, and the bond energy is 498 kJ/mol. Dioxygen is paramagnetic due to the presence of two unpaired electrons.

3.0Methods of Preparation of Dioxygen

Laboratory Method 

There are several methods to prepare dioxygen in the laboratory:

Catalytic Decomposition of Sodium Potassium Chlorate:

• Reaction: 2KClO₃→ 2KCl+3O₂​
• Conditions: This reaction occurs at 420 K when heated with manganese dioxide (MnO₂) as a catalyst.

Thermal Decomposition of Metal Oxides:

• Metal oxides with relatively low electrode potentials, such as mercury and silver, decompose to produce dioxygen.
• Reactions:
• • 2HgO(s) → 2Hg(l)+O₂(g)
  • 2PbO₂(s) → 2PbO(s)+O₂(g)

Thermal Decomposition of Oxygen-Rich Salts:

• Nitrates and permanganates decompose thermally to produce dioxygen.
• Reactions:
• • 2KNO₃→2KNO₂+O₂
  • 2KMnO₄→K₂MnO₄+MnO₂+O₂
  • 2NaNO₃→2NaNO₂+O₂

Decomposition of Hydrogen Peroxide:

• Hydrogen peroxide decomposes to produce dioxygen. Adding manganese dioxide (MnO₂) as a catalyst increases the decomposition rate.
• Reactions:
• • 2H₂O₂(aq)→2H₂O(l)+O₂(g)
• These methods are efficient and commonly used for generating dioxygen in laboratory settings.

Industrial Methods 

Fractional Distillation of Liquefied Air:

• Initial Steps: Carbon dioxide and water vapour are first removed from the air.
• Separation: The remaining air is cooled and liquefied. During fractional distillation, nitrogen (N₂) is distilled off as a vapour due to its lower boiling point, leaving oxygen (O₂) behind in liquid form.

Zeolite Molecular Sieves:

• Process: Clean, dry air is passed through zeolite molecular sieves that absorb nitrogen gas.
• Output: The gas that exits the sieve contains 90%-93% oxygen, which is then collected for use.

Electrolysis of Water:

• Process: Water is subjected to electrolysis, where an electrical current splits the water into its constituent gases.
• Reaction: Hydrogen is released at the cathode, and oxygen is liberated at the anode. This method provides a pure source of dioxygen.

4.0Physical Properties of Dioxygen

Dioxygen’s combination of physical properties and its paramagnetic nature make it vital for various biological and industrial processes.

• Molecular Mass: 31.99 g/mol (commonly approximated to 32 g/mol for convenience).
• Appearance: Colorless gas with no smell.
• Liquid State: Exists as a slightly bluish liquid at -183 °C.
• Solubility in Water: Highly soluble, with 3.08 cm³ dissolving in 100 cm³ of water at 293 K.
• Boiling Point: -182.962 °C.
• Melting Point: -218.79 °C.
• Density: 1.429 g/L, making it heavier than air.
• Magnetic Properties: Paramagnetic due to the presence of unpaired electrons.

5.0Chemical Properties of Dioxygen

Dioxygen (O₂) is the second most electronegative element in the periodic table after fluorine. Its reactions with other elements are generally exothermic. These properties and reactions illustrate dioxygen's high reactivity and versatility in chemical processes.

Some notable chemical properties and reactions of dioxygen include:

• Solubility: Dioxygen is more soluble in cold water than in warm water.
• Toxicity: Pure oxygen can be toxic to humans if inhaled in high concentrations for extended periods.

Reactions with Metals:

• General Reaction: M+O₂→MO₂

E.g. K+O₂→KO₂

• Formation of Metal Oxides:

4M+O₂→2M₂O 

4Na+O₂→2Na₂O

• Formation of Metal Sesquioxides:

4M+3O₂→2M₂O₃

 4Al+3O₂→2Al₂O₃

Reactions with Non-metals:

• C+O₂→CO₂
• P₄+5O₂→P₄O₁₀​

Reactions with Compounds:

2ZnS+3O₂→2ZnO+2SO₂

2SO₂+O₂→2SO₃​ (Catalyst: Vanadium pentoxide, V₂O₅)

• Combustion of Hydrocarbons and Carbohydrates:

CH₄+2O₂→CO₂+2H₂O

C₆H₁₂O₆+6O₂→6CO₂+6H₂O

• Formation of Chlorine Gas:

4HCl+O₂→2Cl₂+2H₂O₄ (Catalyst: CuCl₂)

Action of Silent Electric Discharge:

• Formation of Ozone:
• • 3O₂→2O₃ (When treated with electricity)

Catalytic Oxidation:   

2SO₂  + O₂ → 2SO₃

2CuCl₂ +  4HCl + O₂ → 2Cl₂  + 2H₂

• Simple Oxides: Examples include MgO and Al₂O₃.
• Mixed Oxides: Examples include Pb₃O₄ and Fe₃O₄.

6.0Simple Oxides

A binary compound of oxygen with another element is called an oxide. Oxygen reacts with most elements in the periodic table to form oxides, and one element can form multiple oxides. These oxides vary widely in nature and properties.

Classification of Simple Oxides:

Acidic Oxides:

• Combine with water to form acids. SO₂ + H₂O →H₂SO₃
• SO₂, Cl₂O₇, CO₂, N₂O₅
• Generally, non-metal oxides are acidic, but some metal oxides in high oxidation states also show an acidic character (e.g., Mn₂O₇, CrO₃, V₂O₅).

Basic Oxides:

• Na₂O, CaO, BaO
• Generally, metallic oxides are basic.

Amphoteric Oxides:

• Exhibit both acidic and basic properties.
• React with both acids and bases.
• Example: Al₂O₃

With acids:Al₂O₃+6HCl+9H₂O →2[Al(H₂O)₆]Cl₃

With bases: Al₂O₃  + 6NaOH+3H₂O →2Na₃[Al(OH)₆]

Neutral Oxides:

• Neither acidic nor basic.
• Examples: CO, NO, N₂O

7.0Uses of Dioxygen

• Chemical Synthesis: Dioxygen synthesises various chemicals such as ammonia, methyl alcohol, and acetylene.
• Industrial Combustion: It enhances air for combustion in blast furnaces and flash smelters, particularly for nickel and copper production.
• Rocket Propellants: Liquid oxygen serves as a key component in rocket fuels.
• Metal Welding: Metal welding is crucial for the welding of metals.
• Medical Applications: Oxygen supplementation is used in medicine and the production of medicinal drugs. Oxygen therapy treats heart disorders, pulmonary conditions, and diseases where the body struggles to utilise gaseous oxygen.
• Bacterial Control: Increasing the partial pressure of oxygen can eliminate anaerobic bacteria.
• Space Exploration: It is a low-pressure breathing gas in modern spacesuits.
• Scuba Diving: Artificially delivered oxygen is essential for scuba divers.
• Acrylic Acid Production: Dioxygen is extensively used to manufacture acrylic acid.
• Mountaineering: Oxygen cylinders are carried by mountaineers for high-altitude expeditions.