Important Oxygen Compounds: Revision Guide

Introduction to the Major Compounds of Oxygen

Oxygen, a highly reactive non-metal, forms numerous compounds with almost all other elements. The most important compounds studied at the high school level include water (H₂O), hydrogen peroxide (H₂O₂), and ozone (O₃). These compounds exhibit diverse chemical and physical properties due to the varying oxidation states and bonding environments of oxygen.

Water (H₂O)

Chemical Formula

H₂O

Common Name

Water

Laboratory Preparation

Water can be prepared in the laboratory by the direct combustion of hydrogen gas in oxygen gas.

Balanced Equation

2H₂(g) + O₂(g) → 2H₂O(l)

Hydrogen Peroxide (H₂O₂)

Chemical Formula

H₂O₂

Preparation Process

1. By Barium Peroxide (BaO₂)

Hydrogen peroxide is typically prepared in the laboratory by the action of dilute sulfuric acid on hydrated barium peroxide. BaO₂(s)⋅8H₂O(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O₂(aq) + 8H₂O(l) Barium sulfate (BaSO₄) is insoluble and can be filtered off.

2. By Electrolytic Process (Persulfate Method)

In industries, it is prepared by the electrolysis of 50% H₂SO₄ or an equimolar solution of ammonium sulfate and sulfuric acid, which forms peroxodisulfuric acid (H₂S₂O₈). This is then hydrolyzed to yield H₂O₂.

Electrolysis: 2H₂SO₄(aq) → H₂S₂O₈(aq) + H₂(g) (at anode: 2HSO₄⁻ → H₂S₂O₈ + 2e⁻)

Hydrolysis: H₂S₂O₈(aq) + 2H₂O(l) → 2H₂SO₄(aq) + H₂O₂(aq)

Properties

Physical Properties

• Colorless liquid, slightly more viscous than water.
• High dielectric constant.
• Miscible with water in all proportions.

Chemical Properties

1. Decomposition: Decomposes slowly in the absence of light, but rapidly in the presence of light, metals (e.g., Pt, Ag, Cu), metal oxides (e.g., MnO₂), or dust. 2H₂O₂(aq) → 2H₂O(l) + O₂(g)

2. Acidic Nature: It is a weak acid. H₂O₂(aq) ⇌ H⁺(aq) + HO₂⁻(aq) H₂O₂(aq) + 2NaOH(aq) → Na₂O₂(aq) + 2H₂O(l)

3. Oxidizing Agent: Acts as a powerful oxidizing agent in both acidic and basic media.

   • In acidic medium: H₂O₂(aq) + 2H⁺(aq) + 2e⁻ → 2H₂O(l) 2Fe²⁺(aq) + H₂O₂(aq) + 2H⁺(aq) → 2Fe³⁺(aq) + 2H₂O(l) PbS(s) + 4H₂O₂(aq) → PbSO₄(s) + 4H₂O(l) (Restoration of old oil paintings)
   • In basic medium: H₂O₂(aq) + 2e⁻ → 2OH⁻(aq) 2Cr³⁺(aq) + 3H₂O₂(aq) + 10OH⁻(aq) → 2CrO₄²⁻(aq) + 8H₂O(l) (Chromate formation)

4. Reducing Agent: Can also act as a reducing agent in the presence of strong oxidizing agents.

   • In acidic medium: H₂O₂(aq) → O₂(g) + 2H⁺(aq) + 2e⁻ 5H₂O₂(aq) + 2MnO₄⁻(aq) + 6H⁺(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5O₂(g)
   • In basic medium: H₂O₂(aq) + 2OH⁻(aq) → O₂(g) + 2H₂O(l) + 2e⁻ Cl₂(g) + H₂O₂(aq) + 2OH⁻(aq) → 2Cl⁻(aq) + 2H₂O(l) + O₂(g)

Ozone (O₃)

Chemical Formula

O₃

Brief Guide

Ozone is an allotrope of oxygen, consisting of three oxygen atoms. It is a pale blue gas with a pungent odor. It is much less stable than dioxygen (O₂) and readily decomposes to form dioxygen.

Preparation

Ozone is prepared by passing a silent electric discharge through dry oxygen in an apparatus called an ozonizer. 3O₂(g) ⇌ 2O₃(g) (ΔH = +142 kJ/mol) The silent electric discharge is necessary to prevent the decomposition of ozone formed. Only about 10% ozone is formed; higher concentrations lead to instability.

Properties

1. Strong Oxidizing Agent: Due to its ready decomposition into oxygen and nascent oxygen, it is a powerful oxidizing agent. O₃(g) → O₂(g) + [O](nascent oxygen)

   • Oxidation of Lead Sulphide: PbS(s) + 4O₃(g) → PbSO₄(s) + 4O₂(g)
   • Oxidation of Potassium Iodide: 2KI(aq) + H₂O(l) + O₃(g) → 2KOH(aq) + I₂(s) + O₂(g)
   • Bleaching action: Ozone bleaches coloring matter by oxidation.

2. Thermodynamic Instability: It is thermodynamically unstable with respect to oxygen, as its decomposition to oxygen is exothermic and increases entropy.

Comparative Properties of Oxygen Compounds

Property	Water (H₂O)	Hydrogen Peroxide (H₂O₂)	Ozone (O₃)
Formula	H₂O	H₂O₂	O₃
Nature of Oxygen	-2 oxidation state	-1 oxidation state	0 oxidation state (in elemental form), highly reactive
Stability	Highly stable	Less stable, decomposes readily	Very unstable, readily decomposes to O₂
Oxidizing/Reducing	Neutral/Amphoteric (can act as acid or base)	Both oxidizing and reducing agent	Powerful oxidizing agent
Physical State (RT)	Liquid	Liquid	Pale blue gas
Role in Environment	Universal solvent, life support	Bleaching agent, antiseptic, rocket fuel oxidizer	Protects Earth from UV radiation (stratospheric)