Oxygen (O) - Exam Revision Guide

Introduction

Oxygen (O), the most abundant element by mass in Earth’s crust and the second most abundant element in the atmosphere (20.95% by volume), is fundamental to most life forms and numerous industrial processes. Its high reactivity and unique electronic configuration dictate its role in diverse chemical reactions and biological cycles.

CBSE/JEE Quick Revision Notes

• Symbol: O
• Atomic Number (Z): 8
• Atomic Mass: 15.999 u ≈ 16 u
• Electronic Configuration: 1s² 2s² 2p⁴ ([He] 2s² 2p⁴)
• Valency: 2 (typically)
• Group: 16 (Chalcogens)
• Period: 2
• Block: p-block
• Nature: Non-metal, diatomic gas (O₂), paramagnetic
• Electronegativity (Pauling Scale): 3.44
• First Ionization Enthalpy: 1314 kJ/mol
• Electron Gain Enthalpy: -141 kJ/mol
• Allotropes: Dioxygen (O₂) and Ozone (O₃)
• Common Oxidation States: -2 (most common), -1 (peroxides), -1/2 (superoxides), 0 (elemental), +1 (O₂F₂), +2 (OF₂)

Electron Configuration & Bonding Behavior

Oxygen’s electronic configuration, 1s² 2s² 2p⁴, indicates six valence electrons. To achieve a stable octet, oxygen readily accepts two electrons or shares two electron pairs.

• Electronegativity: Oxygen is the second most electronegative element (after fluorine), contributing to its strong oxidizing properties.
• Covalent Bonding: Typically forms two covalent bonds (e.g., H₂O) or a double bond (e.g., O₂).
• Ionic Bonding: Forms O²⁻ ion with highly electropositive metals (e.g., Na₂O, CaO).
• Oxidation States:
  • -2: Most common, in oxides (H₂O, CO₂, CaO).
  • -1: In peroxides (H₂O₂, Na₂O₂).
  • -1/2: In superoxides (KO₂, RbO₂).
  • 0: In elemental oxygen (O₂, O₃).
  • +1: In dioxygen difluoride (O₂F₂), where F is more electronegative.
  • +2: In oxygen difluoride (OF₂), where F is more electronegative.
• Dioxygen (O₂): Contains a double bond (one sigma, one pi bond). Molecular Orbital Theory explains its paramagnetic nature due to two unpaired electrons in the π* antibonding orbitals.
• Ozone (O₃): An allotrope of oxygen, possessing a bent structure with resonance stabilization.

Crucial Chemical Reactions

Preparation of Dioxygen (O₂)

1. Laboratory Preparation:
   • Decomposition of Potassium Chlorate (KClO₃): 2KClO₃(s) --(MnO₂/Heat)--> 2KCl(s) + 3O₂(g)
   • Decomposition of Hydrogen Peroxide (H₂O₂): 2H₂O₂(aq) --(MnO₂)--> 2H₂O(l) + O₂(g)
   • Decomposition of Metal Oxides (e.g., Ag₂O, HgO) at high temperatures: 2Ag₂O(s) --(Heat)--> 4Ag(s) + O₂(g)
2. Industrial Preparation:
   • Fractional Distillation of Liquid Air: Air is liquefied, then fractionally distilled. Nitrogen (boiling point -196 °C) distills first, leaving behind liquid oxygen (boiling point -183 °C).
   • Electrolysis of Water: 2H₂O(l) --(Electricity)--> 2H₂(g) + O₂(g)

Reactions of Dioxygen (O₂)

Oxygen is a potent oxidizing agent and reacts with most elements and many compounds.

1. Reaction with Metals: Forms basic oxides.
   • 4Na(s) + O₂(g) --> 2Na₂O(s) (Sodium oxide)
   • 2Mg(s) + O₂(g) --> 2MgO(s) (Magnesium oxide)
   • 4Al(s) + 3O₂(g) --> 2Al₂O₃(s) (Aluminium oxide)
   • 3Fe(s) + 2O₂(g) --(Heat)--> Fe₃O₄(s) (Iron(II,III) oxide)
2. Reaction with Non-metals: Forms acidic or neutral oxides.
   • C(s) + O₂(g) --> CO₂(g) (Carbon dioxide, acidic)
   • S(s) + O₂(g) --> SO₂(g) (Sulfur dioxide, acidic)
   • P₄(s) + 5O₂(g) --> P₄O₁₀(s) (Phosphorus pentoxide, acidic)
   • N₂(g) + O₂(g) --(Electric Arc)--> 2NO(g) (Nitrogen monoxide, neutral)
   • 2H₂(g) + O₂(g) --> 2H₂O(l) (Water, neutral)
3. Reaction with Compounds (Combustion): Oxygen supports combustion.
   • Hydrocarbons: CH₄(g) + 2O₂(g) --> CO₂(g) + 2H₂O(g) (Methane combustion)
   • Hydrogen Sulfide: 2H₂S(g) + 3O₂(g) --> 2SO₂(g) + 2H₂O(g)
   • Ammonia: 4NH₃(g) + 5O₂(g) --(Pt/Rh catalyst)--> 4NO(g) + 6H₂O(g) (Ostwald process intermediate)

Industrial and Biological Importance

Industrial Importance

• Metallurgy: Used in steel manufacturing to remove impurities (e.g., carbon, sulfur, phosphorus) from molten iron by oxidation.
• Welding and Cutting: Oxy-acetylene torches produce high temperatures for welding and metal cutting.
• Chemical Synthesis: Used in the production of nitric acid (from ammonia), sulfuric acid, ethylene oxide, and other industrial chemicals.
• Medicine: Administered to patients with respiratory distress, in hyperbaric oxygen therapy.
• Rocket Propellant: Liquid oxygen (LOX) serves as an oxidizer in rocket engines.
• Wastewater Treatment: Used to enhance aerobic biological processes in sewage treatment.

Biological Importance

• Respiration: Essential for aerobic respiration in most living organisms, where it acts as the final electron acceptor in the electron transport chain, releasing energy (ATP). C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
• Photosynthesis (by-product): Oxygen is released into the atmosphere as a by-product of photosynthesis by plants and algae. 6CO₂ + 6H₂O --(Light Energy)--> C₆H₁₂O₆ + 6O₂
• Ozone Layer: Stratospheric ozone (O₃) absorbs harmful ultraviolet (UV) radiation from the sun, protecting life on Earth.
• Decomposition: Involved in the decomposition of organic matter, returning nutrients to the ecosystem.