Hydrogen Peroxide - Structure, Preparation, and Properties of H2O2

What Is Hydrogen Peroxide (H₂O₂)?

Hydrogen peroxide is the simplest kind of peroxide available (oxygen-oxygen single bond). It is a colourless liquid and is used in aqueous solution for safety reasons. It acts as a bleaching agent and is also used as a disinfectant. Concentrated hydrogen peroxide is a very reactive oxygen species and is used as a propellant in rocketry. The chemical formula for hydrogen peroxide is H₂O₂.

Table of Contents

• Overview
• Structure of Hydrogen Peroxide
• Preparation of H₂O₂
• Hydrogen Peroxide Properties
• Concentration of H₂O₂

Hydrogen peroxide is often referred to as water with one more oxygen atom. It is acidic in nature, and PH is about 4.5. It is a 100 per cent degradable compound.

Overview of H₂O₂ – Hydrogen Peroxide

Structure of Hydrogen Peroxide

The structure of hydrogen peroxide is non-planar. H₂O₂ has an open-book structure with O–O spins. The dihedral angle is 111°. The O-O bond length is 145.8 pm, and the O-H bond length is 98.8 pm (which is equal to 9.88 × 10^-13 m). The following diagram clearly shows what an open book structure means.

Hydrogen Peroxide Structure

There are two planes in this structure, and each plane has one O-H bond pair, the angle between both planes is 90.2°.

⇒ Also Read: Chemical Bonding

Preparation of Hydrogen Peroxide

Let us have a look at the various methods of preparation for hydrogen peroxide.

Laboratory Methods of Preparation

When barium peroxide is acidified, and the excess water is removed by the process of evaporation under reduced pressure, we obtain hydrogen peroxide. The following reaction will clarify this:

BaO₂.8H₂O(s) + H₂SO₄(aq) → BaSO₄(s) + H₂O₂(aq) + 8H₂O(l)

Industrial Method of Preparation

Hydrogen peroxide is prepared by the electrolysis of 30% ice-cold H₂SO₄. When acidified sulfate solution is electrolysed at a high current density, peroxodisulphate is obtained. Peroxodisulphate is then hydrolysed to get hydrogen peroxide.

2HSO^–₄(aq) [Electrolysis] → HO₃SOOSO₃H(aq) [Hydrolysis] → 2HSO^–₄(aq)+2H+(aq)+H₂O₂(aq)

Reaction Mechanism

• Electrolyte: 30% dilute H₂SO₄
• Cathode: Pb wire
• Anode: Pt rod

2H₂SO₄ → 2H⁺ + 2HSO^–₄

At Cathode: 2H⁺ + 2e^– → H₂

At Anode: 

2HSO^–₄ → H₂S₂O₈ + 2e^– ⇒ Peroxodi Sulphuric Acid [Marshall’s acid]

H₂S₂O₈ + H₂O → H₂SO₅ + H₂SO₄ ⇒ Peroxomono Sulphuric Acid [Caro’s acid]

H₂SO₅ + H₂O → H₂SO₄ + H₂O₂

Commercial Methods of Preparation

Auto-Oxidation Method:

Auto-Oxidation Method of Preparation of H₂O₂

Properties of Hydrogen Peroxide

Physical Properties

• In the pure state, hydrogen peroxide is an almost colourless (very pale blue) liquid.
• It melts at 272.4 K and has a boiling point of 423 K (extrapolated).
• It is miscible in water in all proportions and forms hydrates.

Chemical Properties

Hydrogen peroxide in both acidic and basic mediums acts as an oxidising as well as a reducing agent. The following reactions will give a clear picture:

Oxidation Reactions of H₂O₂

• Oxidises black Pbs to white PbSO₄. Reaction: Pbs + 4H₂O₂ → PbSO₄ + 4H₂O

• Oxidises KI to Iodine. Reaction: 2KI + H₂O₂ → 2KOH + I₂

• Oxidises nitrites to nitrates. Reaction: NaNO₂ + H₂O₂ → NaNO₃ + H₂O
• Oxidises acidified potassium ferrocyanide. Reaction: 2K₄Fe(CN)₆ + H₂SO₄ + H₂O₂ → 2K₃Fe(CN)₆ + K₂SO4 + 2H₂O

• Oxidises sulphites to sulphates. Reaction: Na₂SO₃ + H₂O₂ → Na₂SO₄ + H₂O

• Oxidises CrO^-2₄ in an acid medium to chromium peroxide CrO₅. Reaction: CrO^-2₄ + 2H⁺ + 2H₂O₂ → CrO₅ + 3H₂O

⇒ Check: Redox Reactions

Bleaching Reactions

It bleaches by oxidation. H₂O₂ bleaches silk, wool, cotton and hair.

Reaction: H₂O₂ → H₂O + [O] Nascent oxygen

Coloured substance + [O] → colourless

Reduction Reactions of Hydrogen Peroxide

• It reduces barium peroxide to monoxide. Reaction: BaO₂ + H₂O₂ → BaO + H₂O + O₂
• It reduces ozone to oxygen. Reaction: O₃ + H₂O₂ → H₂O + 2O₂
• It reduces alkaline potassium ferricyanide. Reaction: 2K₃Fe(CN)₆ + 2KOH + H₂O₂ → 2K₄Fe(CN)₆ + 2H₂O + O₂

Why Is Hydrogen Peroxide Stored in Plastic Containers?

Hydrogen peroxide decomposes when exposed to sunlight, this process is catalysed by traces of alkali metals. Therefore, H₂O₂ is stored in wax-lined glass or plastic containers and kept in the dark.

It should also be kept away from dust particles because dust can induce explosive decomposition of this compound.

How to Determine the Concentration of Hydrogen Peroxide?

Volume Strength of H₂O₂

10 Vol H₂O₂ means 1 mL of H₂O₂ gives 10 mL of O₂ at STP.

Weight Percentage [w/w%]

Let us consider 10 Vol H₂O₂

2H₂O₂ → 2H₂O + O₂

→ 68g of H₂O₂ gives 22.4 L of O₂ at STP.

→ 10 L of O₂ is given by

Amount of hydrogen peroxide = 10 × 68/22.4 = 30.36 g

1000 mL of H₂O₂ → 30.36 g

100 mL → ?

= 30.36%

100 Vol H₂O₂ or 3.036%(w/w) H₂O₂ is known as Perhydrol.

Molarity of H₂O₂

To calculate the molarity, let us consider 10 Vol H₂O₂

Molarity of hydrogen peroxide = 30.36/34 = 0.89 M.

Normality of H₂O₂

Let us consider 10 Vol H₂O₂

= 30.36/17 = 1.78N

Also Read: What Is Hydrogen Peroxide Used for?

Hydrogen Peroxide – Video Lesson